B7 a.pdf Chemistry PDF

## The use of different set-ups for evaporating mixtures * Set-up A is used where the liquid component of the mixture is non-flammable. An example is water. * Set-up B is used where the liquid component of the mixture is highly flammable (inflammable). Examples of flammable liquids are ethanol and...

## The use of different set-ups for evaporating mixtures * Set-up A is used where the liquid component of the mixture is non-flammable. An example is water. * Set-up B is used where the liquid component of the mixture is highly flammable (inflammable). Examples of flammable liquids are ethanol and carbon disulphide. The beaker containing the hot or boiling water is termed a water bath. The use of water bath for heating is termed indirect heating. Indirect heating prevents the flammable vapour of the liquid from being overheated to catch fire. ## Flammable substances Flammable substances bear the Highly Flammable symbol on their packaging. This symbol is part of a group of hazard symbols called warning signs. These signs appear in yellow triangles with black outlines. However, these signs appear in rectangles on reagent containers. ## Never move naked flame towards any substance with this symbol on it. To keep reminding us, safety engineers require all workplaces where flammable substances are stored or sold to also bear the No Naked Flame symbol and No Smoking Symbol. These other hazard symbols are called prohibition signs, that is, do not do those actions. ## Crystallization Crystallization is the formation of crystals of the solute when a small amount of the solvent is left during evaporation. The crystals form and settle at the bottom as the concentrated solution cools. The slower the cooling process, the larger the crystals formed. ## Figure 1.10 Figure 1.10 illustrates the hazard symbols on flammable substances and at where these substances are stored or sold. The terms flammable and inflammable have the same meaning. The prefix in-means moving towards; therefore, an inflammable substance easily moves into flame. ## Distillation Distillation is the process of heating mixtures containing liquids and then condensing the resulting vapour. That is, the process comprises evaporation and condensation. The condensed vapour is called the distillate. Distillation is either simple or fractional. ### Simple distillation Simple distillation is used to separate a liquid solvent from its solution. For example, water is separated from sugar solution by simple distillation. ## The setup for distilling mixtures in the laboratory * The condenser converts the vapour (steam) back into a liquid. * The distillate is collected into a receiver (beaker or conical flask). * An adapter is used to connect tubes of different diameters. ### Fractional distillation Fractional distillation is used when the boiling points of the miscible liquids in the mixture are very close to each other or one another (< 25 °C). Mixtures whose components are separated by fractional distillation include crude oil (petroleum) and liquefied air. * Fractional distillation is used to refine crude oil into its components such as liquefied petroleum gas (LPG), petrol (gasoline), kerosene, diesel, lubricating oils and bitumen. * It is used to separate the components of liquefied air; nitrogen and oxygen. ## Figure 1.12 Figure 1.12 illustrates the set-up for fractional distillation ## Sublimation Sublimation is the process whereby a substance changes directly from solid to gas on heating. The substance again changes directly from gas to solid when cooled. Examples of substances that sublime are dry ice (solid carbon dioxide), ammonium chloride, camphor, iodine crystals and naphthalene. A mixture of any of these subliming substances and a non-subliming one can be separated by heating. ## Figure 1.13 Figure 1.13 shows the set-up for separating a mixture by sublimation. ## For example, if a mixture of ammonium chloride (NH4Cl) and sodium chloride (NaCl) is heated, the ammonium chloride turns directly to vapour but the sodium chloride remains unchanged. When the vapour is cooled, solid ammonium chloride collects free from the sodium chloride. ## Magnetic Separation Magnetic separation is used to retrieve a magnetic substance from a non-magnetic substance. The magnet is brought closer to the mixture to attract the magnetic substance. Iron filings are separated from sulphur powder or ground charcoal by this method. This method is applied in these areas: * Shoemakers and dressmakers use magnets to separate and hold pins and small nails from mixtures. * Scrap dealers use magnets to separate iron and steel from other metals at recycling centres. * Restaurants and pharmaceuticals use magnets to retrieve all pieces of iron and other magnetic substances from the mixture of ingredients used to prepare food and drugs. ## Winnowing Winnowing is the process of separating grains (both bowls of cereal and grain legumes) from the chaff. It is done after threshing. Winnowing is done manually by lifting the mixture upwards and pouring it out gently, allowing the wind to blow off the chaff. ## Today, combine harvesters do all