Physical and Chemical Changes PDF

## 2 Changes ### Learning Outcomes Children will be able to: - differentiate between physical and chemical changes - perform activities related to physical and chemical changes - classify changes such as respiration, preparation of solution of sugar, burning of paper, ripening of fruit, spoiling...

## 2 Changes ### Learning Outcomes Children will be able to: - differentiate between physical and chemical changes - perform activities related to physical and chemical changes - classify changes such as respiration, preparation of solution of sugar, burning of paper, ripening of fruit, spoiling of food materials as physical and chemical changes - discuss that in a chemical change, a new substance with different properties is formed ### UNIT - PHYSICAL AND CHEMICAL CHANGES #### INTRODUCTION All substances around us undergo changes. In some cases, the changes are small and difficult to detect. In other cases, the changes are obvious and easy to detect. These changes generally get accelerated if we heat the substances. Most of these changes can be classified under two headings: 1. Physical changes 2. Chemical changes **Physical changes** are generally temporary in nature and no new substance is formed. **Chemical changes** are generally permanent in nature and new substances are formed, which have entirely new properties. #### EXPERIMENTS TO EXAMINE CHANGES IN A FEW SUBSTANCES **Experiment 1** Take a dry hard glass test tube and put about 2 g of candle wax in it. Heat the test tube gently on a Bunsen flame. What do you observe? The wax melts to form a liquid. Now cool the test tube by holding it in cold water. What do you observe? The molten wax solidifies. Thus, we can say that on heating, wax melts and on cooling, the liquid wax solidifies, but no new product is formed. Similarly, ice, ghee, butter, etc., melt on heating and solidify on cooling. **Experiment 2** Zinc oxide is a white powder. Place about 2 g of zinc oxide in a dry test tube. Heat the test tube strongly. What do you notice after 2 minutes of heating? The zinc oxide changes to yellow colour. Cool the test tube. In a few minutes, the colour of zinc oxide changes to white colour. Thus, we can say that the change in zinc oxide was temporary in nature. Similarly, if we heat lead oxide powder, which is yellow in colour, its colour changes to reddish brown. However, on cooling, its colour changes back to yellow. **Experiment 3** Take about 50 cm³ of water in a beaker. Add a spoonful of common salt in it and stir. You will, observe that common salt dissolves in water. Now evaporate the common salt solution on a heated sand bath. It is observed that water disappears leaving behind common salt. Furthermore, the mass of common salt remains same. Thus, we can say that the above changes are (i) temporary in nature, (ii) no new product is formed, and (iii) there is no change in weight when change takes place. **Experiment 4** Hold a short length of platinum wire or nichrome wire with the help of a pair of tongs. Hold the end of wire in a non-luminous Bunsen flame. What do you observe? The end of wire becomes red hot. Take out the end of wire from the Bunsen flame and allow it to cool. What do you observe? The wire regains its original colour. Thus, we can say that the change brought about in the appearance of the wire is temporary in nature. Similarly, when electric current flows through a bulb, its filament gets white hot and emits light. When current is switched off, the filament does not give off any light. Thus, the change in the appearance of the bulb is temporary in nature. **Experiment 5** Take a bicycle spoke (wire fitted in wheel of bicycle) and rub it with a permanent bar magnet. Roll the iron wire on the iron filings. What do you observe? Iron filings stick to the wire. Thus, we can conclude that wire has changed into a magnet. Now strike the wire against the table for 50 times. Again roll the wire on iron filings. It is observed that iron filings no longer stick to the iron wire. Thus, we can conclude that wire has lost its magnetic property. Thus, the changing of iron wire into a magnet is a temporary change and no new product is formed. **Experiment 6** Hold a short length (about 5 cm) of magnesium ribbon with the help of a pair of tongs. Heat magnesium ribbon with a non-luminous flame for about 2 minutes. What do you observe? The magnesium ribbon burns with a brilliant white flame producing a large amount of heat and light. It leaves behind white ash, which is commonly called magnesium oxide. It does not change back to magnesium on cooling. Thus, we can conclude: 1. The change brought about in magnesium is permanent in nature, 2. a new product is formed which has entirely different properties, 3. and (iii) a large amount of heat and light energy are evolved. **Experiment 7** Fix a candle on a table and light it. What do you observe after a few minutes? 1. The candle burns to give heat and light energy. 