Some Basic Concepts of Chemistry - PDF

Unit 1 Some Basic Concepts of Chemistry Chemistry is the science of molecules and their transformations. It is the science not so much of the one...

Unit 1 Some Basic Concepts of Chemistry Chemistry is the science of molecules and their transformations. It is the science not so much of the one hundred elements but of the infinite variety of molecules After studying this unit, you will be that may be built from them. able to appreciate the contribution of India Roald Hoffmann in the development of chemistry understand the role of chemistry in different spheres of life; Science can be viewed as a continuing human effort to explain the characteristics of three systematise knowledge for describing and understanding states of matter; nature. You have learnt in your previous classes that we classify different substances come across diverse substances present in nature and into elements, compounds and changes in them in daily life. Curd formation from milk, mixtures; formation of vinegar from sugarcane juice on keeping use scientific notations and for prolonged time and rusting of iron are some of the determine significant figures; examples of changes which we come across many times. differentiate between precision and For the sake of convenience, science is sub-divided into accuracy; various disciplines: chemistry, physics, biology, geology, define SI base units and convert etc. The branch of science that studies the preparation, physical quantities from one system of units to another; properties, structure and reactions of material substances explain various laws of chemical is called chemistry. combination; Development of chemistry appreciate significance of atomic Chemistry, as we understand it today, is not a very old mass, average atomic mass, discipline. Chemistry was not studied for its own sake, rather molecular mass and formula mass; it came up as a result of search for two interesting things: describe the terms – mole and molar mass; i. Philosopher’s stone (Paras) which would convert all baser metals e.g., iron and copper into gold. calculate the mass per cent of component elements constituting ii. ‘Elexir of life’ which would grant immortality. a compound; People in ancient India, already had the knowledge of determine empirical formula and many scientific phenomenon much before the advent of molecular formula for a compound modern science. They applied that knowledge in various from the given experimental data; walks of life. Chemistry developed mainly in the form and of Alchemy and Iatrochemistry during 1300-1600 CE. perform the stoichiometric Modern chemistry took shape in the 18th century Europe, calculations. after a few centuries of alchemical traditions which were introduced in Europe by the Arabs. Rationalised 2023-24 Unit 1.indd 1 9/9/2022 4:27:29 PM  2 chemistry Other cultures – especially the Chinese be shown to agree with modern scientific and the Indian – had their own alchemical findings. Copper utensils, iron, gold, silver traditions. These included much knowledge of ornaments and terracotta discs and painted chemical processes and techniques. grey pottery have been found in many In ancient India, chemistry was called archaeological sites in north India. Sushruta Rasayan Shastra, Rastantra, Ras Kriya or Samhita explains the importance of Alkalies. Rasvidya. It included metallurgy, medicine, The Charaka Samhita mentions ancient manufacture of cosmetics, glass, dyes, etc. indians who knew how to prepare sulphuric Systematic excavations at Mohenjodaro in acid, nitric acid and oxides of copper, tin and Sindh and Harappa in Punjab prove that the zinc; the sulphates of copper, zinc and iron story of development of chemistry in India and the carbonates of lead and iron. is very old. Archaeological findings show that baked bricks were used in construction Rasopanishada describes the preparation work. It shows the mass production of of gunpowder mixture. Tamil texts also pottery, which can be regarded as the earliest describe the preparation of fireworks using chemical process, in which materials were sulphur, charcoal, saltpetre (i.e., potassium mixed, moulded and subjected to heat by nitrate), mercury, camphor, etc. using fire to achieve desirable qualities. Nagarjuna was a great Indian scientist. He Remains of glazed pottery have been found in was a reputed chemist, an alchemist and a Mohenjodaro. Gypsum cement has been used metallurgist. His work Rasratnakar deals with in the construction work. It contains lime, the formulation of mercury compounds. He sand and traces of CaCO3. Harappans made has also discussed methods for the extraction faience, a sort of glass which was used in of metals, like gold, silver, tin and copper. A ornaments. They melted and forged a variety book, Rsarnavam, appeared around 800 CE. of objects from metals, such as lead, silver, It discusses the uses of various furnaces, gold and copper. They improved the hardness ovens and crucibles for different purposes. It of copper for making artefacts by using tin describes methods by which metals could be and arsenic. A number of glass objects were found in Maski in South India (1000–900 identified by flame colour. BCE), and Hastinapur and Taxila in North Chakrapani discovered mercury sulphide. India (1000–200 BCE). Glass and glazes were The credit for inventing soap also goes to him. coloured by addition of colouring agents like He used mustard oil and some alkalies as metal oxides. ingredients for making soap. Indians began Copper metallurgy in India dates back to making soaps in the 18th century CE. Oil of the beginning of chalcolithic cultures in the Eranda and seeds of Mahua plant and calcium subcontinent. There are much archeological carbonate were used for making soap. evidences to support the view that technologies The paintings found on the walls of Ajanta for extraction of copper and iron were and Ellora, which look fresh even after ages, developed indigenously. testify to a high level of science achieved in According to Rigveda, tanning of leather ancient India. Varähmihir’s Brihat Samhita is and dying of cotton were practised during a sort of encyclopaedia, which was composed 1000–400 BCE. The golden gloss of the in the sixth century CE. It informs about the black polished ware of northen India could preparation of glutinous material to be applied not be replicated and is still a chemical on walls and roofs of houses and temples. It mystery. These wares indicate the mastery was prepared entirely from extracts of various with which kiln temperatures could be plants, fruits, seeds and barks, which were controlled. Kautilya’s Arthashastra describes concentrated by boiling, and then, treated the production of salt from sea. with various resins. It will be interesting to A vast number of statements and material test such materials scientifically and assess described in the ancient Vedic literature can them for use. Rationalised 2023-24 Unit 1.indd 2 9/9/2022 4:27:29 PM  