three processes in quick succession: reaping (cutting off the cereal heads from the stalks), threshing and winnowing. ## Figure 1.14 Figure 1.14 shows the mechanical harvesting of maize: 'A' shows a combine harvester that is reaping, dehusking, shelling, winnowing and discharging grains into a tractor-trailer and 'B' shows harvested maize ears being loaded onto a thresher which dehusks, shells, winnows and discharges the grains onto the floor. ## The use of a separatory funnel A separatory funnel (separation or separating funnel) is used to evacuate a denser liquid from a mixture of immiscible liquids. An example is a mixture of water and kerosene. ## Figure 1.15 Figure 1.15 shows the parts of a separatory funnel (A) and the separation of a mixture by using it (B). ## How to use a separatory funnel 1. Close the stopcock of the funnel tightly but remove the stopper to keep the mouth open. 2. Pour the liquids into the funnel via the mouth, and fix the stopper. 3. Shake the mixture vigorously and clamp the device onto a retort stand. 4. Allow the device and mixture to stand-still for a while. The liquids settle in distinct layers, with the denser one at the bottom. 5. Remove the stopper and open the stopcock to allow the denser liquid to leave via the jet of the tube. 6. Gently close the tap when the last drop of the denser liquid exits the jet. ## Answering questions on methods of separating mixtures a. (i) State and describe the method for separating a mixture of iron filings and powdered charcoal. (ii) Explain the principle by which the method works. b. A mixture of oil and water can be separated by decantation or by using a separatory funnel. (i) Which principle do the two methods use? (ii) Give one difference between the two methods. c. Describe how to separate the components of these: i. sugar and sand mixture; ii. salt and sugar mixture; iii. vinegar solution. ## Group Work In groups of five learners, each group should perform distilling, filtering, sieving, and others, to separate different kinds of mixtures and present a report of their findings using drawings and written works to the class. ## Describe atoms as composed of sub-atomic particles. An atom is the smallest particle of an element that retains all the chemical properties of the element. Therefore, an atom is usually used to represent the element. For example, atoms represent their elements on the periodic table. The chemical symbol, Na, is either a sodium atom or a sodium element. ## Structurally, an atom consists of a nucleus and shells. * The nucleus is the central part of the atom. It contains protons and neutrons. * The shells are the orbits around the nucleus. They contain electrons. The neutrons, protons and electrons are the subatomic particles. ## Figure 1.16 Figure 1.16 shows the structure of an atom. ## Subatomic particles A subatomic particle is a particle located within an atom. These particles are the neutrons, protons and electrons. They are so tiny that their masses are measured in a unit called atomic mass unit (amu). ## The neutrons are located in the nucleus. ## The protons are located in the nucleus. ## The electrons are located in the orbits (shells). The neutrons and the protons are called nucleons. Therefore, a nucleon is a subatomic particle in the nucleus of an atom. ## Table 1.5 Table 1.5. Properties of the main subatomic particles | Subatomic particle | Symbol | Charge | Mass | |---|---|---|---| | neutron | n | O (neutral) | 1 amu | | proton | p | +1 (positive) | 1 amu | | electron | e | -1 (negative) | 1/1840 amu (negligible) | ## Atomic number and mass number In Basic 7, we learnt that elements are arranged on the periodic table, using atomic numbers (the periodic table in Book 1 is re-produced here as Figure 1.17 with some modifications). In this subsection, we will learn more about atomic number and also explain their relationship to the mass numbers. ## Figure 1.17 Figure 1.17 is the periodic table showing only the first twenty elements. The groups are numbered in roman numerals (from I to VIII). Superscripted numbers are the atomic mass numbers of the elements but the subscripted numbers are the atomic numbers. ## The atomic number is the number of protons in an atom. Its symbol is Z. It uniquely identifies every element on the periodic table. For example, the atomic number of neon is 10 and that of sodium is 11. The atomic number offers us three useful pieces of information about an element: * The atomic number tells us the number of protons the element has. * The atomic number tells us the number of electrons in the element. * The atomic number tells us the location of the element on the periodic table. ## The mass number is the sum of protons and neutrons in an atom. Its symbol is A. We learnt earlier that the neutrons and protons are collectively called nucleons, so the mass number is also known as the nucleon number. For example, the mass number of carbon is 12 and that of oxygen is 16. ## We write the mass number of an element in two different ways: * hyphenate the number in front of its name (e.g., carbon-12 and oxygen-16) * or superscript the number in front of the chemical