Chemistry Term 1 PDF

## Section A ### A1 States of Matter **Objectives** By the end of this topic you will be able to: * Give a definition of matter. * Give the four main ideas of the particulate theory of matter. * Explain why scientists find the particulate theory of matter useful. * Identify the three main states of...

## Section A ### A1 States of Matter **Objectives** By the end of this topic you will be able to: * Give a definition of matter. * Give the four main ideas of the particulate theory of matter. * Explain why scientists find the particulate theory of matter useful. * Identify the three main states of matter. * Explain the relationship between temperature and the motion of particles. **Exam Tip** It is important that you know the definitions of key terms used in Chemistry. These definitions are provided for you in the 'Key fact' boxes throughout the book. **Key Fact** Matter is anything that has mass and occupies space. **Key Fact** The particulate theory of matter states that all matter is made up of particles. **States of Matter** Chemistry is the study of the structure and behaviour of matter. Everything around us is made of matter. Matter has both mass and volume. Air, water, sand, human beings and animals are all matter. Matter exists in various states. The three main states of matter are solid, liquid and gas. ### A1.1 The Particulate Nature of Matter **Matter** As far back as 460 BC a Greek philosopher called Democritus developed the idea that matter consisted of particles. He asked this question: 'If you cut a piece of matter, for example, a piece of gold, in half and then cut it in half again, how many cuts will you have to make before you can cut it no further?' Democritus thought that it ended at some point, the smallest bits of matter, and that these smallest bits of matter, or particles, would be the basic building blocks of matter. Today scientists have added to Democritus' idea and now describe matter and its properties using the particulate theory of matter. **The Particulate Theory of Matter** The particulate theory of matter states that all matter is made of particles. This theory is very useful because it helps us to explain both the physical properties of matter and also the differences between the three states of matter. We will be looking at the three states of matter in detail in Unit A1.3. The particulate theory of matter has four main ideas: * All matter is made of particles. * The particles are in constant, random motion. * There are spaces between the particles. * There are forces of attraction between the particles. **States of Matter** * The difference in density of solids, liquids and gases, e.g. why pebbles sink and bubbles rise in water. * How cooling a liquid can cause it to change into a solid, e.g. when water is placed in a freezer it forms ice. * Why a smell can move throughout a room, e.g. when chicken is frying, it can be smelt at the other side of the kitchen. * Why the pressure of a gas increases with an increase in temperature, e.g. car tyres get harder as you drive. * Why certain vegetables become crisper when soaked in water, e.g. raw potatoes. * Surface tension in liquids, e.g. certain insects can 'walk' on water. **States of Matter** Matter can exist in various forms or states. The three states of matter that are the most common are the solid, liquid and gaseous states. The difference between these states lies in the energy and arrangement of the particles. Particles in the solid state have the least amount of energy, they simply vibrate in their fixed position and they are packed closely together. Particles in the liquid state have medium amounts of energy, they move about slowly and they have small spaces between them. Particles in the gaseous state have the greatest amount of energy, they move about rapidly and they have large spaces between them. You will study this in greater detail in Unit 1.3. The energy of the particles is directly related to the temperature of the particles and matter can change from one physical state to another by changing its temperature. This change of state occurs because increasing the temperature of a substance increases the kinetic energy of the particles in the substance. The greater the kinetic energy the particles possess, the faster they move. Changing state by changing temperature is a physical change. A physical change occurs when the form of the substance is changed without changing its chemical composition, for example, water as a solid, i.e. ice, has exactly the same chemical particles as water in the liquid state and as water in the gaseous state, i.e. water vapour. **Summary Questions** 1. State the three main ideas of the particulate theory of matter. 