Question 1

Question 5A burning candle shows both physical and chemical changes. Explain.

Accepted Answer

When a candle is lighted it, the carbon present in wax oxidises and produces carbon dioxide and water vapours as the new substances and along with it there is evolution of heat and light. This shows a chemical change. The candle also becomes smaller and smaller. When a candle burns wax melts and turns into liquid state 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.

Question 2

Question 6State three differences between evaporation and boiling.

Accepted Answer

EvaporationBoilingEvaporation is a slow process.Boiling is a fast process.It takes place from the surface of the liquid.Evaporation takes place from all parts of the liquid during boiling.It takes place at all temperatures below its boiling point.It takes place at a fixed temperature i.e. at its boiling point.

Question 3

Question 7State four differences between physical and chemical changes.

Accepted Answer

Physical changesChemical changesChange is temporary.Change is permanent.No new substance is formed. There is change only in physical properties.New substances are formed with entirely different properties.Change can be reversed by simple methods.Change cannot be reversed by simple methods.Heat or light may or may not be given out or consumed.Heat or light or both are given out or consumed.

Question 4

Question 8What do you observe when:(a) Water is boiled(b) a piece of paper is burnt(c) some ice cubes are kept in a glass tumbler(d) solid ammonium chloride is heated(e) an iron nail is kept in tap water for few days(f) a spoon of sugar is heated in a pan(g) a lighted match stick is brought near the mouth of the test tube containing hydrogen gas(h) quick lime is dissolved in water(i) little amount of curd is added to a bowl containing warm milk and kept for five hours?

Accepted Answer

(a) When water is boiled it converts into vapours.(b) When a piece of paper is burnt, it changes into ash.(c) When some ice cubes are kept in a glass tumbler, water droplets are seen on the outer wall of the tumbler and after sometime ice cubes changes into water.(d) When solid ammonium chloride is heated, it directly changes into vapours.(e) When an iron nail is kept in tap water for few days a reddish brown coating is seen on the nail.(f) When a spoon of sugar is heated in a pan it first melts and changes its colour to reddish-brown and finally it turns black and gets charred.(g) When a lighted match stick is brought near the mouth of the test tube containing hydrogen gas a pop sound is heard and the flame of the match stick gets extinguished.(h) When quick lime is dissolved in water a large amount of heat energy is evolved along with a hissing sound is produced.(i) The bowl containing milk changes into curd.

Question 5

Question 1Change is the law of nature and occurs in our everyday life, at all times and in all places. Differentiate between the following changes with a suitable example.(a) Desirable and undesirable change(b) Periodic and non-periodic change(c) Slow and fast change(d) Natural and man-made change(e) Reversible and irreversible change

Accepted Answer

(a) Difference between desirable and undesirable change :Desirable changeUndesirable changeDesirable changes produce good results.Undesirable changes are unfavorable.For example, rain bringing water to crops is a desirable change.For example, rain causing destruction by floods is an undesirable change.(b) Difference between periodic and non-periodic change :Periodic changeNon - Periodic changeChanges which occur periodically at regular intervals of time.Changes which do not occur periodically [do not repeat] at regular intervals of time.For example, change of seasons is a periodic changeFor example, change of weather is a non-periodic change.(c) Difference between slow and fast changeSlow changeFast changeChange which occur very slowly over days, months and years.Change which occur very rapidly in seconds or minutes.For example, the growth of a tree from a seedFor example, occurring of lightning.(d) Difference between natural and man-made changeNatural changeMan-made changeChanges brought about by nature.Changes brought by manFor example, growth in man and plants.For example, conversion of iron to steel.(e) Difference between reversible and irreversible changeReversible changeIrreversible changeChanges which are temporary and can be reversed by change in conditions.Changes which are permanent and cannot be reversed by change in conditions.For example, melting of ice and freezing of water.For example, burning of paper.

Question 6

Question 3Give three reasons why melting of wax, is considered a physical change while burning of a candle, a chemical change.

Accepted Answer

Melting of wax is considered a physical change, whereas burning of candle is a chemical change because of the following reasons:(a) Melting of wax is a temporary and reversible change whereas, burning a candle is a permanent and irreversible change.(b) During melting of wax, heat energy is absorbed and on cooling it is evolved forming the original substance i.e., wax. Hence, no marked evolution or absorption of heat takes place. So, it is a physical change.However, during the burning of candle, the wax decomposes. The products formed cannot combine back to give wax. Hence, it is a chemical change.(c) During melting of wax no new substance is formed. The original substance is wax and the final product is also wax. Hence, it is a physical change.During burning of candle new substances are formed. The original substance is the burning candle. The final products are carbon dioxide and water vapour. Hence, it is a chemical change.

