Class-7 Science Chapter-6, Physical and Chemical Changes, Additional Exercises in line with NCERT book / CBSE Syllabus

Chapter 6

Physical and Chemical Changes

Additional Exercises

Exercises from textbook

Activities/Projects

Additional Exercises

Choose the correct answer:

1.    Some substances can be obtained in pure state from their solution by

a.    Galvanization

b.    Rusting

c.     Crystallization (√)

d.    chemical  reaction

2.    Which of these is a chemical change?

a.    Cutting of wood

b.    Melting of ice

c.     Rusting of iron (✓)

d.    Evaporation of water

3.    An example of physical change is;

a.    Photosynthesis

b.    Digestion

c.     Mixing of salt in water

d.    Tearing a piece of paper (✓)

4.    Which of these is physical property of a substance?

a.    Shape and Size

b.    Colour

c.     State

d.    All the above (✓)

5.    Obtaining salt by the evaporation of sea water is _________ process

a.    Galvanization

b.    Crystallization (✓)

c.     Sublimation

d.    Digestion

6.    Lime water turns milky on passing carbon dioxide due to the formation of:

a.    Calcium Carbonate (✓)

b.    Magnesium Carbonate

c.     Calcium Sulphate

d.    Magnesium Sulphate

7.    Change in _______ of a substance occurs in a physical change.

a.    Composition

b.    Chemical Property

c.     Physical Property (✓)

d.    All of these

8.    The substance formed after burning of magnesium ribbon is________

a.    Magnesium

b.    Magnesium rod

c.     Magnesium Oxide (✓)

d.    Magnesium Dioxide

Differentiate between:

Physical change and Chemical change

Physical Change	Chemical Change
Physical change involves the change in physical properties of a substance.	Chemical change involves the change in chemical properties of a substance.
Physical changes may be reversible.	Chemical changes may not be reversible.
No new substances are formed in these changes.	New substances are formed in these changes.

Match the column:

A	B
Physical change	ice plus common salt.
Freezing mixture is	When baking soda is mixed with carbon dioxide.
Salt water	does not involve a change in the property of the matter.
Copper Sulphate	makes the process of Rust formation faster.
Carbon dioxide is formed	also known as blue vitriol

Ans-

A	B
Physical change	does not involve a change in the property of the matter.
Freezing mixture is	ice plus common salt.
Salt water	makes the process of Rust formation faster.
Copper Sulphate	also known as blue vitriol
Carbon dioxide is formed	When baking soda is mixed with carbon dioxide.

State whether the following statements are True or False:

1.    New substance is formed in a physical change. (False)

2.    Burning is always accompanied by production of heat. (False)

3.    For rusting, the presence of both Oxygen and water is essential. (True)

4.    Changes can be of two types physical and chemical. (True)

Give one word for the following:

(1)  A change in which a substance undergoes a change in its physical properties is called a physical change.- Physical Change

(2)  The process of getting crystals of a substance from its solution.-  Crystallization

Encircle the Odd one:

(1)  Canada, Greenland, Iceland, Norway, Sweden, India, Finland (Countries that belong to the polar region)

(2)   Malaysia, Indonesia, Brazil, Kenya, Alaska, Uganda, Nigeria (Countries where Tropical rainforests are found)

Fill in the blanks:

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

(2)  Physical change is generally reversible.

(3)  Neutralisation of an acid with a base is a chemical change.

Answer the following questions:
Q. What is Rust? what do you mean by rusting?

Ans- Rust is a deposit on the surface of iron. If we leave a piece of iron in the open for some time, it acquires a film of brownish substance. This substance is called rust and the process is called rusting.

The process of rusting can be represented by the following equation:

  Iron(Fe)  +  Oxygen (O₂  from air) + Water(H₂O)---> Rust ( Iron oxide Fe₂O₃)

For rusting, the presence of both Oxygen and water (or water vapour) is essential.

Q. What are the ways to prevent rusting?

Ans- To prevent rusting, it is necessary that iron articles do not come in contact with oxygen, or water, or both. Following are the ways to prevent rusting.

1-  To apply a coat of paint or grease. In fact, these coats should be applied regularly to prevent rusting.

2-  To deposit a layer of another metal like chromium or zinc on iron. (Galvanisation)

Q. What happens when a magnesium ribbon is burnt? How do you classify this change?

Ans- Magnesium ribbon burns with a brilliant white light. It leaves behind a powdery ash when completely burnt.

 This is change can be represented by the following equation:

Magnesium (Mg) +  Oxygen (O₂) --->  Magnesium Oxide (MgO)

Magnesium oxide is a new substance formed on burning of magnesium. Therefore this is a chemical change.

Q. What happen when carbon dioxide is passed through limewater?

                                  Or

     What is the standard test of carbon dioxide?

Ans- When carbon dioxide is passed through limewater, calcium carbonate is formed, which makes limewater milky.

 This reaction can be presented by following equation

Carbon dioxide (CO₂) + Lime water [Ca(OH)₂] ---> Calcium Carbonate (CaCO₃) + Water(H₂O)

Turning of limewater into milky is a standard test of carbon dioxide.

Q. What is a chemical change? How are chemical changes useful in our life?.

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

 Chemical changes are useful in our life in many ways for example

(1)  A medicine is the end product of a chain of chemical reactions.

(2)  Plastics and detergents are produced by chemical reaction.

(3)  Iron is extracted from iron ore through a series of chemical changes.

(4)  Burning of coal, wood or leaves is also a chemical change.