2. If a crystal of potassium manganate(vii) is dropped into a beaker of water, the purple colour spreads throughout the water. What features of the particulate theory of matter does this observation provide evidence for? 3. Explain why scientists find the particulate theory of matter useful. 4. What are the three states of matter? 5. What is the relationship between temperature and the movement of particles? ### A1.2 Evidence for the Particulate Theory of Matter **Objectives** By the end of this topic you will be able to: * Explain evidence which supports the particulate theory of matter. * Explain the processes of diffusion and osmosis. * Describe experiments which demonstrate diffusion and osmosis. * Explain the uses of salt and sugar to control garden pests and preserve food items. **Key Fact** Diffusion is the movement of particles from an area of higher concentration to an area of lower concentration until they are evenly distributed. **Exam Tip** It is very important when answering questions in tests or examinations to distinguish between observations and conclusions. If you are asked to give your observations, then you must describe what you would see while the experiment is being performed. If you are asked to state what you would conclude from the experiment, then you must give what you can deduce from the observations. A deduction is made by using data from the experiment to arrive at a conclusion. **Evidence for the Particulate Theory of Matter** In the previous unit we mentioned that scientists find the particulate theory of matter very useful because it allows them to explain the physical properties of matter. At the same time though, scientists have to provide evidence to support their ideas. There are simple practical activities involving diffusion and osmosis which we can perform to provide evidence for the existence and movement of particles. **Diffusion** We have all had experience of being aware of a smell, whether it is walking into a bakery, a cosmetic shop or climbing into a car that contains an air freshener. All of these smells are produced at a point in the shop or car, but the smell seems to travel through the air. This process of the smell travelling through the air is as a result of diffusion. Diffusion occurs because particles of matter are in constant motion and will move from a region of higher concentration to one of lower concentration. **Practical Activity** **Investigating the Particulate Theory of Matter** Your teacher may use this activity to assess: * observation, recording and reporting * analysis and interpretation You will be supplied with a straw, a beaker containing distilled water and a potassium manganate(vii) crystal **Method** 1. Place the straw vertically in the beaker of water until it touches the bottom of the beaker. 2. Drop the crystal of potassium manganate(vii) into the straw without moving the straw. 3. Very carefully remove the straw trying to disturb the water as little as possible. 4. Observe how the purple colour immediately begins to spread throughout the water. 5. Leave the beaker and observe after a few days. Note that the purple colour has spread throughout all the water in the beaker. What conclusion can you draw about: * a the spaces between the water particles * b the movement of the potassium manganate(vii) particles? The potassium manganate(VII) crystal and the water used in the experiment illustrated in Figures 1.2.1 and 1.2.2 are both composed of minute particles. The particles in the crystal are packed closely together and those in the water have very small spaces between them. When the crystal is in the water, the minute crystal particles slowly separate from each other and diffuse into the spaces between the water particles. This continues until all the particles have separated from the crystal and have diffused between the water particles. **Practical Activity** **Investigating Diffusion in Gases** Your teacher may use this activity to assess: * observation, recording and reporting * analysis and interpretation Your teacher will perform the following experiment: **Method** 1. Place a glass tube at least 1 m in length between two retort stands. 2. Soak separate pieces of cotton wool in concentrated ammonia solution and concentrated hydrochloric acid and place them simultaneously at each end of the glass tube. 3. Seal the ends of the glass tube with rubber stoppers. 4. Allow time for the ammonia and hydrogen chloride vapours to diffuse. Observe any changes. 