Q 1 – Which among the following is a physical change?
(a) Cutting a log of wood in small pieces
(b) Burning of wood
(c) Ripening of fruit
(d) Cooking of food
(a) Cutting a log of wood in small pieces
Q 2 – Which of the following is a chemical change?
(a) Bursting of a fire cracker
(b) Germination of seed
(c) Coal formation from buried trees
(d) All of these
(d) All of these
Q 3 – Which is a method to prevent rust?
(a) Crystallization
(b) Sedimentation
(c) Galvanisation
(d) None of these
(c) Galvanisation
Q 4 – What is the formula of rust?
(a) Fe2O3
(b) Fe3O4
(c) FeO
(d) None of these
(a) Fe2O3
Q 5 – Iron pillar near the Qutub Minar in Delhi is famous for the following facts. Which of these facts is responsible for its long stability?
(a) It is more than 7 metres high
(b) It weighs about 6000 kg
(c) It was built more than 1600 years ago
(d) It has not rusted after such a long period
(d) It has not rusted after such a long period
Q 6 – Paheli’s mother made a concentrated sugar syrup by dissolving sugar in hot water. On cooling, crystals of sugar got separated. This indicates a:
(a) physical change that can be reversed
(b) chemical change that can be reversed
(c) physical change that cannot be reversed
(d) chemical change that cannot be reversed
(a) physical change that can be reversed
Q 7 – Galvanisation is a process used to prevent the rusting of which of the following?
(a) Iron
(b) Zinc
(c) Aluminium
(d) Copper
(a) Iron
Q 8 – Match the following:
Column A | Column B |
(i) Rust | (а) Calcium hydroxide (Ca(OH) 2) |
(ii) Vinegar | (b) Ferric oxide (Fe2O3) |
(iii) Baking soda | (c) Copper sulphate (CuSO4) |
(iv) Lime water | (d) Acetic acid (CH3COOH) |
(v) Neela thotha | (e) Sodium hydrogen carbonate (NaHCO3) |
Column A | Column B |
(i) Rust | (b) Ferric oxide (Fe2O3) |
(ii) Vinegar | (d) Acetic acid (CH3COOH) |
(iii) Baking soda | (e) Sodium hydrogen carbonate (NaHCO3) |
(iv) Lime water | (а) Calcium hydroxide (Ca(OH) 2) |
(v) Neela thotha | (c) Copper sulphate (CuSO4) |
Q 9 – Fill in the blanks:
- Changes that lead to the formation of new substances are called ………………….
- Melting of candle wax is a change but its burning is a …………………. change.
- Energy is …………………. in the formation of curd from milk.
- A brown layer formed when an iron article is left exposed in an open area is called ………………….
- The brown layer deposited on an iron nail dipped in copper sulphate solution is due to ………………….
- chemical changes
- physical, chemical
- evolved
- rust
- copper metal
Q 10 – Choose the true and false statements from the following: [6]
- The cooking of rice is a physical change.
- When you cut an apple with knife interactions between the apple and the knife take place.
- When catechu (kaththa) is applied on lime while making a pan (bettle) a chemical change takes place.
- Eruption of volcano is a reversible change.
- Formation of clouds is a reversible change.
- Both oxygen and water are essential for rusting.
- False
- True
- True
- False
- True
- True
Q 11 – Write an example of physical changefor the each of the following:
(a) Change in size
(b) Change in shape
(c) Change in state of a substance
1.) ice melting
2.) water vapourization
b) change of shape
1. crushing a can
2.chopping wood
c) change of state of a substance
1.Rubbing alcohol left uncovered will evaporate quickly into the air. It turns to a gas but maintains the properties of alcohol.
2. Hot molten iron is a liquid and when left to cool will from solid steel and can take many forms.
Q 12 – Name the metals and non-metal which are alloyed together to make stainless steel.
Stainless steel is an alloy formed by mixing iron with nickel, chromium and carbon.
Q 13 – Explain the following:
(a) Explosion of the firecrackers is a chemical change.
