Physical and Chemical Changes Class 7 Extra Questions and Answers

These AP 7th Class Science Important Questions and 5th Lesson Physical and Chemical Changes Class 7 Extra Questions will help students prepare well for the exams.

Class 7 Science Physical and Chemical Changes Extra Questions

Physical and Chemical Changes Class 7 Important Questions

Question 1.
Define a physical change.
Answer:
The change in which the identity of the substance does not change is called a physical change.

Question 2.
Melting of wax is a change where a solid changes to a liquid state. Give one more such change which you observe in your surroundings.
Answer:
Melting of ice is also a change where solid changes into liquid state.

Question 3.
What kind of change is shown by tearing of paper?
Answer:
Tearing of paper is a physical change although, it cannot be reversed.

Question 4.
What are the physical properties of a substance?
Answer:
Shape, size, colour and state of a substance are some of its physical properties of a substance.

Question 5.
What is the colour of copper sulphate solution obtained when iron nails are dipped in it?
Answer:
When iron nails are dipped in copper sulphate solution, then the colour of the solution changes to green.

Question 6.
Define galvanisation.
Answer:
This process of depositing a layer of zinc on iron is called galvanisation.

Question 7.
Name any two methods by which rusting of iron can be prevented.
Answer:
Rusting of iron can be prevented by oiling or painting and by galvanisation.

Question 8.
Which gas is released when baking soda is mixed with vinegar?
Answer:
Carbon dioxide is released when baking soda is mixed with vinegar.

Question 9.
Define rust.
Answer:
When an iron object is left exposed to moist air, it chemically reacts with oxygen and water in the air to form a red-brown flaky substance called rust.

Question 10.
Give any two examples of physical changes and chemical changes.
Answer:
Freezing of water, melting of wax, boiling of water, etc., are examples of physical change. Burning of wood or paper, souring of milk, digestion of food etc., are examples of chemical changes.

Question 11.
What kind of change is crystallisation?
Answer:
Crystallisation is a physical change because no new substance is formed.

Question 12.
What is the nature of aqueous solution of magnesium oxide?
Answer:
The aqueous solution of magnesium oxide is basic in nature, as it turns red litmus to blue.

Question 13.
What are the essential factors responsible for rusting?
Answer:
The essential factors responsible for rusting are :

• Presence of moisture and
• Presence of oxygen in air.

Question 14.
Complete the following reactions:
a) MgO+H₂O →
b) CO₂+Ca(OH)₂ →
Answer:
a) Magnesium oxide (MgO)+ Water H₂O) → Magnesium hydroxide [Mg(OH)₂]
b) Carbon dioxide (CO₂)+ Lime water [Ca(OH)₂] → Calcium Carbonate (CaCO₃) + Water (H₂O)

Question 15.
What is blue vitriol?
Answer:
Vitriol is the generic name for a group of chemical compounds made up of sulfates of various metals, originally iron or copper. Crystals of copper sulphate penta hydrate are blue in colour. Is a solution of copper sulphate in water is commonly known as blue vitriol.

Physical and Chemical Changes Extra Questions

Question 1.
What are the characteristics of a physical change?
Answer:
The characteristics of a physical change are:

• No new substance is formed in this change.
• It is a temporary change and is generally reversible.
• A temporary change in colour may take place.
• Very little energy (heat, etc.) is either absorbed or evolved.

Question 2.
Classify the following processes into physical or chemical changes.
a) Beating of aluminium metal to make aluminium foil
b) Digestion of food
c) Cutting of a log of wood into pieces
d) Burning of crackers
Answer:
Beating of aluminium metal to make aluminium foil and cutting of a log of wood into pieces are physical changes. Digestion of food and burning of crackers are chemical changes.

Question 3.
Write word equations for two chemical reactions with the help of materials given in the box.
Air, copper sulphate, iron, vinegar, iron oxide, carbon dioxide, iron sulphate, copper, lime water, water.
Answer:
The reaction are given below:

• Iron + Air + Water → Iron oxide
• Copper sulphate + Iron→ Iron sulphate + Copper

Question 4.
What are the characteristics of a chemical change?
Answer:
The characteristics of a chemical change are :

• A chemical change is irreversible in nature.
• It is a permanent change.
• A permanent change in colour might occur in a chemical change.
• A new substance is always formed in chemical change.
• The composition of a substance gets changed during a chemical change.
• A lot of energy in the form of a heat, light etc., is either absorbed or given out in a chemical change.