5. Use your observations to explain what happened during the experiment. During the experiment illustrated in Figures 1.2.4 and 1.2.5, the ammonia solution gives off a gas called ammonia gas and the hydrochloric acid gives off a gas called hydrogen chloride gas. The ammonia and hydrogen chloride particles diffuse through the air in the glass tube towards each other. When the particles meet, they collide and react to form a white solid known as ammonium chloride. The ammonium chloride forms a ring inside the glass tube. We can represent the reaction between the ammonia and hydrogen chloride as a chemical equation where (g) and (s) indicate the state of the chemicals involved, (g) indicating a gas and (s) a solid: ammonia + hydrogen chloride → ammonium chloride NH3(g) + HCl(g) → NH4Cl(s) The ammonium chloride forms closer to the cotton wool soaked in hydrochloric acid because the ammonia particles are lighter than the hydrogen chloride particles. Therefore, the ammonia particles move much faster through the air than the hydrogen chloride particles. This experiment provides the following evidence for the particulate theory of matter: * Particles are able to move - the ammonia and hydrogen chloride particles must have moved towards each other to form the white ring. * There are spaces between particles - there must have been spaces between the air, ammonia and hydrogen chloride particles to allow them to move between each other. **Osmosis** Osmosis is a special case of diffusion, which involves the movement of water molecules through a differentially permeable membrane from a region with a lot of water molecules to a region with fewer water molecules. A differentially permeable membrane is a membrane that allows some substances to pass through but not others. You may also find the membrane being called a semi-permeable or selectively permeable membrane. The cell membrane that surrounds biological cells is differentially permeable. A differentially permeable membrane contains minute pores. Water molecules are able to pass through these pores. However, the particles of many other substances, which may be dissolved in the water, are unable to pass through. When two solutions, e.g. sucrose solutions, which have different concentrations, are separated by a differentially permeable membrane, the water molecules will diffuse through the pores in the membrane from the more dilute solution to the more concentrated solution. The sucrose molecules, however, do not move because they are unable to pass through the pores in the membrane. The volume of the more dilute solution decreases and the volume of the more concentrated solution increases. **Key Fact** Osmosis is the movement of water molecules from a region with a lot of water molecules, e.g. a dilute solution or pure water, to a region with fewer water molecules, e.g. a concentrated solution, through a differentially permeable membrane. **Practical Activity** **Investigating Osmosis in Green Paw-Paw** Your teacher may use this activity to assess: * manipulation and measurement * analysis and interpretation You will be supplied with a piece of green paw-paw (the experiment may be done with potato or yam), one beaker filled with distilled water and one beaker filled with concentrated sodium chloride solution. **Method** 1. Cut the piece of green paw-paw into six strips of equal length. 2. Measure and record the length of each strip. 3. Place three of the strips into the beaker containing distilled water and place the other three strips into the beaker containing the concentrated sodium chloride solution. 4. Allow the strips to remain in the solutions for one hour. 5. Remove the strips from the beakers. Feel the strips and take note of the texture of each strip. 6. Measure and record the length of each strip. 7. Explain why the paw-paw strips placed in distilled water become more rigid and have increased in length (consider the direction in which the water molecules move, from the paw-paw into the distilled water or from the distilled water into the paw-paw). 8. Explain why the paw-paw strips placed in concentrated sodium chloride solution become floppy and softer and decrease in length (consider the direction in which the water molecules move, from the paw-paw into the sodium chloride solution or from the sodium chloride solution into the paw-paw). During the experiment illustrated in Figures 1.2.7 and 1.2.8, the cell membranes of the paw-paw cells act as differentially permeable membranes. Water can pass through the cell membranes, either into or out of the cells: * Distilled water has a higher water content (or lower sodium chloride concentration) than the paw-paw cells, therefore water moves into the cells by osmosis, resulting in the paw-paw strip becoming longer and more rigid. * The paw-paw cells have a higher water content than the concentrated sodium chloride solution, therefore water moves out of the cells by osmosis, resulting in the paw-paw strip becoming shorter and softer. **Did You Know?