Some Basic Concepts of Chemistry 3 A number of classical texts, like forces cause interaction between them. He Atharvaveda (1000 BCE) mention some conceptualised this theory around 2500 years dye stuff, the material used were turmeric, before John Dalton (1766-1844). madder, sunflower, orpiment, cochineal and Charaka Samhita is the oldest Ayurvedic lac. Some other substances having tinting epic of India. It describes the treatment of property were kamplcica, pattanga and jatuka. diseases. The concept of reduction of particle Varähmihir’s Brihat Samhita gives size of metals is clearly discussed in Charaka references to perfumes and cosmetics. Samhita. Extreme reduction of particle size is Recipes for hair dying were made from plants, termed as nanotechnology. Charaka Samhita like indigo and minerals like iron power, describes the use of bhasma of metals in the black iron or steel and acidic extracts of sour treatment of ailments. Now-a-days, it has rice gruel. Gandhayukli describes recipes been proved that bhasmas have nanoparticles for making scents, mouth perfumes, bath of metals. powders, incense and talcum power. After the decline of alchemy, Iatrochemistry Paper was known to India in the 17 th reached a steady state, but it too declined due century as account of Chinese traveller I-tsing to the introduction and practise of western describes. Excavations at Taxila indicate that medicinal system in the 20th century. During ink was used in India from the fourth century. this period of stagnation, pharmaceutical Colours of ink were made from chalk, red lead industry based on Ayurveda continued to and minimum. exist, but it too declined gradually. It took It seems that the process of fermentation about 100-150 years for Indians to learn was well-known to Indians. Vedas and and adopt new techniques. During this time, Kautilya’s Arthashastra mention about foreign products poured in. As a result, many types of liquors. Charaka Samhita also indigenous traditional techniques gradually mentions ingredients, such as barks of plants, declined. Modern science appeared in Indian stem, flowers, leaves, woods, cereals, fruits scene in the later part of the nineteenth and sugarcane for making Asavas. century. By the mid-nineteenth century, European scientists started coming to India The concept that matter is ultimately and modern chemistry started growing. made of indivisible building blocks, appeared in India a few centuries BCE as a part of From the above discussion, you have learnt philosophical speculations. Acharya Kanda, that chemistry deals with the composition, born in 600 BCE, originally known by the structure, properties and interection of matter name Kashyap, was the first proponent and is of much use to human beings in daily of the ‘atomic theory’. He formulated the life. These aspects can be best described and theory of very small indivisible particles, understood in terms of basic constituents of which he named ‘Paramãnu’ (comparable matter that are atoms and molecules. That to atoms). He authored the text Vaiseshika is why, chemistry is also called the science of Sutras. According to him, all substances are atoms and molecules. Can we see, weigh and aggregated form of smaller units called atoms perceive these entities (atoms and molecules)? (Paramãnu), which are eternal, indestructible, Is it possible to count the number of atoms spherical, suprasensible and in motion in and molecules in a given mass of matter and the original state. He explained that this have a quantitative relationship between the individual entity cannot be sensed through mass and the number of these particles? any human organ. Kanda added that there We will get the answer of some of these are varieties of atoms that are as different as questions in this Unit. We will further describe the different classes of substances. He said how physical properties of matter can be these (Paramãnu) could form pairs or triplets, quantitatively described using numerical among other combinations and unseen values with suitable units. Rationalised 2023-24 Unit 1.indd 3 9/9/2022 4:27:29 PM  4 chemistry 1.1 IMPORTANCE OF CHEMISTRY many big environmental problems continue to Chemistry plays a central role in science and be matters of grave concern to the chemists. is often intertwined with other branches of One such problem is the management of the science. Green House gases, like methane, carbon dioxide, etc. Understanding of biochemical Principles of chemistry are applicable processes, use of enzymes for large-scale in diverse areas, such as weather patterns, production of chemicals and synthesis of new functioning of brain and operation of a exotic material are some of the intellectual computer, production in chemical industries, challenges for the future generation of manufacturing fertilisers, alkalis, acids, salts, chemists. A developing country, like India, dyes, polymers, drugs, soaps, detergents, needs talented and creative chemists for metals, alloys, etc., including new material. accepting such challenges. To be a good Chemistry contributes in a big way to the chemist and to accept such challanges, one national economy. It also plays an important needs to understand the basic concepts of role in meeting human needs for food, chemistry, which begin with the concept of healthcare products and other material matter. Let us start with the nature of matter. aimed at improving the quality of life. This 1.2 Nature of Matter is exemplified by the large-scale production You are already familiar with the term matter of a variety of fertilisers, improved variety from your earlier classes. Anything which has of pesticides and insecticides. Chemistry mass and occupies space is called matter. provides methods for the isolation of life- Everything around us, for example, book, pen, saving drugs from natural sources and pencil, water, air, all living beings, etc., are makes possible synthesis of such drugs. composed of matter. You know that they have Some of these drugs are cisplatin and mass and they occupy space. Let us recall the taxol, which are effective in cancer therapy. characteristics of the states of matter, which The drug AZT (Azidothymidine) is used for you learnt in your previous classes. helping AIDS patients. Chemistry contributes to a large extent in 1.2.1 States of Matter the development and growth of a nation. With You are aware that matter can exist in three a better understanding of chemical principles physical states viz. solid, liquid and gas. it has now become possible to design and The constituent particles of matter in these synthesise new material having specific three states can be represented as shown in magnetic, electric and optical properties. This Fig. 1.1. has lead to the production of superconducting Particles are held very close to each other ceramics, conducting polymers, optical fibres, in solids in an orderly fashion and there is not etc. Chemistry has helped in establishing much freedom of movement. In liquids, the