2. The candle does not regain its original size when put off. Thus, the burning of a candle is a permanent change which cannot be reversed. Actually, the candle burns to form new products, i.e., carbon dioxide gas and water vapour. **Experiment 8** Take a hard glass test tube and put 2 g of red coloured mercuric oxide in it. Clamp the test tube on an iron stand as shown in Fig. 2.8. Heat the test tube strongly. You will notice that first of all the colour of mercuric oxide changes to black. On further heating, the test tube is filled with fumes. At this moment, hold a glowing wooden splint in the test tube. The wooden splint bursts into flame. Also, tiny silvery droplets are seen sticking to the cooler parts of the test tube. On cooling the test tube, the change is not reversed. Actually, on heating, mercuric oxide decomposes to form mercury and oxygen, i.e., two new products are formed. The mercury vaporises and liquefies on the cooler parts of the test tube. The oxygen is responsible for the glowing splint to burst into flames, because it supports combustion. Thus, strong heating of mercuric oxide is a permanent change which cannot be reversed. Furthermore, new products are formed. **Experiment 9** Take a hard glass test tube and put 2 g of sugar in it. Heat the test tube strongly. What do you observe? The sugar melts and then turns brown. On further heating, it gives off steam which condenses on the cooler parts of the test tube. The residue left in the test tube is black in colour. On cooling, the change does not reverse itself. Thus, heating of sugar is a permanent change. Actually, sugar decomposes to form charcoal which is a form of carbon and is black in colour. It also gives off water in the form of steam From the above experiments, it is clear that there are two types of changes which take place in the substances on heating. In experiments 1 to 5, the changes are temporary in nature and can be reversed. Furthermore, no new substance is formed. Such changes are called physical changes. In experiments 6 to 9, the changes are permanent in nature and cannot be reversed. Furthermore, new substances are formed. Such changes are called chemical changes. #### PHYSICAL CHANGE, ITS CHARACTERISTICS AND EXAMPLES **(a) Definition of physical change** A change which alters some specific property of matter without any change in the composition of its molecules is called physical change. **(b) Characteristics of physical change** 1. No new substance is formed during physical change. On heating, ice melts to form water. On further heating, the water changes into steam. The steam on cooling, changes into water. The water on further cooling, solidifies to form ice. All through the above changes in state, the molecules of ice, water or steam always contain two atoms of hydrogen and one atom of oxygen. Thus, we can conclude that no new substance is formed. Similarly, when we mix common salt to water, common salt solution is formed but no change takes place in its molecules. On evaporation, the water evaporates leaving behind common salt. 2. Physical changes can be generally reversed. Zinc oxide (white in colour), on heating, changes to yellow colour. However, on cooling its colour changes back to white. The wax on heating changes into liquid state. However, liquid wax changes into solid on cooling. Similarly, when a piece of iron is stroked with permanent magnet, it gets magnetised. However, if magnetised iron is striked, it loses its magnetism. 3. There is no change in weight during physical change. 20 g of solid ice on melting will form 20 g of water. If a common salt solution is prepared by dissolving 15 g of common salt in water, then on the evaporation of water, 15 g of common salt is left behind. 4. Only a little heat (if any) is absorbed or given off during a physical change. If we supply a certain amount of heat energy, then molten wax will change into solid wax by giving off the same amount of heat energy. The heat energy supplied during a physical change, is in no way, utilised to change the composition of molecules of a substance. **(c) Everyday examples of physical changes** Some of the common examples of physical changes are given below. 1. Evaporation of water by the heat of the sun. 2. Changing of water into steam by boiling. 3. Freezing of water to ice or solidification of molten wax. 4. Melting of ice or wax or butter or ghee. 5. Production of sound when two materials strike together. 6. Magnetisation of iron. 7. Condensation of water vapours, such as formation of clouds, mist, fog, etc. 8. Glowing of an electric bulb on the passage of electric current. 9. Change of colour due to heat as in case of zinc oxide or lead oxide. 10. Crystallisation of salts from their solutions. 11. Formation of solutions of soluble substances (sugar, salt, etc.) in water. 12. Expansion or contraction of metals on heating or cooling. 13. Shaping of glass by heat. 14. Beating of metals into sheets or drawing metals into wires. #### CHEMICAL CHANGE, ITS CHARACTERISTICS AND EXAMPLES **(a) Definition of chemical change** A change which alters the specific properties of a substance by bringing about a change in its molecular composition, followed by a change in state, is called chemical change. **(b) Characteristics of a chemical change** 1. When a chemical change occurs, new substances with entirely new properties are formed. Candle wax on burning forms entirely new substances, i.e., carbon dioxide gas and steam. Mercuric oxide (red in colour) on strong heating forms new substances, i.e., mercury and oxygen gas. Sugar on strong heating forms new substances, i.e., carbon and steam. 2. Chemical change cannot be easily reversed. The carbon dioxide and steam formed during the burning of the candle cannot be converted into wax by altering the conditions of experiment. Carbon and water vapour, formed during the heating of sugar, cannot be recombined to form sugar. Magnesium oxide formed during the burning of magnesium cannot be easily changed to original metal and oxygen. 3. There is usually a change in weight during chemical reaction. When magnesium is burnt in air, then the weight of white ash (magnesium oxide) is more than magnesium metal. For every 3 g of magnesium metal, 5 g of magnesium oxide is formed. When sugar is burnt, for every 7 g of sugar approximately 3 g of sugar charcoal is left. Similarly, when iron rusts, the weight of rusted iron is more than that of original metal, because the oxygen combines with iron. 4. Lot of heat is usually given off or absorbed during a chemical change. When magnesium burns in air, it produces a large amount of heat and light energy. When mercuric oxide decomposes to form mercury and oxygen, it absorbs a large amount of heat energy. When sugar decomposes to sugar charcoal and steam, it absorbs a large amount of heat energy. **(c) Everyday examples of chemical changes** Following are the common examples of chemical changes : 1. Cooking of food. 2. Food turning bad after a few days. 3. Curdling of milk. 4. Fading the colours of clothes. 