** Osmosis works in the same way in your cells as it does in the paw-paw. If you sweat a lot you lose water. This lowers the amount of water in your blood and osmosis takes place and starts to pull water out of your cells. For this reason it is very important to drink lots of water on a hot day or when you exercise. ### A1.3 Practical Uses of Osmosis **Practical Uses of Osmosis** We use the principles of osmosis in various ways. These include controlling garden pests and preserving food items. **Controlling Garden Pests** Slugs and snails, being herbivores, are serious garden pests which ravage many of our precious plants. The skin of these pests is a lot more permeable than the skin of most other animals. This means that they need to keep themselves moist to prevent water evaporating from their bodies causing them to dehydrate and die. We make use of these facts to control slugs and snails in our gardens by using sodium chloride (table salt). **Sodium chloride is deliquescent**, which means that it absorbs water readily and dissolves. When sodium chloride is sprinkled on slugs and snails, it absorbs some of the moisture surrounding their bodies and dissolves forming a concentrated solution. This causes water inside their bodies to move out and into the solution by osmosis. If the slugs and snails lose more water than their bodies can tolerate, they die from dehydration. **Preserving Food Items** Both sodium chloride and sugar are used to preserve food items, e.g. meat, fish, fruits and vegetables. We are all familiar with salt fish, salt pork, crystallised fruits, guava jelly and glacé cherries. Both salt and sugar work in the same way to preserve foods: * They withdraw water from the cells of the food items by osmosis. This makes the water unavailable for the chemical reactions in cells which cause decay. Without these reactions occurring, the food items do not decay. * They also withdraw water from the microorganisms that bring about decay, e.g. bacteria and fungi. This prevents these organisms from growing and causing the food items to decay. ### A1.3 The Three States of Matter **The Three States of Matter** You have learnt already that matter exists in three states: solid, liquid and gas. The three states of matter have noticeable differences in their physical properties. Physical properties are characteristics that can be observed or measured without changing the chemical composition of a substance. Shape, volume, density, compressibility, solubility, melting point and boiling point are all examples of physical properties. The different physical properties of the three states can be explained by the particulate theory of matter. Table 1.3.1 summarises the physical properties of the three states of matter and the arrangement of particles in the three states. | Property | Solid | Liquid | Gas | |---------|-------|--------|-------| | Shape and Volume | Solids have a fixed shape and a fixed volume. | Liquids do not have a fixed shape, but they have a definite volume, Liquids take the shape of the part of the container that they occupy and the surface is always horizontal. | Gases do not have a fixed shape or volume. A gas will take up the space of the container it is placed in. The shape and volume of a gas is, therefore, the shape and volume of the entire container it is in. | | Density | Most solids have a high density. | The density of liquids is usually lower than the density of solids. | Gases have a low density. | | Compressibility | Solids are very difficult to compress. | Liquids can be compressed very slightly when pressure is applied. | Gases are easy to compress. | | Arrangement of the particles | The particles are packed closely together, usually in a regular pattern. | The particles are randomly arranged and have small spaces between them. | The particles are randomly arranged and have large spaces between them. | | Forces of attraction between the particles | The particles have very strong forces of attraction between them. | The forces of attraction between the particles are not as strong as those between the particles of a solid. | The particles have very weak forces of attraction between them. | | Energy and movement of the particles | Particles in a solid have very small amounts of kinetic energy. The particles vibrate in their fixed position. | Particles in a liquid have more kinetic energy than particles in a solid. The particles move about slowly. | Particles in a gas have large amounts of kinetic energy. The particles move about freely and rapidly. | | Arrangement of particles | Particles are packed in a near perfect order| Particles are close together but not in a regular order| Particles are far apart | **Changing State** Matter can be changed from one state to another by heating or cooling. A change of state is, therefore, caused by a change in temperature and consequently a change in the kinetic energy of the particles. For example, in order to change water into ice we need to put the water into the freezer, i.e. we need to remove heat energy. Changing the state of a substance without changing its chemical composition is a physical change. The different changes of state are summarised in Figure 1.3.1. **Did You Know?