symbol for the element (e.g., 12C and 16O). ## Figure 1.18 Figure 1.18 illustrates how to present mass numbers in both typed and hand-written text. ## The neutron number is the number of neutrons in an atom. Its symbol is N. Most elements have atoms which have different neutron numbers. For example, some carbon atoms have 6 neutrons each while others have 7 or 8 each. The differences in neutron numbers have two implications. * Any element that has different neutron numbers also has different mass numbers. * Both neutron number and mass number cannot be used to identify elements. ## Figure 1.19 Figure 1.19 illustrates how to present the neutron number in both typed and hand-written text. ## Textbox 1.3. Isotopes and nuclides * Atoms of the same element with different mass numbers are called isotopes. * The atomic number and the mass number may be combined on the same chemical symbol. * An atom whose proton number and mass number are both specified is called a nuclide. * Because the atomic number is unique for each element, the mass number is used to represent atoms of the same element that have different neutron numbers. * Mass number (A) = atomic number (Z) + neutron number (N) A= Z+N ## Activity: 1.6 Solving questions involving atomic and mass numbers 1. An atom has 6 protons and 7 neutrons. What is its mass number? proton number (Z) = 6 neutron number (N) = 7 mass number (A) = ? A = Z + N = 6 + 7 = 13 The mass number is 13. 2. An atom of an element is represented as 37M. Find the number of electrons and neutrons in the atom. mass number (A) = 37 atomic number (Z) = 17 neutron number (N) = ? number of electrons = 17 (The number of electrons = number of protons) = atomic number A = Z + N 37 = 17 + N N = 37 - 17 = 20 The number of neutrons is 20. ## Table 1.6. Table 1.6. Differences between atomic number and mass number | Atomic number | Mass number | |---|---| | is the number of protons in an atomic nucleus | is the sum of protons and neutrons in an atomic nucleus | | is unique for every element | varies for elements with different neutron numbers | | is equal to the number of electrons in neutral atoms. | is unrelated to the number of electrons. | ## How elements are arranged on the periodic table, using their atomic numbers In Basic 7, we established that elements are arranged on the periodic table according to their atomic numbers. In a previous section, we mentioned that we use atomic numbers to identify the exact locations of elements on the periodic table. Do you remember we said that potassium is the 19th element on the periodic table and that its exact location is Group 1 of Period 4? In this section, we will learn how to use the electron configuration to arrange elements on the periodic table. ## Electron configuration ### What is electron configuration? Electron configuration is the distribution of electrons in the shells of an atom or element. The shells are named with letters of the alphabet beginning with K for the first shell. Each of the shells holds a certain maximum number of electrons. For example, the K shell holds up to 2 electrons. ## Table 1.7 Table 1.7. Shells in the first 20 elements and the maximum number of electrons each holds. | Shell name | Shell number (n) | Number of electrons | |---|---|---| | K | 1 | 2 | | L | 2 | 8 | | M | 3 | 8 | ## Textbox 1.5 Textbox 1.5. The formula for the number of electrons a shell holds The formula used to find the maximum number of electrons a shell holds is 2n² (where n = shell number). From the formula, the M shell holds a maximum of 18 electrons. However, the M shell holds a maximum of 8 electrons for the first twenty elements having it (i.e., from sodium to calcium). Any extra electron after the eighth one enters the N shell. Thus, the electron configuration of calcium (20Ca) is 2,8,8,2. Conversely, the electron configuration of krypton (36Kr) is 2,8,18,3, and that for zinc (30Zn) is 2,8,18,2. ## How do we present electron configurations? We present electron configurations in writing and drawings. ## Figure 1.20 Figure 1.20 shows how to present electron configuration of elements in handwritten text. The no-space rule also applies to typed text. ## How to write the electron configurations of atoms and ions 1. Determine the number of electrons in the atom or element. The number of electrons in an atom is equal to its atomic number (number of protons). For example, aluminium has 13 electrons and oxygen has 8 electrons. 2. Distribute the electrons in the shells. Start from the K shell. ## Electron configurations of some elements are given below: * lithium (Li) = 2,1 * aluminium (Al) = 2,8,3 * oxygen (O) = 2,6 * argon (Ar) = 2,8,8 * neon (Ne) = 2,8 * sulphur (S) = 2,8,6 * sodium (Na) = 2,8,1 * potassium (K) = 2,8,8,1 ## Separate the numbers in the electron configuration with commas but insert no spaces between them: * sodium = 2,8,1 * sulphur = 2,8,6 ## Textbox 1.5. Terms associated with electron configuration * The core-shell is a shell between the nucleus and the last shell. For example, sodium has three shells; namely, K, L and M. The K and L shells are the core shells. These shells are also called inner shells.