Question 7

Question 9State why addition of sodium chloride to water is considered a physical change, while addition of dilute sulphuric acid to zinc is considered a chemical change.

Accepted Answer

Addition of sodium chloride (salt) to water is considered a physical change because:(a) It is a temporary and reversible change. On adding, the salt dissolves in water and forms a salt solution. On evaporation of the salt solution the original salt remains behind. Hence, it is a reversible change.(b) No new substances are formed. Original and the final product is the same [NaCl].Addition of dilute sulphuric acid to zinc is a chemical change because :Zinc + dil. Sulphuric Acid ⟶ Zinc Sulphate + Hydrogen gas(a) It is a permanent and irreversible change.(b) New substances zinc sulphate and hydrogen gas are formed.

Question 8

Question 10Photosynthesis is a natural process by which green plants manufacture food in the presence of sunlight.(a) Are any new products formed during the above process?(b) Can the change be reversed or is irreversible?(c) State the conclusions which can be drawn, to represent photosynthesis as a physical or a chemical change.

Accepted Answer

(a) Glucose and oxygen are formed during photosynthesis. Hence, new products are formed.(b) Carbon dioxide and water cannot be obtained back, from glucose, hence, the change is irreversible.(c) Photosynthesis is a chemical change as it is irreversible and leads to the formation of new products.

Question 9

Question 11Give reasons why - separation of mixtures e.g., iron from a mixture of iron and sulphur is a physical change, but heating a mixture of iron and sulphur is considered a chemical change.

Accepted Answer

As separation of iron and sulphur from its mixture is areversible process,no new products are formed andmixture has the same properties as iron and sulphur individually have. Hence, it is a physical change.However, on heating of iron and sulphur,a new product iron sulphide is formedthis new product [iron sulphide] has completely different properties from iron and sulphur.it is an irreversible process. Hence, it is a chemical change.

Question 10

Question 12Explain in brief the involvement of energy in - physical & chemical changes.

Accepted Answer

Physical changes use energy to change the state of matter.Chemical changes release or absorb energy when changing a substance into a new substance.Energy is released during decomposition of a substance, whileEnergy is absorbed during formation of a new substance.

Question 11

Question 2State whether the following are physical or chemical changes.(a) glowing of a bulb(b) burning of sugar(c) heating of water(d) growing of a piglet into a pig(e) burning of wood(f) passing electric current through a heater's rod(g) water cycle in nature(h) respiration in living beings(i) shaping a piece of glass(j) lightning(k) dissolving sugar in water(l) heating a mixture of iron fillings and sulphur(m) mixing oil with water(n) cutting wood into small pieces(o) photosynthesis(p) Digestion of food(q) Melting of wax(r) Boiling of an egg(s) Slaking of lime

Accepted Answer

(a) glowing of a bulb — Physical change(b) burning of sugar — Chemical change(c) heating of water — Physical change(d) growing of a piglet into a pig — Chemical change(e) burning of wood — Chemical change(f) passing electric current through a heater's rod — Physical change(g) water cycle in nature — Physical change(h) respiration in living beings — Chemical change(i) shaping a piece of glass — Physical change(j) lightning — Chemical change(k) dissolving sugar in water — Physical change(l) heating a mixture of iron fillings and sulphur — Chemical change(m) mixing oil with water — Physical change(n) cutting wood into small pieces — Physical change(o) photosynthesis — Chemical change(p) Digestion of food — Chemical change(q) Melting of wax — Physical change(r) Boiling of an egg — Chemical change(s) Slaking of lime — Chemical change

Question 12

Question 4Fill in the blanks with the correct words for each of the statements given below:Sublimation is ............... [considered/not considered] a change of state of matter.Sublimation involves a ............... [chemical/physical] change.During sublimation a solid on heating turns directly into ............... [liquid/vapour] & on cooling back to ............... [solid/liquid].Ammonium chloride, a sublimable solid undergoes ............... [synthesis/dissociation] on heating.Sublimation of iodine is an example of ............... [physical/chemical] change.

Accepted Answer

Sublimation is considered a change of state of matter.Sublimation involves a physical change.During sublimation a solid on heating turns directly into vapour & on cooling back to solid.Ammonium chloride, a sublimable solid undergoes dissociation on heating.Sublimation of iodine is an example of physical change.

Question 13

Question 5iWhen we add water to the following substances, which one will show a chemical change?saltsugaroilquick lime

Accepted Answer

quick limeReason — Quick lime reacts with water to form calcium hydroxide and a large amount of heat is produced which is a chemical change.

Physical and Chemical Changes

Exercise

Introduction

Objective Type Questions