** SCUBA divers make use of the fact that gases are very easy to compress. An average sized SCUBA diving tank holds about 2250 litres of compressed air. To understand this, think of a milk carton. Most milk cartons hold one litre of milk, therefore, a SCUBA diving tank holds the same volume of air as 2250 empty milk cartons! **Melting** When a solid is heated, the particles gain kinetic energy and begin to vibrate more vigorously. Eventually the particles are able to overcome the strong forces of attraction between them and they move more freely and further apart forming a liquid, i.e. the solid melts. The temperature remains constant while the solid is melting because all the heat energy being supplied is used to overcome the forces of attraction between the solid particles. This constant temperature is known as the **melting point**. **Key Fact** Melting point is the constant temperature at which a solid changes into a liquid. **Evaporation** When a liquid is heated, the particles gain kinetic energy and move faster. Some of the particles near the surface of the liquid have enough kinetic energy to overcome the forces of attraction between them and are able to leave the liquid and become a vapour. These particles are said to evaporate. The particles that leave the liquid take lots of energy with them, leading to a cooling of the liquid. **Did You Know?** When we sweat and the water in the sweat evaporates from our skin, it takes energy with it causing our bodies to feel cooler. If we put alcohol on our skin, it evaporates even faster than water because it has a lower boiling point than water. This makes our skin feel even colder than when we sweat. **Boiling** When a liquid is heated its temperature eventually reaches a certain point where it starts to boil. At this point the liquid particles have gained enough kinetic energy and started to move fast enough to change into a gas both within the liquid and at its surface. The temperature remains constant while the liquid is boiling because the heat energy being supplied is used to overcome the forces of attraction between the liquid particles. This constant temperature is known as the **boiling point**. **Key Fact** Boiling point is the constant temperature at which a liquid changes into a gas. **Boiling differs from evaporation in two ways.** * Boiling occurs at a specific temperature, whereas evaporation can take place at any temperature. * Boiling takes place throughout the liquid, whereas evaporation takes place only at the surface of the liquid. **Condensation** When the temperature of a gas is lowered, the particles lose kinetic energy and begin to move more slowly. The forces of attraction between the particles become stronger causing the particles to move closer together forming a liquid, i.e. the liquid condenses. **Freezing** When the temperature of a liquid is lowered, the particles lose kinetic energy and begin to move more slowly. The forces of attraction between the particles become stronger causing the particles to move even closer together forming a solid, i.e. the liquid freezes. The temperature at which this occurs is called the **freezing point**. **Key Fact** Freezing point is the constant temperature at which a liquid changes into a solid. **The freezing point of a pure substance has the same value as the melting point**, e.g. water has a melting point and a freezing point of 0 °C. **Sublimation** When the forces of attraction between the particles in a solid are weak, the addition of a small amount of heat can cause the solid to change directly into a gas, without passing through the liquid state. If the gas is then cooled it will change directly back to the solid. When a substance changes directly from a solid to a gas or a gas to a solid it is said to **sublime**. **Examples of substances which undergo sublimation are iodine, carbon dioxide (known as 'dry ice'), ammonium chloride and naphthalene. Moth balls or camphor balls are made of naphthalene. Solid air fresheners also sublime releasing their fragrances into the air.** **Practical Activity** **Observing Sublimation in Iodine** Your teacher may use this activity to assess: * observation, recording and reporting You will be supplied with a test tube, a small iodine crystal, a piece of cotton wool and a pair of tongs. **Method** 1. Place the iodine crystal into the test tube and place the cotton wool into the mouth of the test tube. 2. Hold the test tube with tongs at a 45° angle and heat the bottom of the tube in the flame of a Bunsen burner until all the iodine crystal has sublimed. 3. Observe what happens as the iodine is heated. 