NCERT Solutions Std 7 Science Physical and Chemical Changes

Physical and Chemical Changes

Exercise:

 

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

(a) Photosynthesis

(b) Dissolving sugar in water

(c) Burning of coal

(d) Melting of wax

(e) Beating aluminium to make aluminium foil

(f ) Digestion of food

Physical Changes Chemical Changes
Dissolving sugar in water

Melting of wax

Beating aluminium to make aluminium foil

Photosynthesis

Burning of coal

Digestion of food

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

(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

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

False.

The formation of manure from leaves is a chemical change.

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

True.

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

False.

Iron and rust are two different substances.

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

True.

3). Fill in the blanks in the following statements:

(a) When carbon dioxide is passed through lime water, it turns milky due to the formation of calcium carbonate.

(b) The chemical name of baking soda is sodium hydrogen carbonate.

(c) Two methods by which rusting of iron can be prevented are painting and galvanisation.

(d) Changes in which only physical properties of a substance change are called physical changes.

(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.

     The reaction of baking soda with lemon juice is as follows.

Baking soda + lemon juice        carbon dioxide + other substances

As new substance formed it is a chemical change.

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 change: melting of wax

Chemical change: burning of vapour of wax to give heat, light and carbon dioxide.

     The burning of LPG in our kitchen is another example in which both chemical and physical changes take place.

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

     A new substance is formed with different taste and this change is irreversible. Therefore, setting of curd is a chemical change.

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

            Burning of wood is a chemical change because in burning, a new substance is formed.

     Cutting wood into small pieces is a physical change because here no new substance is formed. Only the size of the wood is changed.

8). Describe how crystals of copper sulphate are prepared.

          Steps to prepare crystals of copper sulphate are as follows.

(i). Take a cupful of water in a beaker and add a few drops of dilute sulphuric acid.

(ii). Heat the water.

(iii). When it starts boiling add copper sulphate powder slowly while stirring continuously.

(iv). Continue adding copper sulphate powder till no more powder can be dissolved.

(v). Filter the solution. Allow it to cool. Do not disturb the solution when it is cooling.

(vi). Look at the solution after some time. You will see the crystals of copper sulphate.

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

            Rusting happens when iron gate exposed to air and moisture.  We can prevent iron articles from coming in contact with oxygen, or water, or both. One simple way is to apply a coat of paint or grease. Another way is to deposit a layer

of metal like chromium or zinc on iron.

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

            If the content of moisture in the air is high, which means if it is more humid, rusting becomes faster. Humidity is more in coastal areas than in desert areas. Therefore, rusting of iron objects is faster in coastal areas than in deserts.

11). The gas we use in the kitchen is called liquified petroleum gas (LPG). In the cylinder, it exists 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.

(i) Process – A is a chemical change.

(ii) Process – B is a chemical change.

(iii) Both processes A and B are chemical changes.

(iv) None of these processes is a chemical change.

Answer: Process – B 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.

(i) Process – A is a chemical change.

(ii) Process – B is a chemical change.

(iii) Both processes A and B are chemical changes.

(iv) None of these processes is a chemical change.

Answer: Both processes A and B are chemical changes.

Additional Questions

1). State differences between physical changes and chemical changes.

Physical Change Chemical Change
1. A change in which a substance undergoes a change in its physical properties is called a physical change 1. A change in which a new substance is formed is called a chemical change.
2. A physical change is generally reversible.

 

2. A chemical change is generally irreversible.
3. In such a change no new substance is formed. 3. new substance is formed in chemical change.
4. cutting of paper, spinning wheels of a cycle, inflating a balloon, Melting of wax etc are physical changes. 4. digestion of food in our body, ripening of fruits, fermentation of grapes, etc., are chemical changes.

Fill in the blanks.

1). Chemical changes are very important in our lives.

2). A chemical change is also called as a chemical reaction.

3). Burning is always accompanied by production of heat.

4). Burning of any substance is a chemical change.

5). Large crystals of pure substances can be formed by crystallisation.

Write whether ‘True’ or ‘False’.

1). A physical change is generally irreversible.

 False.

2). New substances are formed in physical change.

False.

3). Almost every article of iron, kept in the open gets rusted.

 True.

4). Souring of milk is a change.

True.

5). If you leave a piece of iron in the open for some time, it acquires a film of brownish substance.

True.

6). During a chemical change sound may be produced.

True.

7). Burning of magnesium ribbon is a physical change.

False.

8). Burning of any substance is a physical change.

False.

Define

1). Physical change

          A change in which a substance undergoes a change in its physical properties is called a physical change.

2). Chemical change

          A change in which one or more new substances are formed is called a chemical change.

3). Rusting of iron.

           If you leave a piece of iron in the open for some time, it acquires a film of brownish substance. This is called rusting of iron.

4). Galvanisation.

          The process of depositing  a layer of a metal like chromium or zinc on iron to prevent iron from rusting is called as galvanisation.

5). Crystallisation

          Large crystals of pure substances can be formed from their solutions. The process is called crystallisation.

Answer the following in short.

1). What are the physical properties of a substance?

          Properties such as shape, size, colour and state of a substance are called its physical properties.

2). Mention the characteristics of physical change.

         Characteristics of physical changes are as follows.

i). Only the physical properties like shape, size, colour or state of a substance change.

ii). No new substance is formed.

iii). Generally, physical changes are reversible.

3). Mention the characteristics of chemical change.

          Characteristics of chemical changes are as follows.

i). New substances are formed.

ii). New substance is completely different from the original substance.

iii). Generally, chemical changes are irreversible.

4). Mention the things that accompany a chemical change.

          In addition to new products, the following may accompany a chemical change:

i). Heat, light or any other radiation (ultraviolet, for example) may be given off or absorbed.

ii).  Sound may be produced.

iii). A change in smell may take place or a new smell may be given off.

iv). A colour change may take place.

v). A gas may be formed. 

Click here for the solutions of

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2). Nutrition in Animals

3). Fibre to Fabric

4). Heat

5). Acids, Bases and Salts

6). Physical and Chemical Changes

7). Weather, Climate and Adaptations of Animals to Climate

8). Winds, Storms and Cyclones

9). Soil

10). Respiration in Organisms

11). Transportation in Animals and Plants

12). Reproduction in Plants

13). Motion and Time

14). Electric Currents and its Effects

15). Light

16). Water: A Precious Resource

17). Forests: Our Lifeline

18). Wastewater Story

 

 

 

 

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