Class 7 Science Chapter 6 Important Questions

Class 7 Science Chapter 6 Important Questions and Keywords of Physical and Chemical Change updated for CBSE academic session 2020-2021. In class 7 Science Chapter 6 Extra practice question answers the entire NCERT Textbook is covered.

All the important topics from the notes and text book are taken for exam practice. It provides a better practice for unit tests and periodic exams.

Class 7 Science Chapter 6 Important Questions 2020-2021

Class: 7Science
Chapter: 6Physical and Chemical Changes
Contents:Important Extra Questions with Answers

Class 7 Science Chapter 6 Important Questions Set – 1

What do you mean by physical change? Explain with example.

Properties such as shape, size, colour and state of a substance are called its physical properties. A change in which a substance undergoes a change in its physical properties is called a physical change. A physical change is generally reversible. In such a change no new substance is formed.
For example, dissolution of sugar in water, glowing of an electric bulb, tearing of paper etc.

Make a list of ten changes you have noticed around you.

Every day we come across many changes in our surroundings. This change may involve one or more substances. For example, lighting of bulb, motion of fan, sound produced by radio, changing of day and night, changes in the shape of moon, changing in the size of the baby dog, blooming of flower, melting of ice, vaporisation of water, lightning.

What is chemical change? Explain with example.

A change in which the composition and chemical properties of the substance get changes, called chemical change. This change is permanent and irreversible.
For example: burning of a candle, formation of curd from milk, ripening of fruits etc.

    • Most physical changes are reversible. Comment.
      In physical change, no new product is formed. So, these changes can be reversed easily. Thus, most physical changes are reversible. Dissolution of sugar in water is a physical change. We can get back sugar and water by evaporation. Freezing of ice, from water is a physical change. On melting ice, we can get back water again.
    • “Chemical changes are very important in our life”. Discuss.
      ANSWER: Chemical changes are very important in our lives. All new substances are formed as a result of chemical changes. For example, if a metal is to be extracted from an ore, such as iron from iron ore, we need to carry out a series of chemical changes. A medicine is the end product of a chain of chemical reactions. Useful new materials, such as plastics and detergents, are produced by chemical reactions. Indeed, every new material is discovered by studying chemical changes.

Class 7 Science Chapter 6 Important Questions Set –2

What happens when a new substance is formed in a chemical reaction?

One or more new substances are produced in a chemical change. In addition to new products, the following may accompany a chemical change:
When a new substance is formed in chemical reaction heat, light or any other radiation (ultraviolet, for example) may be given off or absorbed. Sound may be produced. A change in smell may take place or a new smell may be given off. A colour change may take place. A gas may be formed.

What do you mean by “rusting”?

If a piece of iron is left in the open for some time, it acquires a film of brownish substance. This substance is called rust. The process by which rust is formed on an iron object is called rusting. For rusting, the presence of moisture (or water vapour) and oxygen are essential.
Rust of iron is chemically different from iron.

How can you show that on dissolving the ash of magnesium in water, forms a new substance?

A small piece of a thin strip or ribbon of magnesium. Bring the tip near a candle flame. It burns with a brilliant white light. When it is completely burnt, it leaves behind a powdery ash. Collect the ash and mix it with a small amount of water. Stir the mixture (aqueous solution) well. On dissolving the ash in water, it forms a new substance.
Magnesium oxide (MgO) + Water (H2O) → Magnesium hydroxide [Mg(OH)2]
So, magnesium oxide is a new substance formed on burning of magnesium. Magnesium hydroxide is another new substance formed by mixing magnesium oxide with water.




    1. Explain how painting of an iron gate prevents it from rusting.
      We know that rusting take place in the presence of oxygen and water. If we somehow, prevent the oxygen and water from coming in contact with iron articles, there will be no rusting. Hence, if iron gates are painted all over, then they will not come in contact with oxygen and water and consequently no rusting will occur on the gates.
    2. What happens when iron nails are dipped in copper sulphate solution? Explain with an activity.
      Dissolve about a teaspoonful of copper sulphate (blue vitriol or neela thotha) in about half a cup of water in a glass tumbler or a beaker. Add a few drops of dilute sulphuric acid to the solution. We get a blue coloured solution. Save a small sample of the solution in a test tube or a small glass bottle. Drop a nail or a used shaving blade into the remaining solution. We should wait for half an hour or so. Observing the colour of the solution and compare it with the colour of the sample solution saved separately. Take out the nail or the blade. The changes that we notice are due to a reaction between copper sulphate and iron. The change of colour of the solution from blue to green is due to the formation of iron sulphate, a new substance. The brown deposit on the iron nail is copper, another new substance. We can write the reaction as:
      Copper sulphate solution (blue) + Iron → Iron sulphate solution (green) + Copper (brown deposit)

Class 7 Science Chapter 6 Important Questions Set –3

“Ozone layer is a protective shield in the atmosphere”. Comment.

The ozone layer in our atmosphere protects us from the harmful ultraviolet radiation which come from the sun. Ozone absorbs this radiation and breaks down to oxygen. Oxygen is different from ozone. If ultraviolet radiation were not absorbed by ozone, it would reach the earth’s surface and cause harm to us and other life forms. Ozone acts as a natural shield against this radiation.