4. Remove the tube from the flame and let it cool. 5. Observe what happens as the tube is cooling. During the experiment illustrated in Figure 1.3.3, as the iodine crystal is heated, it sublimes and forms purple iodine vapour which diffuses up the test tube. The top of the tube is much cooler and when the vapour reaches the top, it sublimes back to a solid, forming a ring of iodine crystals around the inside of the tube. **Heating and Cooling Curves** If the temperature of a pure solid is measured at intervals as it is heated and changes state to a liquid and then to a gas, and the temperature is plotted on a graph against time, a heating curve is obtained. The heating curve for water is shown in Figure 1.3.4. The curve shows that as heating occurs, the temperature of the substance increases. However, the graph has two horizontal sections where the temperature remains constant for a period of time even though heating continues. These happen when there is a change of state. The first change of state is where melting occurs and the temperature remains constant at the melting point of the substance until all the substance has melted, e.g. for water this is 0 °C. The second change of state is where boiling occurs and the temperature remains constant at the boiling point of the substance until all the substance has boiled, e.g. for water this is 100 °C. If the temperature of a gas is measured at intervals as it is cooled and changes state to a liquid and then to a solid, and the temperature is plotted on a graph against time, a cooling curve is obtained. The cooling curve for water is shown in Figure 1.3.5. Like heating curves, cooling curves have two horizontal sections. The first is where the state changes from gas to liquid and the second is where it changes from liquid to solid. **Summary Questions** 1. Complete the table below. The first row is completed as an example of what is required. | Name given to change of state | Change of state | Energy added or removed to change state | | ------------------------ | -------------- | ----------------------------------------- | | Melting | Solid to liquid | Added | | | Liquid to gas | | | | Gas to liquid | | | | Liquid to solid | | | | Solid to gas | | 2. What are the main differences between evaporation and boiling? 3. a Explain what occurs during sublimation. b Give three examples of solids which undergo sublimation. 4 Explain what a heating curve shows. ## Section A2 ### A2 Mixtures and their separation **Objectives** By the end of this topic you will be able to: * Distinguish between pure substances and mixtures. * Explain the difference between an element, a compound and a mixture. * Give examples of elements, compounds and mixtures. * Explain the difference between a homogeneous and a heterogeneous mixture. **Mixtures and their Separation** Elements, compounds and mixtures form a part of our everyday lives. When we wrap our food in aluminium foil we are using an element. When we place salt on our food we are eating a compound. When we drink a cold soda we are drinking a mixture. It may be useful to know how to separate some of these mixtures into their component parts. An example of this is the purification of drinking water. ### A2.1 Elements, Compounds and Mixtures **Matter can be classified into two main groups** * pure substances * mixtures **Pure substances have the following general characteristics.** * They have a fixed, constant composition. * Their properties are fixed and constant. * The component parts of a pure substance cannot be separated by physical means. **Mixtures have the following general characteristics.** * They have a variable composition. * Their properties are variable since their components retain their own, individual properties. * The component parts of mixtures can be separated by physical means. **Pure substances can be further classified into elements and compounds.** **Mixtures can be further classified into homogeneous mixtures and heterogeneous mixtures. The tree diagram in Figure 2.1.1 shows the breakdown of these groups.** **Pure Substances** A pure substance is composed of only one type of material and has the following fixed properties: * a sharply defined, constant melting point or freezing point * a sharply defined, constant boiling point * a constant density. **To determine if a substance is pure or not, its melting point or boiling point is determined.** Any impurities in a pure substance will usually lower its melting point and cause it to melt over a wider temperature range. Impurities will usually raise the boiling point of a pure substance and cause it to boil over a wider temperature range. **Paper chromatography (Unit 2.4) may also be used.** If a substance is pure it will produce only one single spot on a chromatogram. If it is not pure it will produce more than one spot. **Practical Activity** **Comparing the Boiling Points of Pure Water and Sodium Chloride Solution** Your teacher may use this activity to assess: * observation, recording and reporting Your teacher will perform the following demonstration. **Method** 1. Place 2 cm³ of distilled water in a test tube. 2. Place an inverted closed end capillary tube into the test tube with the open end facing downwards. 