industries which manufacture utility goods, particles are close to each other but they can like acids, alkalies, dyes, polymesr metals, move around. However, in gases, the particles etc. These industries contribute in a big way are far apart as compared to those present in to the economy of a nation and generate solid or liquid states and their movement is employment. easy and fast. Because of such arrangement In recent years, chemistry has helped in of particles, different states of matter exhibit dealing with some of the pressing aspects the following characteristics: of environmental degradation with a fair (i) Solids have definite volume and definite degree of success. Safer alternatives to shape. environmentally hazardous refrigerants, (ii) Liquids have definite volume but do like CFCs (chlorofluorocarbons), responsible not have definite shape. They take the for ozone depletion in the stratosphere, have shape of the container in which they are been successfully synthesised. However, placed. Rationalised 2023-24 Unit 1.indd 4 9/9/2022 4:27:29 PM  Some Basic Concepts of Chemistry 5 Fig. 1.1 Arrangement of particles in solid, liquid Fig. 1.2 Classification of matter and gaseous state (iii) Gases have neither definite volume nor completely mix with each other. This means definite shape. They completely occupy particles of components of the mixture are the space in the container in which they uniformly distributed throughout the bulk of are placed. the mixture and its composition is uniform throughout. Sugar solution and air are the These three states of matter are examples of homogeneous mixtures. In interconvertible by changing the conditions contrast to this, in a heterogeneous mixture, of temperature and pressure. the composition is not uniform throughout Solid liquid Gas and sometimes different components are On heating, a solid usually changes to visible. For example, mixtures of salt and a liquid, and the liquid on further heating sugar, grains and pulses along with some changes to gas (or vapour). In the reverse dirt (often stone pieces), are heterogeneous process, a gas on cooling liquifies to the liquid mixtures. You can think of many more and the liquid on further cooling freezes to examples of mixtures which you come across the solid. in the daily life. It is worthwhile to mention here that the components of a mixture can 1.2.2. Classification of Matter be separated by using physical methods, In Class IX (Chapter 2), you have learnt that such as simple hand-picking, filtration, at the macroscopic or bulk level, matter can crystallisation, distillation, etc. be classified as mixture or pure substance. Pure substances have characteristics These can be further sub-divided as shown different from mixtures. Constituent particles in Fig. 1.2. of pure substances have fixed composition. When all constituent particles of a Copper, silver, gold, water and glucose are substance are same in chemical nature, it some examples of pure substances. Glucose is said to be a pure substance. A mixture contains carbon, hydrogen and oxygen in contains many types of particles. a fixed ratio and its particles are of same A mixture contains particles of two or composition. Hence, like all other pure more pure substances which may be present substances, glucose has a fixed composition. in it in any ratio. Hence, their composition is Also, its constituents—carbon, hydrogen variable. Pure substances forming mixture and oxygen—cannot be separated by simple are called its components. Many of the physical methods. substances present around you are mixtures. Pure substances can further be classified For example, sugar solution in water, air, into elements and compounds. Particles tea, etc., are all mixtures. A mixture may of an element consist of only one type of be homogeneous or heterogeneous. In a atoms. These particles may exist as atoms or homogeneous mixture, the components molecules. You may be familiar with atoms Rationalised 2023-24 Unit 1.indd 5 9/9/2022 4:27:30 PM  6 chemistry and molecules from the previous classes; however, you will be studying about them in detail in Unit 2. Sodium, copper, silver, hydrogen, oxygen, etc., are some examples of elements. Their all atoms are of one type. However, the atoms of different elements Water molecule Carbon dioxide are different in nature. Some elements, (H2O) molecule (CO2) such as sodium or copper, contain atoms Fig. 1.4 A depiction of molecules of water and as their constituent particles, whereas, in carbon dioxide some others, the constituent particles are molecules which are formed by two or more elements are present in a compound in a fixed atoms. For example, hydrogen, nitrogen and and definite ratio and this ratio is characteristic oxygen gases consist of molecules, in which of a particular compound. Also, the properties two atoms combine to give their respective of a compound are different from those of its molecules. This is illustrated in Fig. 1.3. constituent elements. For example, hydrogen and oxygen are gases, whereas, the compound formed by their combination i.e., water is a liquid. It is interesting to note that hydrogen burns with a pop sound and oxygen is a supporter of combustion, but water is used as a fire extinguisher. 1.3 Properties of Matter and their Measurement 1.3.1 Physical and chemical properties Every substance has unique or characteristic properties. These properties can be classified into two categories — physical properties, such as colour, odour, melting point, boiling point, density, etc., and chemical properties, like composition, combustibility, ractivity with Fig. 1.3 A representation of atoms and molecules acids and bases, etc. When two or more atoms of different Physical properties can be measured elements combine together in a definite ratio, or observed without changing the identity the molecule of a compound is obtained. or the composition of the substance. The Moreover, the constituents of a compound measurement or observation of chemical cannot be separated into simpler substances properties requires a chemical change to by physical methods. They can be separated occur. Measurement of physical properties by chemical methods. Examples of some does not require occurance of a chemical change. The examples of chemical properties compounds are water, ammonia, carbon are characteristic reactions of different dioxide, sugar, etc. The molecules of water substances; these include acidity or basicity, and carbon dioxide are represented in Fig. 1.4. combustibility, etc. Chemists describe, Note that a water molecule comprises interpret and predict the behaviour of two hydrogen atoms and one oxygen atom. substances on the basis of knowledge of their Similarly, a molecule of carbon dioxide physical and chemical properties, which are contains two oxygen atoms combined with determined by careful measurement and one carbon atom. Thus, the atoms of different experimentation. In the following section, we Rationalised 2023-24 Unit 1.indd 