Q. How does Ozone in atmosphere act as a natural Shield against the ultraviolet radiation from Sun?

Ans- There is a layer of ozone in our atmosphere. Ozone absorbs UV radiations and breaks down into oxygen.

Ozone------ UV rays -----à Oxygen

In this way  ozone prevents UV rays from reaching earth’s surface and causing harm to us and other life forms

Q.What is galvanization ? why is it useful?

 galvanization is the process of depositing a layer of zinc on iron.

 the iron pipes we use in our homes to carry water are galvanised. it is useful to prevent rusting.

Q. Define crystallization.

Ans- The process of getting crystals of a substance from its solution is called crystallization.

Give reason. Why?

Q. You are advised not to play with fireworks?

Ans- Explosion of a firework is a chemical change. It produces heat, light, sound and unpleasant gases that pollute the atmosphere. That is why we are advised not to play with fireworks.

Q. An apple acquires a brown colour, if it is not consumed immediately.

Ans- We observe a change in colour if we cut a fresh slice of apple and keep it away for sometime. This change in colour is due to the formation of new substances as a result of chemical changes.

Q. What is stainless steel?

Ans- Stainless steel is an alloy made by mixing of iron, carbon and metals like chromium, Nickel and magnet.  This alloy does not Rust.

Exercises from Textbook

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

Ans- Photosynthesis, Dissolving sugar in water, Burning of coal, Melting of wax, Beating aluminium to make aluminium foil digestion of food

Change	Type of change
Photosynthesis	Chemical change
Dissolving sugar in water	Physical change
Burning of coal	Chemical change
Melting of wax	Physical change
Beating aluminium to make aluminium foil	Physical change
Digestion of food	Chemical change

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

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

Correct statement -cutting of a  log of wood involve a change in physical properties like shape and size of wood but in this change no new substance is formed. therefore cutting a log of wood into pieces is a physical change.

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

Correct statement- formation of manure from leaves involves various chemical changes by the action of microbes.  chemically manure is a different substance from leaves. therefore it is a chemical change.

3. Iron pipes coated with zinc do not get rusted easily. (True)
4. Iron and rust are the same substances. (False)

Correct statement- rust is formed by the action of Oxygen and water on the surface of iron. rust and iron are not same substances.

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

Q. Fill in the blanks in the following statements:

1. When carbon dioxide is passed through lime water it turns milky due to the formation of calcium carbonate.
2.  the chemical name of baking soda is Sodium Bicarbonate.
3.  two methods by which rusting of iron can be prevented are painting and galvanisation.
4.  changes in which only physical properties of a substance change are called physical changes.
5.  changes in which new substances are formed are called chemical changes.

Q. When baking soda is mixed with lemon juice, Bubbles are formed with the evolution of a gas. what type of change is it? Explain.

Ans- When baking soda is mixed with lemon juice, bubbles  are formed with the evolution of carbon dioxide gas. this is a chemical change as it involves the formation of new substance (sodium ion, citric acid ion, water and carbon dioxide) along with release of heat and carbon dioxide.

Q. 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 physical and chemical changes take place.

Ans- During the burning of candle both physical and chemical changes occur simultaneously.

 physical change- wax gets melted and vaporized.

 chemical change- carbon dioxide, light and heat are produced by burning of papers of wax.

Chewing of food involves both physical and chemical changes. Breaking of a bite of food into  smaller pieces is a physical change while the action of saliva on the starch is a chemical change.

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

Ans- Setting of curd is a chemical change because;

1-    Curd is a new substance formed.

2-    Curd has a different taste, form and properties than milk.

3-    This change is Irreversible.

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

Ans-

 Burning of wood

Burning of wood is a chemical process because it involves the formation of charcoal with the release of carbon dioxide, heat and light.

 Burning of wood in presence of oxygen-------> charcoal+ carbon dioxide+ heat+ light

Cutting of wood

Cutting of a wood involves a change in physical properties like shape and size of wood but in this change, no new substance is formed. Therefore cutting a log of wood into pieces is a physical change.

Q. Describe how crystals of Copper Sulphate are prepared.

Ans- Crystals of Copper Sulphate are prepared by the process of crystallization which can be stated in following steps.

1-  Take a cap full of water in a beaker and add a few drops of sulphuric acid.

2-  Heat the water.

3-  When it starts boiling, add Copper Sulphate powder slowly while stirring continuously.

4-  Continue adding copper sulphate powder till no more powder can be dissolved.

5-  Filter the solution.

6-  Allow it to cool down. Do not disturb it when it is cooling.

7-  After sometime, we see the crystals of Copper Sulphate and the beaker.

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

Ans- For rusting presence of air and water is necessary. Paint forms a protective layer on the surface of iron, not letting iron come in contact with air and water. Thus painting of an iron gate prevents it from rusting.

Q. The gas we use in kitchen is called liquefied 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 ia chemical change

(ii) Process-B is a chemical change (✓)

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

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

Q. 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 ia chemical change

(ii) Process-B is a chemical change

(iii) Both processes A and B are chemical change. (✓)

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

Extended learning - Activities and Projects

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

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 sme amount as in other bottles. In each bottle put few similar iron nails so that they are completely under water. Add a teaspoonful of cooking oil to the water in bottle C sothat 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.

3.    Prepare crystals of alum.

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?

Did you know?

Near the Qutub Minar in Delhi stands an iron pillar, which is more than 7 meters high. It weighs more than 6000 kg. It was built more than 1600 years ago. After such a long period it has not rusted. For its quality of rust resistance, it has been examined by scientists from all parts of the world. It tells something about the advances India had made in metal technology as back as 1600 years ago.

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