Changes Around Us Physical and Chemical Class 7 Extra Questions and Answers Science Chapter 5

Students can keep Class 7 Science Extra Questions and Class 7th Curiosity Chapter 5 Changes Around Us Physical and Chemical Class 7 Important Extra Question Answer handy for quick reference during exams.

Class 7 Science Chapter 5 Changes Around Us Physical and Chemical Extra Questions

Class 7 Science Chapter 5 Extra Questions on Changes Around Us Physical and Chemical

Changes Around Us Physical and Chemical Class 7 Very Short Question Answer

Question 1.
What are the two main kinds of changes?
Answer:
There are two kinds of changes:
(i) Physical changes
(ii) Chemical changes.

Question 2.
If you cut off a paper into various pieces, is there any change in the property of the paper?
Answer:
No, there is no change in the chemical property of paper.

Question 3.
Melting of wax is a change where a solid changes to liquid state. Give one more such change which you observe in your surroundings. Answer:
Melting of ice cubes or melting of ice cream.

Question 4.
What do you mean by rusting?
Answer:
The process of formation of rust on iron is called rusting.

Question 5.
Name the gas released when baking soda is mixed with vinegar.
Answer:
Carbon dioxide

Question 6.
What happens when carbon dioxide is passed through lime water? Write the equation of the reaction.
Answer:
Lime water becomes milky due to formation of calcium carbonate.

Question 7.
Define combustion.
Answer:
A chemical reaction in which a substance reacts with oxygen and produces heat and/ or light is called combustion.

Question 8.
Define the term bioluminescence.
Answer:
Some insects like fireflies emit light in late evenings, which is produced by a chemical change. This type of light production (without heat) in living organisms is called bioluminescence.

Question 9.
What happens when we add beetroot extract to water?
Answer:
Colour of water changes to pink.

Question 10.
Melting of butter is reversible or irreversible change?
Answer:
Reversible change

Changes Around Us Physical and Chemical Class 7 Short Question Answer

Question 1.
What are physical properties?
Answer:
The properties such as shape, size, colour and state of a substance, that can be observed are called physical properties.

Question 2.
What are physical changes?
Answer:
The changes in which a substance undergoes a change in its physical properties are called physical changes.
Physical changes are generally reversible. In such changes, no new substances are formed. For example, melting of ice, formation of steam, etc.

Question 3.
What is chemical change?
Answer:
A change in which one or more new substances are formed is called chemical change. A chemical change is caused by a chemical reaction. For example, formation of curd from milk, burning of paper, etc.

Question 4.
Explain the activity of burning of magnesium ribbon. Represent the change by an equation. Ans. Get a small and thin strip or ribbon of magnesium. Clean its tip with sand paper. Bring the tip near a candle flame. It burns with a brilliant white light. After combustion white powdery ash is left, it is called magnesium oxide.

Question 5.
Explain the following:
(a) Lime water turns milky on passing carbon dioxide gas into it.
(b) Bubbles are produced when acetic acid is added to a solution of sodium hydrogen carbonate.
Answer:
(a) On passing carbon dioxide gas through lime water, the lime water turns milky due to the formation of a white coloured insoluble calcium carbonate.
(b) Bubbles are produced when acetic acid is added to a solution of sodium hydrogen carbonate due to the formation of carbon dioxide gas.

Question 6.
Write the characteristics of a chemical change.
Answer:
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.

Question 7.
List conditions under which combustion can take place.
Answer:
Conditions necessary for combustion are:

1. Presence of a combustible substance.
2. Attainment of ignition temperature.
3. Proper supply of air to provide oxygen.

Question 8.
What are combustible substances? Give two examples.
Answer:
The substances that undergo combustion reaction are called combustible substances. For example, paper, kerosene.

Question 9.
The gas we use in the 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 bums (Change-B). Identify change A and change B.
Answer:
(i) Change-A is a physical change.
(ii) Change-B is a chemical change.

Changes Around Us Physical and Chemical Class 7 Long Question Answer

Question 1.
Which gas is released when vinegar and baking soda are mixed? What happens when this gas is passed through lime water?
Answer:
When baking soda and vinegar are mixed with each other then carbon dioxide gas is released. When carbon dioxide gas 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:

Question 2.
If you leave a piece of iron 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 from iron is a chemical change?
(d) Give two other examples of a similar type of change.
Answer
(a) Yes, it is quite different from iron. Rust is formed due to the oxidation of iron in the presence of moist air.
(b) No, this is an irreversible chemical change.
(c) Yes, it is a chemical change because rust is a new substance formed when iron undergoes chemical reaction with the water particles and oxygen present in air and this change cannot be reversed back.
(d) Cooking of food and curdling of milk are other examples of chemical reaction that cannot be reversed.