6 9/9/2022 4:27:30 PM  Some Basic Concepts of Chemistry 7 will learn about the measurement of physical properties. Maintaining the National Standards of Measurement 1.3.2 Measurement of physical properties The system of units, including unit Quantitative measurement of properties is definitions, keeps on changing with time. reaquired for scientific investigation. Many Whenever the accuracy of measurement of a properties of matter, such as length, area, particular unit was enhanced substantially volume, etc., are quantitative in nature. Any by adopting new principles, member nations quantitative observation or measurement is of metre treaty (signed in 1875), agreed represented by a number followed by units to change the formal definition of that in which it is measured. For example, length unit. Each modern industrialised country, of a room can be represented as 6 m; here, including India, has a National Metrology Institute (NMI), which maintains standards of 6 is the number and m denotes metre, the measurements. This responsibility has been unit in which the length is measured. given to the National Physical Laboratory Earlier, two different systems of (NPL), New Delhi. This laboratory establishes measurement, i.e., the English System experiments to realise the base units and and the Metric System were being used derived units of measurement and maintains National Standards of Measurement. These in different parts of the world. The metric standards are periodically inter-compared system, which originated in France in late with standards maintained at other National eighteenth century, was more convenient as Metrology Institutes in the world, as well it was based on the decimal system. Late, as those, established at the International need of a common standard system was felt Bureau of Standards in Paris. by the scientific community. Such a system was established in 1960 and is discussed in governmental treaty organisation created by a detail below. diplomatic treaty known as Metre Convention, 1.3.3 The International System of Units (SI) which was signed in Paris in 1875. The International System of Units (in The SI system has seven base units French Le Systeme International d’Unités and they are listed in Table 1.1. These units — abbreviated as SI) was established by pertain to the seven fundamental scientific the 11th General Conference on Weights and quantities. The other physical quantities, Measures (CGPM from Conference Generale such as speed, volume, density, etc., can be des Poids et Measures). The CGPM is an inter- derived from these quantities. Table 1.1 Base Physical Quantities and their Units Base Physical Symbol for Name of Symbol for Quantity Quantity SI Unit SI Unit Length l metre m Mass m kilogram kg Time t second s Electric current I ampere A Thermodynamic T kelvin K temperature Amount of n mole mol substance candela Iv cd Luminous intensity Rationalised 2023-24 Unit 1.indd 7 9/9/2022 4:27:30 PM  8 chemistry The definitions of the SI base units are These prefixes are listed in Table 1.3. given in Table 1.2. Let us now quickly go through some of The SI system allows the use of prefixes to the quantities which you will be often using indicate the multiples or submultiples of a unit. in this book. Table 1.2 Definitions of SI Base Units The metre, symbol m is the SI unit of length. It is defined by taking the fixed numerical value of the speed of light in vacuum Unit of length metre c to be 299792458 when expressed in the unit ms–1, where the second is defined in terms of the caesium frequencyV Cs. The kilogram, symbol kg. is the SI unit of mass. It is defined by taking the fixed numerical value of the planck constant h to Unit of mass kilogram be 6.62607015×10–34 when expressed in the unit Js, which is equal to kgm2s–1, where the metre and the second are defined in terms of c and V Cs. The second symbol s, is the SI unit of time. It is defined by taking the fixed numerical value of the caesium frequency V Cs, Unit of time second the unperturbed ground-state hyperfine transition frequency of the caesium-133 atom, to be 9192631770 when expressed in the unit Hz, which is equal to s–1. The ampere, symbol A, is the SI unit of electric current. It is defined by taking the fixed numerical value of the elementary Unit of electric ampere charge e to be 1.602176634×10–19 when expressed in the unit current C, which is equal to As, where the second is defined in terms of V Cs. The kelvin, symbol k, is the SI unit of thermodynamic temperature. It is defined by taking the fixed numerical value Unit of of the Boltzmann constant k to be 1.380649×10–23 when thermodynamic kelvin expressed in the unit JK–1, which is equal to kgm2s–2k–1 where temperature the kilogram, metre and second are defined in terms of h, c and V Cs. The mole, symbol mol, is the SI unit of amount of substance. One mole contains exactly 6.02214076×10 23 elementary entities. This number is the fixed numerical value of the Avogadro constant, NA, when expressed in the unit mol–1 and Unit of amount mole is called the Avogadro number. The amount of substance, of substance symbol n, of a system is a measure of the number of specified elementary entities. An elementary entity may be an atom, a molecule, an ion, an electron, any other particle or specified group of particles. The candela, symbol cd is the SI unit of luminous intensity in a given direction. It is defined by taking the fixed numerical value of the luminous efficacy of monochromatic radiation Unit of luminous Candela of frequency 540×1012 Hz, Kcd, to be 683 when expressed Intensity in the unit lm·W–1, which is equal to cd·sr·W–1, or cd sr kg–1 m–2s3, where the kilogram, metre and second are defined in terms of h, c and V Cs. Rationalised 2023-24 Unit 1.indd 8 9/9/2022 4:27:30 PM  Some Basic Concepts of Chemistry 9 Table 1.3 Prefixes used in the SI System Multiple Prefix Symbol 10–24 yocto y 10 –21 zepto z 10–18 atto a 10–15 femto f 10 –12 pico p 10 –9 nano n 10 –6 micro µ 10–3 milli m 10–2 centi c 10 –1 deci d 10 deca da 10 2 hecto h 103 kilo k 10 6 mega M Fig. 1.5 Analytical balance 109 giga G 10 12 tera T SI system, volume has units of m3. But again, 10 15 peta P in chemistry laboratories, smaller volumes 1018 exa E are used. Hence, volume is often denoted in 1021 zeta Z cm3 or dm3 units. 10 24 yotta Y A common unit, litre (L) which is not an SI unit, is used for measurement of volume 1.3.4 Mass and Weight of liquids. Mass of a substance is the amount of matter 1 L = 1000 mL, 1000 cm3 = 1 dm3 present in it, while weight is the force Fig. 1.6 helps to visualise these relations. exerted by gravity on an object. The mass of a substance is constant, whereas, its weight may vary from one place to another due to change in gravity. You should be careful in using these terms. The mass of a substance can be determined accurately in the laboratory by using an analytical balance (Fig. 1.5). The SI unit of mass as given in Table 1.1 is kilogram. However, its fraction named as gram (1 kg = 1000 g), is used in laboratories due to the smaller amounts of chemicals used in chemical reactions. 