Question 1

Question 2Classify 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

Accepted 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 2

Question 4Give 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.

Accepted 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 3

Question 5Give four differences between physical and chemical changes.

Accepted Answer

Physical ChangeChemical ChangeIn 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.

Question 4

Question 1Select the correct answer from the choice in bracket to complete each sentence:A change in which a substance retains its identity is a ............... [physical/chemical] change.Matter is added or removed during a ............... [chemical/physical] change.Energy required for completion of a physical change is ............... [released/absorbed] when the change is reversed.Dissolution of lead nitrate in water is deemed as a ............... [physical/chemical] change.A change in which matter undergoes a change but the total mass of substance is unaltered is a ............... [physical/chemical] change.

Accepted Answer

A change in which a substance retains its identity is a physical change.Matter is added or removed during a chemical change.Energy required for completion of a physical change is released when the change is reversed.Dissolution of lead nitrate in water is deemed as a physical change.A change in which matter undergoes a change but the total mass of substance is unaltered is a physical change.

Question 5

Question 3Give reasons why the following are considered as chemical changes.Copper carbonate on heat gives copper oxide and carbon dioxide.A bright light is seen evolved when a strip of magnesium is heated.Sulphur when burnt in air evolves a gaseous acidic product.An iron nail kept open in the atmosphere rusts.A piece of magnesium strip is dropped into a beaker containing dilute hydrochloric acid.

Accepted Answer

Copper carbonate on heating gives copper oxide and carbon dioxide. As new products are formed and the change is irreversible hence it is a chemical change.When a strip of magnesium is heated, magnesium oxide is produced and the bright light seen is the evolution of energy in the form of light. Hence, it is considered a chemical change.Sulphur burns in air to produce a new gaseous acidic product [sulphur dioxide]. Hence, the change is a chemical change.Iron nail kept in air forms rust [iron [III] oxide]. Iron [III] oxide is a new product and has different chemical composition and properties as compared to that of an iron nail. Hence, the reaction is a chemical change.When a piece of magnesium strip is dropped into a beaker containing dilute hydrochloric acid, magnesium chloride is formed and hydrogen gas is liberated. As new products are formed and the change is irreversible hence it is a chemical change.

Physical and Chemical Changes

Exercise

Objective Type Questions