3. Place a thermometer in the tube. 4. Half fill a 250 cm³ beaker with oil and place the above test tube assembly in the oil bath so that the surface of the water in the test tube is beneath the surface level of the oil. 5. Heat the beaker gently over a Bunsen burner, stirring constantly to ensure that heating is even. Continue heating until a rapid stream of bubbles emerges from the capillary tube. This stream of bubbles indicates that the water in the test tube is boiling. 6. Remove the heat source and observe the stream of bubbles. When the last bubble emerges from the capillary tube, record the temperature. 7. Reheat the oil bath and repeat the cooling process twice more. Record the temperature reading after each trial and average all three temperatures. This is the boiling point of distilled water. 8. Repeat the procedure using a sodium chloride solution in place of the water. 9. What conclusion can you draw about the boiling points of pure water and sodium chloride solution? **Did You Know?** In many countries where snow and ice pose a problem on the roads in winter, rock salt (sodium chloride) is spread on the roads to melt the ice. The salt dissolves in the film of water on the surface of the ice, this lowers its freezing point to below the temperature of the ice and the ice starts to melt. Salt can lower the freezing point of water to about -18 °C, the freezing point of saturated sodium chloride solution. **Elements** An element is the simplest form of matter. It cannot be broken down into anything simpler by ordinary chemical or physical means. We say 'ordinary chemical means' to exclude nuclear reactions. The smallest particle in an element that has the same properties as the element is an atom. Each element is composed of only one kind of atom. **Key Fact** An element is a pure substance that cannot be broken down into any simpler substances by any ordinary chemical or physical means. **Examples of elements are iron (Fe), which** is composed of only iron atoms, copper (Cu), which is composed of only copper atoms and oxygen (O2), which is composed of only oxygen atoms. **Compounds** Compounds are composed of more than one kind of atom. These atoms are combined together chemically, they are always present in the same proportions by mass and they cannot be separated by physical means. A compound can be represented by a chemical formula, which indicates the elements that the compound is made up of and the ratio in which they have combined, e.g. the chemical formula of water is H2O. **Key Fact** A compound is a pure substance that contains two or more different types of element which are bonded together chemically in fixed proportions and in such a way that their properties have changed. **Examples of compounds are water**, which is composed of hydrogen and oxygen in a ratio of 2:1, sodium chloride (NaCl), which is composed of sodium and chlorine in a ratio of 1:1 and methane (CH4), which is composed of carbon and hydrogen in a ratio of 1: 4. The properties of a compound are fixed and are different from the properties of the individual elements that form the compound. For example, hydrogen and oxygen are both gases at room temperature, however, water is a liquid. **Mixtures** Mixtures are composed of two or more substances which are not chemically combined, therefore their components can be separated by physical means. Some of the physical methods for separating mixtures, which we will be investigating in Unit 2.4, are filtration, evaporation, crystallisation, distillation, fractional distillation and chromatography. In a mixture the component parts are not in a fixed ratio and they retain their own, individual physical properties. **Homogeneous Mixtures** A homogeneous mixture is one in which the properties and composition are uniform throughout the mixture. The component parts cannot be distinguished from each other. A solution is a homogeneous mixture. Examples of homogeneous mixtures are air, salt dissolved in water and metal alloys such as brass, a mixture of copper and zinc. **Heterogeneous Mixtures** A heterogeneous mixture is a non-uniform mixture, for example, a mixture in which the component parts are in different states. The component parts can be distinguished from each other, although not always with the naked eye. Suspensions and colloids are heterogeneous mixtures. Examples of heterogeneous mixtures are salt and sand, mayonnaise, and muddy water. The diagrams in Figure 2.1.4 show how elements, compounds and mixtures can be distinguished by looking at the particles that make up the substance. * If there