Exercise
Question 1
Define:
(a) a physical change
(b) a chemical change
Answer
(a) A physical change is a change in which no new substance is formed and the chemical composition of the original substance remains the same, even though some of its physical properties like state, color, shape, size, appearance etc. may change.
(b) A chemical change is a permanent change in which new substances are formed whose chemical composition and physical and chemical properties are different from those of the original substance.
Question 2
Classify the following as a physical or a chemical change:
(a) Drying of wet clothes.
(b) Manufacture of salt from sea water.
(c) Butter getting rancid.
(d) Boiling of water
(e) Burning of paper
(f) Melting of wax
(g) Burning of coal
(h) Formation of clouds
(i) Making of a sugar solution
(j) Glowing of an electric bulb
(k) Curdling of milk
(l) Rusting of iron
(m) Roasting of potatoes
(n) Formation of alloys
Answer
(a) Drying of wet clothes — Physical change
(b) Manufacture of salt from sea water — Physical change
(c) Butter getting rancid — Chemical change
(d) Boiling of water — Physical change
(e) Burning of paper — Chemical change
(f) Melting of wax — Physical change
(g) Burning of coal — Chemical change
(h) Formation of clouds — Physical change
(i) Making of a sugar solution — Physical change
(j) Glowing of an electric bulb — Physical change
(k) Curdling of milk — Chemical change
(l) Rusting of iron — Chemical change
(m) Roasting of potatoes — Chemical change
(n) Formation of alloys — Physical change
Question 3
Fill in the blanks:
(a) The process of a liquid changing into a solid is called ............... .
(b) A change which alters the composition of a substance is known as a ............... change.
(c) There is no change in the ............... of the substance during a physical change.
(d) The reaction in which energy is evolved is called ............... .
Answer
(a) Freezing
(b) Chemical
(c) composition
(d) exothermic reaction
Question 4
Give reasons:
(a) Freezing of water to ice and evaporation of water are physical changes.
(b) Burning of a candle is both a physical and a chemical change.
(c) Burning of paper is a chemical change.
(d) Cutting of a cloth piece is a physical change, though it cannot be reversed.
Answer
(a) Freezing of water to ice and evaporation of water are physical changes because the changes are reversible and the chemical composition of the substance remains the same. No new substance if formed. Only change of state occurs i.e. water (liquid) is converted to ice (solid) in freezing and in evaporation, water (liquid) is changed to vapour (gaseous) state.
(b) When a candle is lighted it produces carbon dioxide and water vapour as the new substances and along with it there is evolution of heat and light. This shows a chemical change. When a candle burns wax melts that falls on floor and again it solidifies which shows a physical change. So burning of a candle is both a physical and a chemical change.
(c) When a piece of paper is burnt a new substance ash is produced which cannot be changed back into paper. This change is permanent and irreversible, so it is a chemical change.
(d) Cutting a cloth piece does not produce any new substances. There is no change in the chemical composition of the cloth piece after cutting. Hence, even though it is irreversible, still it is a physical change.
Question 5
Give four differences between physical and chemical changes.
Answer
| Physical Change | Chemical Change |
|---|---|
| In a physical change, no new substance is formed and the chemical composition of the substance remains the same. These are changes only in physical properties and state. | In a chemical change a new substance with entirely different chemical composition and properties is formed. |
| The change can be reversible or irreversible. | The change is permanent. |
| The change can be reversed by simple physical methods. | The change cannot be reversed by simple physical methods. |
| Heat or light energy may or may not be released or absorbed. | Heat or light energy are given out or absorbed. |