(b) Neutralization reaction is a chemical change.
The explosion of a firecracker is a chemical change. When we crack open a firecracker, which we should not do at home, we would see that it is a mixture of substances, and may contain metal fillings. Now, the combustion of these substances produces the distinct color and sound.
Q 14 –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. When a candle burns, both physical and chemical changes occur
(i) Physical change: melting of wax, vapourisation of melted wax.
(ii) Chemical change: Burning of vapours of wax to give carbon dioxide, heat and light.
LPG is another example in which physical change occurs when LPG comes out of cylinder and is converted from liquid to gaseous state and a chemical change occurs when gas burns in air.
Q 15 – Why is the burning of a candle considered a chemical change?
Candles are made of wax and a long thread of cotton (called wick of the candle). While candle is burnt, the molten wax goes up through the thread and undergoes combustion to form carbon dioxide and water vapour. The ‘wick’ of the candle gets changed to a black mass.
Over the process, heat and light energy is given out. It is not possible to
(i) recover the burnt wax again,
(ii) recover the thread again.
Hence, the burning of a candle is a chemical change.
Q 16 – State the differences between chemical and physical changes.
Chemical change | Physical change |
(i) It is generally a irreversible process. | (i) It is generally a reversible process. |
(ii) A new substance is formed. | (ii) No new substance is formed. |
(iii) Properties of a substance changes. | (iii) Properties of a substance does not changes. |
(iv) Energy is given out or absorbed during the change. | (iv) No energy is given out or absorbed during the change. |
Q 17 – Most of the physical changes are reversible but some are zirreversible. Explain the statement with examples
. In physical change, a substance undergoes a change in its physical properties only. Physical change occurs when there is a force applied, change in temperature, etc., on a substance. Most of the physical changes are reversible like rolling a dough into a chapati and then again bringing back into a dough.
Making a toy aeroplane by folding the paper and then again unfolding the toy aeroplane to recover the page. But when a physical change cannot be reversed or we cannot bring the substance back to its original shape, size, state, etc., is known as irreversible change. For example, baking chapati or making a toy aeroplane by cutting the paper instead of folding it.
Q 18 – Why lime water turns milky on passing carbon dioxide gas into it?
Chemical formula of lime water is calcium hydroxide which is a colourless solution. When we pass carbon dioxide gas into lime water, it forms white coloured insoluble calcium carbonate.
Lime water + Carbon dioxide ➝ Calcium carbonate + Water
Q 19 – Sarita while helping her mother in kitchen work noticed that the slices of potato and brinjal have acquired a brown colour. She asked her mother reason behind this change. Her mother said whenever we keep cut pieces of these vegetables open for long time they acquire such colour.
(a) Are cutting vegetables and change in colour, physical or chemical changes?
(b) Why vegetables acquired brown colour?
(c) Do this brown colouration indicate that the vegetables are spoilt. Can we reverse this process? Why?
(d) Can you suggest ways to avoid this colouration of vegetables?
(e) What value of Sarita is shown here?
(a) Cutting vegetables is physical change and change in colour of vegetables is chemical change.
(b) Vegetables acquire brown colour due to chemical reaction and thus, formation of new substance.
(c) No, vegetables are not spoilt and we cannot reverse this process because it is a chemical change.
(d) 1. By not keeping cut vegetables for long time to exposed air.
2. By dipping cut vegetables in cold water.
(e) Sarita is helping in nature, thoughtful and inquisitive.
Q 20 – When we keep a piece of iron in the open area for few days, a brownish, flaky substance, called rust, is deposited on it.
(a) Is rust different from iron?
(b) Is formation of rust from iron a chemical change?
(c) Can you change rust back into iron by some simple method?
(c) Give some other examples of a similar type of changes.
(a) Yes. Rust is oxide of iron.
(b) Yes, it is a chemical change as properties of iron has changed.
(c) No.
(d) 1. Setting of curd into milk.
2. Burning of magnesium ribbon in air.
3. Cooking of food.