Science STD 7 Chapter 5: Physical and Chemical Changes - Exercises

Science STD 7 Chapter 5: Physical and Chemical Changes - Exercises

1. Classify the changes involved in the following processes as physical or chemical changes:

  1. (a) Photosynthesis

    Chemical change

  2. (b) Dissolving sugar in water

    Physical change

  3. (c) Burning of coal

    Chemical change

  4. (d) Melting of wax

    Physical change

  5. (e) Beating aluminium to make aluminium foil

    Physical change

  6. (f) Digestion of food

    Chemical change

2. State whether the following statements are true or false. In case a statement is false, write the corrected statement in your notebook.

  1. (a) Cutting a log of wood into pieces is a chemical change. (True/False)

    False. Cutting a log of wood into pieces is a physical change.

  2. (b) Formation of manure from leaves is a physical change. (True/False)

    False. Formation of manure from leaves is a chemical change.

  3. (c) Iron pipes coated with zinc do not get rusted easily. (True/False)

    True.

  4. (d) Iron and rust are the same substances. (True/False)

    False. Rust is a new substance formed from iron and oxygen. It is different from iron.

  5. (e) Condensation of steam is not a chemical change. (True/False)

    True.

3. Fill in the blanks in the following statements:

  1. (a) When carbon dioxide is passed through lime water, it turns milky due to the formation of calcium carbonate.
  2. (b) The chemical name of baking soda is sodium hydrogencarbonate.
  3. (c) Two methods by which rusting of iron can be prevented are painting and galvanisation.
  4. (d) Changes in which only physical properties of a substance change are called physical changes.
  5. (e) Changes in which new substances are formed are called chemical changes.

4. When baking soda is mixed with lemon juice, bubbles are formed with the evolution of a gas. What type of change is it? Explain.

This is a chemical change. When baking soda (sodium hydrogencarbonate) is mixed with lemon juice (citric acid), a chemical reaction occurs. The bubbles formed are due to the evolution of a new substance, carbon dioxide gas. In a chemical change, one or more new substances are formed.

5. When a candle burns, both physical and chemical changes take place. Identify these changes. Give another example of a familiar process in which both the chemical and physical changes take place.

Physical changes: The melting of solid wax into liquid wax is a physical change. The liquid wax vaporises, and this is also a physical change.

Chemical changes: The burning of wax vapour to produce heat, light, carbon dioxide, and water vapour is a chemical change. The production of a new substance (carbon dioxide) along with heat and light confirms that it is a chemical change.

Another example of a process with both physical and chemical changes is the cooking of an egg. The physical change is the liquid egg yolk and white becoming solid. The chemical change is the change in the composition of the egg, creating new substances with different properties, such as a change in smell and taste.

6. How would you show that setting of curd is a chemical change?

The setting of curd from milk is a **chemical change** because a new substance is formed with different properties. Curd cannot be converted back into milk. The change in taste and composition from milk to curd is irreversible, which is a characteristic of a chemical change.

7. Explain why burning of wood and cutting it into small pieces are considered as two different types of changes.

Cutting wood into small pieces is a **physical change**. The physical properties like size and shape of the wood change, but no new substance is formed. The chemical composition of the wood remains the same.

Burning of wood is a **chemical change**. In this process, wood reacts with oxygen to produce new substances like ash, carbon dioxide, and water vapour. This change is irreversible, and new products are formed along with the release of heat and light.

8. Describe how crystals of copper sulphate are prepared.

The process of preparing crystals of a pure substance from its solution is called **crystallisation**. To prepare copper sulphate crystals:

  1. Take a cupful of water in a beaker and add a few drops of dilute sulphuric acid to it.
  2. Heat the water. When it starts boiling, add copper sulphate powder slowly while stirring continuously.
  3. Continue adding the powder until no more can be dissolved.
  4. Filter the solution and let it cool without disturbing it.
  5. After some time, you will see blue crystals of copper sulphate forming in the solution.

9. Explain how painting of an iron gate prevents it from rusting.

Rusting of iron requires the presence of both oxygen and water (or water vapour). Painting an iron gate prevents it from rusting by creating a protective barrier between the iron and the air/moisture. This coating stops the iron from coming into contact with oxygen and water, which are essential for the rusting process. For the protection to last, the paint coat should be applied regularly.

10. Explain why rusting of iron objects is faster in coastal areas than in deserts.

Rusting of iron objects is faster in coastal areas because the air there has a high content of moisture and salts. The presence of high humidity and salts accelerates the chemical reaction between iron, oxygen, and water, leading to faster rust formation. In deserts, the air is dry and the humidity is low, which slows down the rusting process.

11. The gas we use in the kitchen is called liquified petroleum gas (LPG). In the cylinder it exist as a liquid. When it comes out from the cylinder it becomes a gas (Change - A) then it burns (Change - B). The following statements pertain to these changes. Choose the correct one.

  1. Change A is a chemical change.
  2. Change B is a chemical change.
  3. Both changes A and B are chemical changes.
  4. None of these changes is a chemical change.

12. Anaerobic bacteria digest animal waste and produce biogas (Change - A). The biogas is then burnt as fuel (Change B). The following statements pertain to these changes. Choose the correct one.

  1. Change A is a chemical change.
  2. Change B is a chemical change.
  3. Both changes A and B are chemical changes.
  4. None of these changes is a chemical change.

Suggested Activities and Projects

1. Describe two changes that are harmful. Explain why you consider them harmful. How can you prevent them?

Outline: This activity involves identifying harmful changes and thinking about their prevention. Examples could include:

  1. Rusting of iron: Rust weakens iron objects like bridges and ships, which can lead to huge monetary losses and safety hazards. It can be prevented by applying a coat of paint, grease, or by galvanisation (coating with zinc).
  2. Spoiling of food: Spoiled food can cause food poisoning and other health problems if consumed. It can be prevented by refrigeration, proper storage, or using preservatives.

2. Take three glass bottles with wide mouths. Label them A, B and C. Fill about half of bottle A with ordinary tap water. Fill bottle B with water which has been boiled for several minutes, to the same level as in A. In bottle C, take the same boiled water and of the same amount as in other bottles. In each bottle put a few similar iron nails so that they are completely under water. Add a teaspoonful of cooking oil to the water in bottle C so that it forms a film on its surface. Put the bottles away for a few days. Take out nails from each bottle and observe them. Explain your observations.

Outline: This experiment is designed to show the conditions necessary for rusting. The nails in bottle A (tap water) will rust because both oxygen and water are present. The nails in bottle B (boiled water) will rust very slowly or not at all because boiling the water removed most of the dissolved oxygen. The nails in bottle C (boiled water with oil) will also not rust because the oil layer prevents atmospheric oxygen from dissolving in the water, thus separating the iron nails from oxygen and water.

3. Prepare crystals of alum.

Outline: This activity is a practical demonstration of crystallisation. You would dissolve alum powder in hot water to create a saturated solution. By allowing this solution to cool undisturbed for a day or two, pure, large crystals of alum would form as the solvent evaporates. This visually demonstrates that crystallisation is a physical change where a pure substance is separated from its solution in crystal form.

4. Collect information about the types of fuels used for cooking in your area. Discuss with your teachers/parents/others which fuels are less polluting and why.

Outline: This project involves researching and discussing different types of cooking fuels. You would learn about fuels like LPG, wood, kerosene, and biogas. The discussion would focus on which fuels are cleaner and why. For example, LPG is considered less polluting than wood because it burns more completely, producing less smoke and harmful pollutants that affect air quality and health.