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

When wood is cut into small little pieces, no new substances are formed. Therefore the cutting of wood is a physical change but when wood is burnt, carbon dioxide is formed along with the generation of heat. From the burnt wood, we cannot get back the original wood. Hence, the burning of wood is a chemical change.

Why do you consider explosion of cracker a chemical change?

When a cracker explodes, heat, light and smoke come out due to the formation of new substances. Thus, it is a chemical change.

    • A fraction of ship’s iron part has to be replaced every year. Explain.
      Ships are made of wood and iron and a part of them remains in water. On the part above water, water keeps clinging. This party is exposed to air. Moreover, the water of sea contains many salts which make the process of rust formation faster. Therefore, ships suffer a lot of damage from rusting in-spite of being painted regularly. So, a fraction of ship’s iron has to be replaced every year.
    • Identify the type of change and state what happens when sugar is heated.
      When sugar is heated strongly, it becomes foggy due to water vapour and a black powder is left behind. The black substance is charcoal. This is called charring of sugar and it cannot be reversed. Thus, this is a chemical change.



Class 7 Science Chapter 6 Important Questions Set –4

When a candle burns both physical and chemical change occur. Explain.

It is true that when a candle burns, both physical and chemical change occur. Initially candle burns to produce heat and light. It is a chemical change because candle burns with the formation of gases. Due to the heat produced, some candles simultaneously melts. This is a physical change as the molten candle wax behaves like original candle.

Describe how crystals of copper sulphate are prepared?

Take a cup full of water in a beaker and add a few drops of dilute sulphuric acid. Heat the water. When it starts boiling, add copper sulphate powder slowly. Continue to add copper sulphate powder till no more powder can be dissolved. During this process continuously stir the solution. Filter the solution. Leave it for cooling. Look it after sometime, you can see the crystals of copper sulphate.

Which gas is released when vinegar and baking soda are mixed? What happens when this gas is passed through lime water?

When baking soda and vinegar are mixed with each other than carbon dioxide gas is released. When carbon dioxide is passed through lime water, then calcium carbonate is formed. Calcium carbonate makes lime water appear milky. The turning of lime water into milky solution is a standard test of carbon dioxide.
The following reactions take place:
i)Vinegar + Baking soda -> Carbon dioxide + Other substances.
ii)Carbon dioxide (CO₂) + Limewater Ca(OH)₂ -> Calcium carbonate (CaCO₃) + Water (H₂O)

Write the characteristics of a chemical change.
Characteristics of a chemical change:

    1. Mostly chemical reactions are irreversible.
    2. They cause release or absorption of energy.
    3. New substances with new properties are formed.
    4. The properties of reactants are different from the products.



What is rusting? How do we prevent it?
When iron reacts with oxygen and water vapour (or moisture) it creates rust, which is totally different from iron. Rusting of iron is a chemical change. Methods of preventing rust formation rusting of an iron object can be prevented or reduced by not allowing air and moisture to come in contact with the surface of iron objects.
Following are the methods of prevention from rusting:

    • i) By putting oil or paint over the surface of iron implements.
    • ii) By galvanization process, in which a thin coating of zinc giving on iron sheets to prevent rusting.
    • iii) By tinning process, using a coating of molten tin to avoid corrosion.
    • iv) By electroplating, in this method metal surface are covered with another metal by using electric current. And thus, corrosion is stopped.
    • v) By anodizing process, in which metals like aluminium, copper are coated electrically with a thin and strong film of their oxides which protect them from corrosion.
    • vi) By using anti-rust solution, in which alkaline chromate and alkaline phosphate solution act as anti- rust solution. When iron objects are dipped into a boiling and strongly alkaline solution of sodium phosphate, a protective insoluble film of iron phosphate is formed on them. This film protects the article from rusting.

Class 7 Science Chapter 6 Important Questions Set –5

What do you mean by “galvanization” and “electroplating”?

The process of depositing a layer of zinc on iron is called “galvanization” and “electroplating” is the deposition of a metallic coating by passing electric current through dissolved metal ions on the object to be coated.

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

Setting of curd is a chemical change since a new substance (lactic acid) is formed which differs in taste from the original substance (fats and proteins) i.e. milk. Also the change is irreversible (curd cannot be converted into milk).

What do you mean by “alloying” and “corrosion”?

Mixing of metals with other metal or non-metals in molten form is called “alloying” and “corrosion” is the damage of metals due to air, water, salts, acids and moisture.

What do you know about crystallization? Explain.
As we know that salt can be obtained by the evaporation of sea water. The salt obtained in this manner is not pure and its crystals are small. The shape of the crystals cannot be seen clearly. However, large crystals of pure substances can be formed from their solutions. The process is called crystallisation. Generally, crystal refers a solid structure with flat sides. It is an example of a physical change.



Explain why rusting of iron objects is faster in coastal areas than in deserts?
For rusting, the presence of both oxygen and moisture (water vapour) is required. In coastal areas the quantity of moisture content in air is high (highly humid) as compared to desert regions (low humid), so the rusting becomes faster in coastal areas.