1.3.5 Volume Volume is the amont of space occupied by a Fig. 1.6 Different units used to express substance. It has the units of (length)3. So in volume Rationalised 2023-24 Unit 1.indd 9 9/9/2022 4:27:31 PM  10 chemistry In the laboratory, the volume of liquids fahrenheit) and K (kelvin). Here, K is the or solutions can be measured by graduated SI unit. The thermometers based on these cylinder, burette, pipette, etc. A volumetric scales are shown in Fig. 1.8. Generally, flask is used to prepare a known volume of a the thermometer with celsius scale are solution. These measuring devices are shown calibrated from 0° to 100°, where these two in Fig. 1.7. temperatures are the freezing point and the boiling point of water, respectively. The fahrenheit scale is represented between 32° to 212°. The temperatures on two scales are related to each other by the following relationship: 9 F  C  32 5 The kelvin scale is related to celsius scale as follows: K = °C + 273.15 It is interesting to note that temperature below 0 °C (i.e., negative values) are possible in Celsius scale but in Kelvin scale, negative Fig. 1.7 Some volume measuring devices temperature is not possible. 1.3.6 Density The two properties — mass and volume discussed above are related as follows: Mass Density = Volume Density of a substance is its amount of mass per unit volume. So, SI units of density can be obtained as follows: SI unit of density = kg = or kg m–3 m3 This unit is quite large and a chemist often expresses density in g cm–3, where mass is expressed in gram and volume is expressed in cm3. Density of a substance tells us about Fig. 1.8 Thermometers using different temperature scales how closely its particles are packed. If density is more, it means particles are more closely 1.4 Uncertainty in Measurement packed. Many a time in the study of chemistry, one 1.3.7 Temperature has to deal with experimental data as well as There are three common scales to measure theoretical calculations. There are meaningful temperature — °C (degree celsius), °F (degree ways to handle the numbers conveniently and Rationalised 2023-24 Unit 1.indd 10 9/9/2022 4:27:32 PM  Some Basic Concepts of Chemistry 11 present the data realistically with certainty to Reference Standard the extent possible. These ideas are discussed below in detail. After defining a unit of measurement such as the kilogram or the metre, scientists agreed 1.4.1 Scientific Notation on reference standards that make it possible As chemistry is the study of atoms and to calibrate all measuring devices. For getting molecules, which have extremely low masses reliable measurements, all devices such as metre sticks and analytical balances have and are present in extremely large numbers, been calibrated by their manufacturers a chemist has to deal with numbers as large to give correct readings. However, each of as 602, 200,000,000,000,000,000,000 for the these devices is standardised or calibrated molecules of 2 g of hydrogen gas or as small as against some reference. The mass standard 0.00000000000000000000000166 g mass of is the kilogram since 1889. It has been a H atom. Similarly, other constants such as defined as the mass of platinum-iridium Planck’s constant, speed of light, charges on (Pt-Ir) cylinder that is stored in an airtight particles, etc., involve numbers of the above jar at International Bureau of Weights magnitude. and Measures in Sevres, France. Pt-Ir was chosen for this standard because it is highly It may look funny for a moment to resistant to chemical attack and its mass write or count numbers involving so many will not change for an extremely long time. zeros but it offers a real challenge to do Scientists are in search of a new simple mathematical operations of addition, standard for mass. This is being attempted subtraction, multiplication or division with through accurate determination of Avogadro such numbers. You can write any two constant. Work on this new standard focuses numbers of the above type and try any one on ways to measure accurately the number of the operations you like to accept as a of atoms in a well-defined mass of sample. One such method, which uses X-rays to challenge, and then, you will really appreciate determine the atomic density of a crystal of the difficulty in handling such numbers. ultrapure silicon, has an accuracy of about This problem is solved by using scientific 1 part in 106 but has not yet been adopted to notation for such numbers, i.e., exponential serve as a standard. There are other methods notation in which any number can be but none of them are presently adequate to represented in the form N × 10n, where n is an replace the Pt-Ir cylinder. No doubt, changes exponent having positive or negative values are expected within this decade. and N is a number (called digit term) which The metre was originally defined as the varies between 1.000... and 9.999.... length between two marks on a Pt-Ir bar kept at a temperature of 0°C (273.15 K). In Thus, we can write 232.508 as 1960 the length of the metre was defined as 2.32508 × 102 in scientific notation. Note that 1.65076373 × 106 times the wavelength of while writing it, the decimal had to be moved light emitted by a krypton laser. Although to the left by two places and same is the this was a cumbersome number, it preserved exponent (2) of 10 in the scientific notation. the length of the metre at its agreed value. The metre was redefined in 1983 by CGPM Similarly, 0.00016 can be written as as the length of path travelled by light in 1.6 × 10 –4. Here, the decimal has to be vacuum during a time interval of 1/299 792 moved four places to the right and (–4) is the 458 of a second. Similar to the length and exponent in the scientific notation. the mass, there are reference standards for While performing mathematical operations other physical quantities. on numbers expressed in scientific notations, the following points are to be kept in mind. Rationalised 2023-24 Unit 1.indd 11 9/9/2022 4:27:32 PM  12 chemistry Multiplication and Division mass obtained by an analytical balance is These two operations follow the same rules slightly higher than the mass obtained by which are there for exponential numbers, i.e. using a platform balance. Therefore, digit 4 placed after decimal in the measurement by platform balance is uncertain. 