5. Germination of seeds. 6. Ripening of fruit. 7. Lighting of a match stick by striking match head at the side of a match box. 8. Digestion of food within our bodies. 9. Respiration by humans, plants and animals. 10. Decaying of old pieces of wood. 11. Rusting of iron. 12. Blackening of silver-ware. 13. Hardening of cement to form concrete block. 14. Fermentation of sugar solution to alcohol. 15. Burning of paper, wood, coal, kerosene oil, petrol or liquefied petroleum gas. 16. Spoiling of food materials. #### DIFFERENCES BETWEEN PHYSICAL CHANGE AND CHEMICAL CHANGE | Physical Change | Chemical Change | | ----------- | ----------- | | The change takes place only in state, colour, texture, electrical and magnetic properties. However, the same substance remains | The change takes place in state, colour, texture, etc., along with the change in molecular properties. New substances, with new chemical properties, are produced. | | No new substance is produced. | New substances with entirely new chemical properties are formed. | | Specific properties of the substance do not change. | Specific properties of a substance change completely. | | It is a temporary change and can be reversed. | It is a permanent change and cannot be reversed. | | There is no net absorption or release of energy. | There is always net absorption or release of energy. | #### TYPES OF CHANGE INVOLVED WHEN THERE IS A CHANGE IN STATE OF MATTER On heating, a solid changes into liquid. This process in which a solid changes into liquid is called **melting**. The temperature at which a solid changes into liquid at atmospheric pressure is called its **melting point**. For example, the melting point of ice is 0°C. The melting point of tungsten is highest, that is, 3414°C. Higher the melting point, higher is the force of attraction between its particles (atoms or molecules). It means that the force of attraction between the atoms of tungsten is the strongest. In a solid, the particles occupy fixed positions. They simply vibrate in their fixed positions. They cannot leave their place. On heating a solid, its particles start vibrating vigorously. At the melting point, the particles of solid acquire sufficient energy to overcome the strong forces of attraction between them and move away from their positions. Thus the solid changes into liquid. At melting point, the substance exists in both solid and liquid state. In a liquid, the particles are free to move but there is enough force of attraction between the particles which binds them together. On heating, a liquid changes into a gas. This process of changing of a liquid rapidly into a gas, on heating, is called **boiling**. The temperature at which a liquid boils and changes rapidly into a gas at atmospheric pressure is called its **boiling point**. For example, the boiling point of water is 100°C. Higher the boiling point, stronger is the force of attraction between the particles. The boiling point of acetone is 55°C. When a liquid is heated, its particles start moving faster. At its boiling point, its particles gain sufficient kinetic energy to overcome the forces of attraction between them and separate into individual particles. The liquid changes into a gas which has least inter-particle forces of attraction. When a gas is cooled, it changes into liquid. The process of changing a gas into liquid, on cooling, is called **condensation**. For example, steam changes into water. In all these changes, there is change in state of matter only, and no new substance is formed. Thus, these changes are examples of physical change. #### TYPES OF CHANGE INVOLVED WHEN THERE IS A CHANGE OF ENERGY Whenever any physical or chemical change takes place, energy is involved. Either the energy is absorbed or released in one form or other. The changes that involve absorption of energy are known as **endothermic changes**. For example: cooking of food or melting of ice needs heat energy. Therefore, they are endothermic changes. The changes that involve evolution or release of energy are known as the **exothermic changes**. For example, burning of matchstick, or glowing of an electric bulb releases light energy. Therefore, they are exothermic changes. When a substance is dissolved in water, heat energy is either released or absorbed. If the heat energy is released during the dissolution (exothermic), the solution becomes hot and if the heat energy is absorbed during the dissolution (endothermic), the solution becomes cold. **Experiment 10: To show the Dissolution of Ammonium Chloride in Water** To show that the dissolution of ammonium chloride in water is an endothermic change. We have to take a test tube and fill it half with water. Add ammonium chloride salt in it (Fig. 2.10 a)). Then hold the test tube in your hands (Fig. 2.10 b)). After some time, we will feel that the test tube has become cooler. This is because ammonium chloride absorbs energy from water and makes it cool. Thus, we can conclude that the dissolution of ammonium chloride in water is an endothermic change. **Experiment 11: To show the Dissolution of Quick Lime in the Water** Take a small quantity of quicklime (calcium oxide) in a beaker. Slowly add enough water to the beaker so that the quicklime remains submerged in the water. Leave the beaker undisturbed for some time. After some time, on touching the beaker you will feel that it is hot. This shows that the dissolution of quicklime (calcium oxide) in water is an exothermic change.