is only one kind of atom, then it is an element. * If there are two or more kinds of atoms joined together in the same ratio, then it is a compound. * If there is a combination of two or more elements and/or compounds, then it is a mixture. ### A2.2 Solutions, Suspensions and Colloids **Solutions, Suspensions and Colloids** Solutions, suspensions and colloids form part of our everyday lives. For example, sea water is a solution, muddy water is a suspension and milk and fog are both colloids. **Solutions** A solution is a homogeneous mixture. The major component of a solution is known as the solvent and the minor component is known as the solute. Some solutions may contain more than one solute, e.g. sea water. The solute and solvent can be gases, liquids or solids. When a gas or a solid dissolves in a liquid, the liquid is always the solvent, e.g. in a mixture of salt in water, salt is the solute and water is the solvent. **Key Fact** A solution is a homogeneous mixture consisting of two or more components, one of which is usually a liquid. **Table 2.2.1 gives examples of various types of solutions.** | Solute | Solvent | Example | Components of the solution | |---|---|---|---| | Solid | Liquid | Sea water | Sodium chloride in water | | Gas | Liquid | Soda Water | Carbon dioxide in water | | Solid | Solid | Brass | Zinc in copper | | Liquid | Liquid | White Rum | Ethanol in water | | Gas | Gas | Air | Oxygen, water vapour, argonn and carbon dioxide in nitrogen | **A Saturated Solution** A saturated solution is one that contains as much solute as can be dissolved at a particular temperature in the presence of undissolved solute. You will study saturated solutions in more detail in Unit 2.3. **Suspensions** A suspension is a heterogeneous mixture containing minute particles which are visible to the naked eye. If left undisturbed, the particles in a suspension eventually settle. The components of a suspension can be separated by filtration. **Key Fact** A suspension is a heterogeneous mixture in which minute, but visible, particles are dispersed in another substance, usually a liquid. **Examples of Suspensions** * Dust in air is a suspension of a solid in a gas. * Powdered chalk in water is an example of a suspension of a solid in a liquid. * Muddy water is another example of a solid suspended in a liquid. **Colloids** A colloid is a heterogeneous mixture containing particles that are intermediate in size between those of a solution and those of a suspension. The particles in a colloid cannot be seen even with a microscope, and if left undisturbed they do not settle. The properties of a colloid are intermediate between those of a solution and those of a suspension. **Key Fact** A colloid is a heterogeneous mixture in which the particles of one substance are dispersed in another substance, usually a liquid. The dispersed particles are smaller than those of a suspension, but larger than those of a solution. **Examples of Colloids** * Smoke in air is a colloid of a solid in a gas, also known as a solid aerosol. * Fog and aerosol sprays in air are colloids of a liquid dispersed in a gas, also known as liquid aerosols. * Milk and mayonnaise are colloids of a liquid dispersed in a liquid, also known as emulsions. * Gelatine and jelly are colloids of a solid dispersed in a liquid, also known as gels. **A comparison of the distinguishing properties of solutions, suspensions and colloids is given in Table 2.2.2.** | Property | Solution | Suspension | Colloid | |---|---|---|---| | Particle size | Very small (less than one nanometre in diameter) | Large enough so that they are visible to the naked eye (greater than 1000 nanometres in diameter) | Greater than that of a solution but they are not visible to the naked eye (between 1 and 1000 nanometres in diameter) | | Type of mixture | Homogeneous | Heterogeneous | Heterogeneous | | Appearance | Generally transparent | Usually opaque, some are translucent | Usually opaque, some are translucent | | Can the components be separated by filtration? | No | Yes | Yes | | Do the components separate out after the mixture has been standing for a while? | No | Yes | No | | Transmission of light | Transmits light appearing transparent | Will scatter light | Does not transmit light, it is opaque | **Practical Activity** **Comparing the Properties of a Solution, a Suspension and a Colloid** Your teacher may use this activity to assess: * observation, recording and reporting * manipulation and measurement. You will be supplied with two beakers, a filter funnel held in a retort stand, filter paper, distilled water, calcium hydroxide powder, copper(II) sulfate and gelatine. **Method** 1. Half fill the beaker with water. 2. Place a large spatula full of calcium hydroxide powder into the water and mix vigorously. 3. Hold the mixture up to the light and look