5.6  10   6.9  10  = 5.6  6.9 10  5 8 58 The uncertainty in the experimental = 5.6  6.9  1013 or the calculated values is indicated by = 38..64  1013 mentioning the number of significant = 3.864  1014 figures. Significant figures are meaningful digits which are known with certainty plus 9.8  10   2.5  10  = 9.8  2.5 10    2 6 2  6 one which is estimated or uncertain. The = 9.8  2.5 10  2  6 uncertainty is indicated by writing the certain digits and the last uncertain digit. Thus, if we = 24.50  10 8 write a result as 11.2 mL, we say the 11 is = 2.450  10 7 certain and 2 is uncertain and the uncertainty would be +1 in the last digit. Unless otherwise 2.7  10 3 5  104 5.5   = 2.7  5.5 10 3  4 = 0.4909  10 7 stated, an uncertainty of +1 in the last digit is always understood. = 4.909  10 8 There are certain rules for determining the number of significant figures. These are Addition and Subtraction stated below: For these two operations, first the numbers are (1) All non-zero digits are significant. For written in such a way that they have the same example in 285 cm, there are three exponent. After that, the coefficients (digit significant figures and in 0.25 mL, there terms) are added or subtracted as the case are two significant figures. may be. (2) Zeros preceding to first non-zero digit Thus, for adding 6.65×104 and 8.95×103, are not significant. Such zero indicates exponent is made same for both the numbers. the position of decimal point. Thus, Thus, we get (6.65×104) + (0.895×104) 0.03 has one significant figure and Then, these numbers can be added as follows 0.0052 has two significant figures. (6.65 + 0.895)×104 = 7.545×104 (3) Zeros between two non-zero digits Similarly, the subtraction of two numbers can are significant. Thus, 2.005 has four be done as shown below: significant figures. (2.5 × 10–2) – (4.8 ×10–3) (4) Zeros at the end or right of a number are significant, provided they are on = (2.5 × 10–2) – (0.48 × 10–2) the right side of the decimal point. For = (2.5 – 0.48)×10–2 = 2.02 × 10–2 example, 0.200 g has three significant figures. But, if otherwise, the terminal 1.4.2 Significant Figures zeros are not significant if there is no Every experimental measurement has decimal point. For example, 100 has some amount of uncertainty associated only one significant figure, but 100 has with it because of limitation of measuring three significant figures and 100.0 has instrument and the skill of the person making four significant figures. Such numbers the measurement. For example, mass of an are better represented in scientific object is obtained using a platform balance notation. We can express the number and it comes out to be 9.4g. On measuring 100 as 1×102 for one significant figure, the mass of this object on an analytical 1.0×102 for two significant figures and balance, the mass obtained is 9.4213g. The 1.00×102 for three significant figures. Rationalised 2023-24 Unit 1.indd 12 9/9/2022 4:27:32 PM  Some Basic Concepts of Chemistry 13 (5) Counting the numbers of object, for Here, 18.0 has only one digit after the decimal example, 2 balls or 20 eggs, have infinite point and the result should be reported only significant figures as these are exact up to one digit after the decimal point, which numbers and can be represented by is 31.1. writing infinite number of zeros after placing a decimal i.e., 2 = 2.000000 or Multiplication and Division of 20 = 20.000000. Significant Figures In numbers written in scientific notation, In these operations, the result must be all digits are significant e.g., 4.01×102 has reported with no more significant figures as three significant figures, and 8.256×10–3 has in the measurement with the few significant four significant figures. figures. However, one would always like the results to be precise and accurate. Precision and 2.5×1.25 = 3.125 accuracy are often referred to while we talk Since 2.5 has two significant figures, about the measurement. the result should not have more than two Precision refers to the closeness of significant figures, thus, it is 3.1. various measurements for the same quantity. While limiting the result to the required However, accuracy is the agreement of a number of significant figures as done in the particular value to the true value of the above mathematical operation, one has to result. For example, if the true value for a keep in mind the following points for rounding result is 2.00 g and student ‘A’ takes two off the numbers measurements and reports the results as 1.95 1. If the rightmost digit to be removed is g and 1.93 g. These values are precise as they more than 5, the preceding number is are close to each other but are not accurate. increased by one. For example, 1.386. If Another student ‘B’ repeats the experiment we have to remove 6, we have to round and obtains 1.94 g and 2.05 g as the results it to 1.39. for two measurements. These observations 2. If the rightmost digit to be removed is are neither precise nor accurate. When the less than 5, the preceding number is not third student ‘C’ repeats these measurements changed. For example, 4.334 if 4 is to and reports 2.01 g and 1.99 g as the result, be removed, then the result is rounded these values are both precise and accurate. upto 4.33. This can be more clearly understood from the 3. If the rightmost digit to be removed is 5, data given in Table 1.4. then the preceding number is not changed Table 1.4 Data to Illustrate Precision if it is an even number but it is increased and Accuracy by one if it is an odd number. For example, Measurements/g if 6.35 is to be rounded by removing 5, we 1 2 Average (g) have to increase 3 to 4 giving 6.4 as the result. However, if 6.25 is to be rounded Student A 1.95 1.93 1.940 off it is rounded off to 6.2. Student B 1.94 2.05 1.995 1.4.3 Dimensional Analysis Student C 2.01 1.99 2.000 Often while calculating, there is a need to Addition and Subtraction of convert units from one system to the other. Significant Figures The method used to accomplish this is called The result cannot have more digits to the right factor label method or unit factor method of the decimal point than either of the original or dimensional analysis. This is illustrated numbers. 12.11 below. 18.0 Example 1.012 A piece of metal is 3 inch (represented by in) 31.122 long. What is its length in cm? Rationalised 2023-24 Unit 1.indd 13 9/9/2022 4:27:32 PM  14 chemistry Solution The above is multiplied by the unit factor We know that 1 in = 2.54 cm 1 m3 2 m3 2  1000 cm 3  6 3  3  2  10 3 m 3 From this equivalence, we can write 10 cm 10 1 in 2.54 cm Example = 1= 2.54 cm 1 in How many seconds are there in 2 days? Solution 1 in 2.54 cm Here, we know 1 day = 24 hours (h) Thus, equals 1 and 2.54 cm 1 in 1 day 24 h or = 1= 24 h 1 day also equals 1. Both of these are called unit then, 1h = 60 min factors. If some number is multiplied by these 1h 60 min unit factors (i.e., 1), it will not be affected or = 1= 60 min 1h otherwise. so, for converting 2 days to seconds, Say, the 3 in given above is multiplied by the unit factor. So, i.e., 2 days – – – – – – = – – – seconds 2.54 cm The unit factors can be multiplied in 3 in = 3 in × = 3 × 2.54 cm = 7.62 cm 1 in series in one step only as follows: 24 h 60 min 60 s Now, the unit factor by which multiplication 2 day × × × 2.54 cm 1 day 1h 1 min is to be done is that unit factor ( in 1 in = 2 × 24 × 60 × 60 s = 172800 s the above case) which gives the desired units i.e., the numerator should have that part 1.5 Laws of Chemical which is required in the desired result. Combinations It should also be noted