Question 3.
Give two examples for each of the following cases:
(a) Physical changes which are reversible.
(b) Physical changes which are not reversible.
(c) Chemical changes.
Answer:
(a) Dissolving sugar in water, folding of paper
(b) Tearing of paper, cutting of wood
(c) Cooking of food, burning of wood

Question 4.
Explain weathering of rocks.
Answer:
Weathering is the process by which rocks are broken down into smaller pieces, eventually forming soil. It occurs through both physical and chemical changes. Physical weathering occurs when large rocks break into smaller pieces without changing their chemical composition. This can be caused by change in temperature due to climate, the growth of tree roots, or the freezing of water within cracks in the rocks. Chemical weathering happens when water or chemicals in the water react with the minerals in rocks, changing their composition. For example, a black coloured rock called basalt, which contains iron, undergoes a chemical change. When exposed to water or moist air for a long time, the iron reacts with oxygen to form iron oxide, which gives the rock a red colour.

Question 5.
Select the chemical or physical changes which are reversible by putting reverse arrow in the following boxes:

Answer:

Changes Around Us Physical and Chemical Class 7 Skill Based Questions

Question 1.
Observe the following figure and answer the following questions:

(i) What does this figure show?
Answer:
This figure shows the action of an acid over sodium carbonate.

(ii) Name the gas released.
Answer:
The gas released is carbon dioxide.

(iii) Write the effect of the gas on lime water.
Answer:
The lime water becomes milky.

(iv) Write the chemical equation of the reaction.
Answer:

Question 2.
Is air essential for combustion? Explain with the help of an activity.
Answer:
Take a candle, light it and fix it on a table. Allow the candle to burn steadily. Now, carefully cover the burning candle with a glass jar. We will observe that after some time, the flame flickers and eventually goes out. This occurs because the oxygen inside the jar is consumed by the burning candle, and without a continuous supply of oxygen from the air, combustion cannot be sustained. This experiment shows that air, specifically oxygen, is essential for combustion.

Fig. Candle, (a) burning (b) covered with a glass tumbler

Changes Around Us Physical and Chemical Class 7 Case Based Questions

I. A cleaned magnesium ribbon about 3-4 cm long is burnt using a burner by holding it with a pair of tongs and the ash so formed is collected in a watch-glass. The magnesium ribbon is kept away as far as possible from eyes.
Question 1.
What is the colour of the powder collected in a watch glass ?
Answer:
White

Question 2.
Write the changes that determines the occurrence of chemical reaction.
Answer:
Change in state, change in temperature or change in colour determines the occurrence of a chemical reaction.

Question 3.
What is the chemical name of the powder collected in a watch glass ?
Answer:
Magnesium oxide (MgO)
OR
Which type of change occurs in the above experiment?
Answer:
Chemical change

Changes Around Us Physical and Chemical Class 7 Suggested Projects and Activities

1. Start by collecting chalk dust from the floor near the chalkboard. If needed, you can also crush a small piece of chalk to make dust. Add a small amount of water to the dust to form a paste. Mould the paste into the shape of a chalk stick and let it dry. Were you able to make chalk from the dust?

2. Watch the activities in the kitchen and identify any changes that can be reversed. Can you determine whether these are physical or chemical changes?

Class 7 Changes Around Us Physical and Chemical Extra Questions for Practice

Question 1.
The substances which have very low ignition temperature will
(a) catch fire easily
(b) will not catch fire
(c) catch fire after some time
(d) none of these.

Question 2.
Which of the following is rust?
(a) lime water
(b) iron oxide
(c) vinegar
(d) baking soda

Question 3.
Formation of manure from leaves is a
(a) physical change
(b) chemical change
(c) both physical and chemical change
(d) none of these

Question 4.
Read the given statements and select the correct option.
Assertion (a): Combustion is a physical change.
Reason (R): A new substance is formed during this process.
(a) Both A and R are true and R is the correct explanation of A.
(b) Both A and R are true but R is not the correct explanation of A
(c) A is true but R is false.
(d) A is false but R is true.

II. Very Short Answer Type Questions (1 mark each)

Question 6.
What is rust?

Question 7.
Give two examples of physical changes.

Question 8.
Making popcorns from corns is what type of change?

Question 9.
Why lime water turns milky on passing carbon dioxide gas to it?

Question 10.
Define ignition temperature.

III. Short Answer Type-1 Questions (2 marks each)

Question 11.
When baking soda is mixed with lemon juice, bubbles are formed with the evolution of a gas. What type of change is it? Explain

Question 12.
Burning of candle involves both physical and chemical change. Justify this statement.

Question 13.
Define the term ‘bioluminescence’. Give one example.

Question 14.
Explain why burning of wood and cutting it into small pieces are considered as two different types of changes.

Question 15.
Classify the following changes into physical change, chemical change, reversible change or irreversible change.
(i) Folding of paper to make a boat.
(ii) Cooking chapati from wheat flour.

IV. Short Answer Type-ii Questions (3 marks each)

Question 16.
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

Question 17.
Setting of curd from milk is a chemical change. Explain.

Question 18.
Give two examples each of natural slow change and desirable change.

Question 19.
Burning of crackers is a chemical change. Justify the given statement.

Question 20.
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.

V. Long Answer Type Questions (5 marks each)

Question 21.
Explain the process of weathering of rocks and how it involves both physical and chemical changes.

Question 22.
Define combustion. What are the prerequisite conditions for the combustion reaction to occur.

Question 23.
Most of the physical changes are reversible. Give some examples with reasons.

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