Question 5.
Magnesium ribbon burns in air and changes to white substance, i.e., magnesium oxide. When magnesium oxide dissolves in water, what type of change take place? Give reason in support of your answer. Express the change in the form of equation.
Answer:
Mixing of ash obtained by the burning of magnesium with water is a chemical change. When magnesium is burnt in air, it forms magnesium oxide in the form of white ash.
Magnesium (Mg)+ Oxygen (O₂) → Magnesium oxide (MgO)
When magnesium oxide dissolves in water, it forms a new substance, magnesium hydroxide.
Magnesium oxide (MgO)+Water (H₂O) → Magnesium hydroxide Mg(OH)₂
So, it is a chemical change.

Question 6.
Compare the energy changes involved during a physical and a chemical change.
Answer:
Yes, all types of changes involves energy changes. During a change, energy is either evolved or absorbed. In case of chemical change the absorption and evolution of energy will takes place. In case of physical change the absorption and evolution of energy does not takes place.

Question 7.
What is a chemical change?
Answer:
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. Most of the chemical changes are irreversible. They involve in evolution of heat, light, gas or sound. For example burning of paper or wood, digestion of food etc.

Question 8.
Define crystallisation. Give an example.
Answer:
The process of obtaining crystals of pure substances from their solutions is called crystallisation. It is a method used to purify solids. It is a physical change. For example : Copper sulphate crystals can be obtained from the solution of pure copper sulphate. The solution is known as blue vitriol.

Question 9.
Explain the following:
a) Lime water turns milky on passing carbon dioxide gas through it.
b) Bubbles are produced when acetic acid is added to a solution of sodium hydrogen carbonate.
Answer:
a) When carbon dioxide (CO₂) reacts with the lime water [Ca(OH)₂], calcium carbonate (CaCO₃) and water (H₂O) is formed.
Ca(OH})₂+CO₂ → CaCO₃+H₂O

b) When baking soda (NaHCO₃) is added to acetic acid (CH₃COOH), then carbon dioxide (CO₂) gas is formed which causes appearance of bubbles. CH₃ COOH+NaHCO₃ → CO₂₊ other substance.

Question 10.
Explain why process of digestion is a chemical change.
Answer:
In the process of digestion, the various food materials break down to form new substances which can be absorbed by the body. So, the process of digestion is a chemical change.

Question 11.
Give an example of a chemical reaction for which of the following reactions?
a) A change in colour is observed
b) A gas is involved
c) Change in taste is observed
d) Sound is produced
e) Light is produced
f) Heat is produced
Answer:
a) Reaction between copper sulphate solution and iron metal.
b) Reaction between baking soda and vinegar (carbon dioxide is evolved).
c) Setting of curd from milk. Taste of milk changes to sour in curd.
d) Burning of crackers.
e) Burning of fuel produces light.
f) Reaction between hydrochloric acid and sodium hydroxide produces heat.

Question 12.
Is burning always a desirable change?Explain.
Answer:
No. it is not always a desirable change. It is desirable during cooking of food, burning of candle, burning of fossil fuels to drive engines and other machines. However, it is undesirable at other times as it causes a lot of damage, E.g: forest fire causes a lot of damage to plants, animals and our ecosystem. Therefore, it is not always a desirable change.

Extra Questions of Physical and Chemical Changes Class 7

Question 1.
State the differences between physical and chemical changes.
Answer:
The differences between physical and chemical changes are:

S.No.	Physical change	Chemical change
1)	It is generally a reversible process.	It is generally a irreversible process.
2)	No new substance is formed.	A new substance is formed.
3)	It is a temporary change.	It is a permanent change.
4)	Only physical state and physical properties of the substance are changed.	The physical as well as the chemical properties of the substance are changed.
5)	There is no energy change in these changes.	Energy is either absorbed or evolved in these changes.
6)	Example : Melting of ice.	Example : Burning of paper.