in the above The combination of elements example that units can be handled just like to form compounds is other numerical part. It can be cancelled, governed by the following five divided, multiplied, squared, etc. Let us study basic laws. Antoine Lavoisier one more example. (1743–1794) 1.5.1 Law of Conservation Example of Mass A jug contains 2 L of milk. Calculate the This law was put forth by Antoine Lavoisier volume of the milk in m3. in 1789. He performed careful experimental Solution studies for combustion reactions and reached Since 1 L = 1000 cm3 to the conclusion that in all physical and and 1m = 100 cm, which gives chemical changes, there is no net change in 1m 100 cm mass duting the process. Hence, he reached = 1= to the conclusion that matter can neither be 100 cm 1m created nor destroyed. This is called ‘Law To get m3 from the above unit factors, the of Conservation of Mass’. This law formed first unit factor is taken and it is cubed. the basis for several later developments in  1m  3 chemistry. Infact, this was the result of exact 1 m3  1  1 3  100 cm   measurement of masses of reactants and 106 cm 3 products, and carefully planned experiments Now 2 L = 2 ×1000 cm3 performed by Lavoisier. Rationalised 2023-24 Unit 1.indd 14 9/9/2022 4:27:33 PM  Some Basic Concepts of Chemistry 15 1.5.2 Law of Definite Proportions are produced in a chemical This law was given by, a reaction they do so in a French chemist, Joseph simple ratio by volume, Proust. He stated that a given provided all gases are at compound always contains the same temperature and exactly the same proportion of pressure. elements by weight. Thus, 100 mL of hydrogen Joseph Louis Proust worked with two Joseph Proust combine with 50 mL of Gay Lussac samples of cupric carbonate (1754–1826) oxygen to give 100 mL of — one of which was of natural water vapour. origin and the other was synthetic. He found Hydrogen + Oxygen → Water that the composition of elements present in it 100 mL 50 mL 100 mL was same for both the samples as shown below: Thus, the volumes of hydrogen and % of % of % of oxygen which combine (i.e., 100 mL and copper carbon oxygen 50 mL) bear a simple ratio of 2:1. Natural Sample 51.35 9.74 38.91 Gay Lussac’s discovery of integer ratio Synthetic Sample 51.35 9.74 38.91 in volume relationship is actually the law of definite proportions by volume. The law of Thus, he concluded that irrespective of the definite proportions, stated earlier, was with source, a given compound always contains respect to mass. The Gay Lussac’s law was same elements combined together in the same explained properly by the work of Avogadro proportion by mass. The validity of this law in 1811. has been confirmed by various experiments. It is sometimes also referred to as Law of 1.5.5 Avogadro’s Law Definite Composition. In 1811, Avogadro proposed that equal volumes of all gases at the same temperature 1.5.3 Law of Multiple Proportions and pressure should contain equal number This law was proposed by Dalton in 1803. of molecules. Avogadro made a distinction According to this law, if two elements can between atoms and molecules which is combine to form more than one compound, quite understandable in present times. If the masses of one element that combine with we consider again the reaction of hydrogen a fixed mass of the other element, are in the and oxygen to produce water, we see that ratio of small whole numbers. two volumes of hydrogen combine with one For example, hydrogen combines with volume of oxygen to give two volumes of water oxygen to form two compounds, namely, water without leaving any unreacted oxygen. and hydrogen peroxide. Note that in the Fig. 1.9 (Page 16) each Hydrogen + Oxygen → Water box contains equal number of 2g 16g 18g molecules. In fact, Avogadro Hydrogen + Oxygen → Hydrogen Peroxide could explain the above result by considering the molecules 2g 32g 34g to be polyatomic. If hydrogen Here, the masses of oxygen (i.e., 16 g and 32 g), which combine with a fixed mass of hydrogen and oxygen were considered (2g) bear a simple ratio, i.e., 16:32 or 1: 2. as diatomic as recognised now, then the above results Lorenzo Romano 1.5.4 Gay Lussac’s Law of Gaseous are easily understandable. Amedeo Carlo Volumes However, Dalton and others Avogadro di Quareqa edi This law was given by Gay Lussac in 1808. believed at that time that Carreto He observed that when gases combine or atoms of the same kind (1776–1856) Rationalised 2023-24 Unit 1.indd 15 9/9/2022 4:27:34 PM  16 chemistry Fig. 1.9 Two volumes of hydrogen react with one volume of oxygen to give two volumes of water vapour cannot combine and molecules of oxygen or Dalton’s theory could explain the laws hydrogen containing two atoms did not exist. of chemical combination. However, it could Avogadro’s proposal was published in the not explain the laws of gaseous volumes. It French Journal de Physique. In spite of being could not provide the reason for combining correct, it did not gain much support. of atoms, which was answered later by other After about 50 years, in 1860, the first scientists. international conference on chemistry was 1.7 Atomic and Molecular Masses held in Karlsruhe, Germany, to resolve various ideas. At the meeting, Stanislao After having some idea about the terms Cannizaro presented a sketch of a course of atoms and molecules, it is appropriate here chemical philosophy, which emphasised on to understand what do we mean by atomic the importance of Avogadro’s work. and molecular masses. 1.6 Dalton’s Atomic Theory 1.7.1 Atomic Mass Although the origin of the idea that matter is The atomic mass or the mass of an atom is composed of small indivisible particles called actually very-very small because atoms are ‘a-tomio’ (meaning, indivisible), dates back extremely small. Today, we have sophisticated to the time of Democritus, techniques e.g., mass spectrometry for a Greek Philosopher (460– determining the atomic masses fairly 370 BC), it again started accurately. But in the nineteenth century, emerging as a result of several scientists could determine the mass of one experimental studies which atom relative to another by experimental led to the laws mentioned means, as has been mentioned earlier. above. Hydrogen, being the lightest atom was In 1808, Dalton published John Dalton arbitrarily assigned a mass of 1 (without ‘A New System of Chemical (1776–1884) any units) and other elements were assigned Philosophy’, in which he masses relative to it. However, the present proposed the following : system of atomic masses is based on 1. Matter consists of indivisible atoms. carbon-12 as the standard and has been agreed upon in 1961. Here, Carbon-12 is 2. All atoms of a given element have identical one of the isotopes of carbon and can be properties, including identical mass. Atoms represented as 12C. In this system, 12C is of different elements differ in mass. assigned a mass of exactly 12 atomic mass 3. Compounds are formed when atoms of unit (amu) and masses of all other