Question 2.
What is rusting ? Suggest and explain any three methods for prevention of rusting.
Answer:
Iron reacts with oxygen and moisture present in the atmosphere to form a brown, flaky substance called rust. Rust of iron is undesirable because the layer of rust formed falls off, exposing the metal to further rusting. As a result, iron object becomes weak with the passage of time.
Iron ( Fe ) + Oxygen (O₂ from the air) + Water (H₂O) → Rust (iron oxide Fe₂O₃)

• Methods to prevent rusting: Applying oil or grease on the surface inhibits the iron surface from, coming into touch with the atmosphere, which prevents rusting.
• Galvanisation: A layer of a more reactive metal, such as zinc, is deposited on the surface of iron to protect it from rust.
• Painting: Applying a layer of paint to the metal’s surface stops it from coming into contact with the atmosphere once more.
• Alloying: To make alloys, iron can be combined with other metals and non-metals. Rust resistance is a feature of alloys. Stainless steel is an important iron alloy.

Question 3.
Answer the following questions
a) How ozone layer acts as a protective shield ?
b) What is stainless steel? How is stainless steel made? State an important property of stainless steel.
Answer:
a) The ozone layer protects us from the harmful ultraviolet radiations which come from the sun. Ozone absorbs ultraviolet radiations coming from the sun and breaks down to form oxygen. In this way, ozone layer absorbs harmful ultraviolet radiations.
b) Stainless steel is an alloy of iron. When iron is mixed (or alloyed) with carbon, chromium and nickel, then stainless steel is obtained. Stainless steel does not rust at all.

Question 4.
Why temperature is important in our daily lives?
Answer:
Chemical changes are very important in our lives because 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 studying chemical changes.

Question 5.
What happens when vinegar is added to baking soda? What happens when this gas is passed through lime water?
a) Vinegar + Baking soda Carbon dioxide + Other solutions
b) Carbon dioxide (CO₂)+Lime water [Ca(OH)₂] → Calcium Carbonate (CaCO₃)+Water (H₂O)
Answer:
When baking soda and vinegar are mixed with each other, carbon dioxide gas is released. When carbon dioxide is passed through lime water, calcium carbonate is formed. Calcium carbonate to makes lime water appear milky. The turning of lime water into milky solution is a standard test of carbon dioxide. The following reactions takes place.

Question 6.
What happens when iron nails are dipped in copper sulphate solution?
Answer:
When iron is dipped in copper sulphate solution, a brown layer of copper gets deposited on the surface of the iron nails after some time. This happens due to the reaction between the copper sulphate and iron. Also, the colour of the copper sulphate changes from blue to green due to the formation of copper sulphate.
Iron nail + Copper sulphate solution (blue) → Iron sulphate solution (Green) + Copper (brown)

Question 7.
Give two examples for each of the following cases:
a) Physical changes which are reversible.
b) Physical changes which are not reversidele.
c) Chemical changes
Answer:
a) (i) Folding of paper
(ii) Melting of ice.

b) (i) Tearin of paper
(ii) Breaking of glass

c) (i) Reaction between vinegar and baking soda
(ii) Burning of a matchstick

Case Based / Data – Based questions

Sneha bought big iron table for their shop. Sneha leaves the table in the open for a few days, ‘it acquires a film of brownish substance, called rust.
a) Do you think rust is different from iron?
b) Can you change rust back into iron by some simple method?
c) Do you think formation of rust on iron is a chemical change?
d) Give two other examples of a similar type of change.
Answer:
a) Yes, rust is iron oxide (Fe₂O₃). Thus, rust and iron are not the same substance.

b) No, rusting of iron is a chemical change because in this reaction, a new substance, rust (iron oxide) is formed. It cannot be reversed by any simple method.

c) Yes, rusting of iron is a chemical change. During the rusting of iron, it combines with the oxygen in the presence of water (moisture) to form a new compound ‘iron oxide’. This iron oxide is a rust.
Iron + Oxygen + Water → Iron oxide (Rust)
It is a permanent change which cannot be reversed back.
So, rusting of iron is a chemical change.

d) Two other examples are:

• Setting of curd from milk.
• Burning of magnesium ribbon to form magnesium oxide.