atoms are different elements combine in a fixed ratio. given relative to this standard. One atomic 4. Chemical reactions involve reorganisation mass unit is defined as a mass exactly equal of atoms. These are neither created nor to one-twelfth of the mass of one carbon – 12 destroyed in a chemical reaction. atom. Rationalised 2023-24 Unit 1.indd 16 9/9/2022 4:27:34 PM  Some Basic Concepts of Chemistry 17 And 1 amu = 1.66056×10–24 g 1.7.3 Molecular Mass Mass of an atom of hydrogen Molecular mass is the sum of atomic masses = 1.6736×10–24 g of the elements present in a molecule. It is obtained by multiplying the atomic mass Thus, in terms of amu, the mass of each element by the number of its atoms of hydrogen atom = and adding them together. For example, molecular mass of methane, which contains one carbon atom and four hydrogen atoms, = 1.0078 amu can be obtained as follows: = 1.0080 amu Molecular mass of methane, Similarly, the mass of oxygen - 16 (16O) (CH4) = (12.011 u) + 4 (1.008 u) atom would be 15.995 amu. = 16.043 u At present, ‘amu’ has been replaced by Similarly, molecular mass of water (H2O) ‘u’, which is known as unified mass. = 2 × atomic mass of hydrogen + 1× atomic When we use atomic masses of elements mass of oxygen in calculations, we actually use average atomic masses of elements, which are = 2 (1.008 u) + 16.00 u explained below. = 18.02 u 1.7.2 Average Atomic Mass 1.7.4 Formula Mass Many naturally occurring elements exist Some substances, such as sodium chloride, as more than one isotope. When we take do not contain discrete molecules as their into account the existence of these isotopes constituent units. In such compounds, and their relative abundance (per cent positive (sodium ion) and negative (chloride ion) occurrence), the average atomic mass of entities are arranged in a three-dimensional that element can be computed. For example, structure, as shown in Fig. 1.10. carbon has the following three isotopes with relative abundances and masses as shown against each of them. Isotope Relative Atomic Mass Abundance (amu) (%) 12 C 98.892 12 13 C 1.108 13.00335 14 C 2 ×10 –10 14.00317 Fig. 1.10 Packing of Na+ and Cl– ions From the above data, the average atomic in sodium chloride mass of carbon will come out to be: (0.98892) (12 u) + (0.01108) (13.00335 u) + It may be noted that in sodium chloride, (2 × 10–12) (14.00317 u) = 12.011 u one Na+ ion is surrounded by six Cl– ion and Similarly, average atomic masses for vice-versa. other elements can be calculated. In the The formula, such as NaCl, is used to periodic table of elements, the atomic masses calculate the formula mass instead of mentioned for different elements actually molecular mass as in the solid state sodium represent their average atomic masses. chloride does not exist as a single entity. Rationalised 2023-24 Unit 1.indd 17 9/9/2022 4:27:35 PM  18 chemistry Thus, the formula mass of sodium chloride is This number of entities in 1 mol is so atomic mass of sodium + atomic mass of chlorine important that it is given a separate name and = 23.0 u + 35.5 u = 58.5 u symbol. It is known as ‘Avogadro constant’, or Avogadro number denoted by NA in honour Problem 1.1 of Amedeo Avogadro. To appreciate the Calculate the molecular mass of glucose largeness of this number, let us write it with (C6H12O6) molecule. all zeroes without using any powers of ten. Solution 602213670000000000000000 Molecular mass of glucose (C6H12O6) Hence, so many entities (atoms, molecules or = 6 (12.011 u) + 12 (1.008 u) + any other particle) constitute one mole of a 6 (16.00 u) particular substance. = (72.066 u) + (12.096 u) + We can, therefore, say that 1 mol of hydrogen (96.00 u) atoms = 6.022 × 1023 atoms = 180.162 u 1 mol of water molecules = 6.022 × 1023 water 1.8 Mole concept and Molar Masses molecules Atoms and molecules are extremely small 1 mol of sodium chloride = 6.022 ×1023 formula in size and their numbers in even a small units of sodium chloride amount of any substance is really very large. To handle such large numbers, a unit of Having defined the mole, it is easier to convenient magnitude is required. know the mass of one mole of a substance Just as we denote one dozen for 12 items, or the constituent entities. The mass of one score for 20 items, gross for 144 items, we mole of a substance in grams is called its use the idea of mole to count entities at the molar mass. The molar mass in grams is microscopic level (i.e., atoms, molecules, numerically equal to atomic/molecular/ particles, electrons, ions, etc). formula mass in u. In SI system, mole (symbol, mol) was introduced as seventh base quantity for the Molar mass of water = 18.02 g mol–1 amount of a substance. Molar mass of sodium chloride = 58.5 g mol–1 The mole, symbol mol, is the SI unit of amount of substance. One mole contains 1.9 Percentage Composition exactly 6.02214076 × 1023 elementary entities. So far, we were dealing with the number of This number is the fixed numerical value of entities present in a given sample. But many the Avogadro constant, NA, when expressed a time, information regarding the percentage in the unit mol–1 and is called the Avogadro of a particular element present in a compound number. The amount of substance, symbol is required. Suppose, an unknown or new n, of a system is a measure of the number of compound is given to you, the first question specified elementary entities. An elementary entity may be an atom, a molecule, an ion, an electron, any other particle or specified group of particles. It may be emphasised that the mole of a substance always contains the same number of entities, no matter what the substance may be. In order to determine this number precisely, the mass of a carbon–12 atom was determined by a mass spectrometer and found to be equal to 1.992648 × 10–23 g. Knowing that one mole of carbon weighs 12 g, the number of atoms in it is equal to: 12 g / mol 12C 1.992648  10 23 g /12 C atom  6.0221367  1023 atoms/mol Fig. 1.11 One mole of various substances Rationalised 2023-24 Unit 1.indd 18 9/9/2022 4:27:35 PM  Some Basic Concepts of Chemistry 19 you would ask is: what is its formula or what 1.9.1 Empirical Formula for Molecular are its constituents and in what ratio are they Formula present in the given compound? For known An empirical formula represents the simplest compounds also, such information provides a whole number ratio of various atoms present check whether the given sample contains the in a compound, whereas, the molecular same percentage of elements as present in a formula shows the exact number of different pure sample. In other words, one can check types of atoms present in a molecule of a the purity of a given sample by analysing this compound. data. If the mass per cent of various elements Let us understand it by taking the example present in a compound is known, its empirical of water (H2O). Since water contains hydrogen formula can be determined. Molecular formula and oxygen, the percentage composition of can further be ob