ICSE Class 9 Chemistry Chapter 05 Physical and Chemical Changes

Physical And Chemical Changes

Scope Of Syllabus

Definitions and distinction between Physical and Chemical changes.

Simple experiments like dissolution of sugar in water, burning of paper should be shown to make the concepts of physical and chemical change clear. More examples of such type may be given.

Conditions for chemical change.

Close contact, heat, light, electricity, pressure, catalysts with examples.

Types of chemical change.

Direct combination; decomposition; displacement; double decomposition with examples.

Energy changes in a chemical change.

Exothermic and endothermic reactions with examples - evolution/absorption of heat, light and electricity.

Burning: Definition and conditions of burning.

Definition; conditions for burning (combustible substance, supporter of combustion and ignition temperature); comparison of respiration and burning; burning of magnesium or candle to show that substances gain weight on burning; students to be made aware of how the balance of O2 and CO2 is maintained in nature.

Important Points To Remember

1. The changes in energy are always accompanied by some kind of change (chemically or physically). During the reaction the old bonds in the reactants break and the new bonds in the products are formed. During this breaking and making of bonds there is a difference in energy. If the energy required to break the bonds is greater than the energy required to make the bonds, then the energy is given out to the surroundings. Such reactions or changes are exothermic. But if the energy required to break the bonds is less than the energy required to make the bonds, then the energy is absorbed from the surroundings. Such reactions are said to be endothermic reactions.

2. Many changes occur in our natural environment like evaporation of water, condensation of water vapours, rainfall, etc. In the above mentioned changes only the change in state is involved. Such changes are referred to as physical changes.

3. The physical change may be defined as the change which occurs in size, shape, colour, texture, state, magnetic or electrical condition but the molecular composition remains totally unaltered, i.e., no new product is formed as a result of physical change.

Teacher's Note

When ice melts into water, the molecules rearrange but the molecular composition stays the same - this happens in our freezer and kitchen every day.

The characteristics of the physical change are as follows:

(i) No new product is formed. The molecular composition remains totally unaltered. During the physical change only the arrangement of molecules gets altered leading to the change in state. For example,

Ice on melting gives Water

Solid becomes Liquid

The molecular compositions of ice and water remain same.

(ii) The change is temporary and reversible. If the cause producing the change is removed, then the reaction gets reversed. For example, ice on melting forms water and water on freezing produces ice.

(iii) No energy changes take place as a result of physical change. The energy required to bring about a physical change is generally equal to the amount of energy required to reverse the change. Therefore there is no change in energy.

(iv) The mass of the substance remains same during a physical change. There is no involvement of mass during physical change but only energy is added or removed. So, no matter is added or removed during a physical change and hence the mass of the substance remains the same.

4. Examples of physical changes are

(i) Melting of wax - (ii) Boiling of water

(iii) Heating of zinc oxide - (iv) Freezing of water

(v) Heating of camphor - (vi) Melting of ice

(vii) Heating of ammonium chloride - (viii) Dissolution of sugar in water

(ix) Dissolution of sulphur in carbon disulphide - (x) Cutting of wood.

5. A change during which the molecular composition gets totally altered, i.e. a change in which always a new product is formed is called a chemical change.

The characteristics of the chemical change are as follows:

(i) Chemical change results in the formation of a new product. Entirely a new product is formed with a complete difference in the molecular composition.

(ii) The change is permanent and irreversible. It means that the chemical change cannot be reversed by altering or changing the experimental conditions.

(iii) The mass of the substance gets altered during a chemical change. During a chemical change either the mass is added or removed. It results in either the increase in weight or decrease in weight of the substance.

(iv) Energy changes take place during a chemical change. There is a difference in energy in the breaking of old bonds in reactants and making of new bonds in products. Either the energy is released or absorbed during a chemical change.

If the energy is released then the reaction is exothermic and if the energy is absorbed then the reaction is endothermic.

For example, when paper is burnt, it forms carbon dioxide and water vapours along with energy in the form of heat and light.

6. Examples of chemical changes are

(i) Burning of candle - (ii) Burning of wood or paper

(iii) Souring of milk - (iv) Rancidification of butter

(v) Burning of camphor - (vi) Digestion of food

Teacher's Note

When we cook food or burn wood, completely new substances form with different properties - this is why cooked food tastes different and burnt wood becomes ash.

(vii) Rusting of iron - (viii) Photosynthesis

(ix) Electrolysis of water - (x) Combustion of fuel.

7. There are certain substances which undergo physical and chemical changes simultaneously. For example,

(i) Heating of zinc carbonate

\[\text{ZnCO}_3 \xrightarrow{\Delta} \text{ZnO} + \text{CO}_2\]

(Chemical change)

\[\text{ZnO} \xrightarrow{\Delta} \text{ZnO}\]

white - yellow

(Physical change)

(ii) Heating of sodium nitrate

\[2\text{NaNO}_3(s) \xrightarrow{\Delta} 2\text{NaNO}_3(l)\]

(Physical change)

\[2\text{NaNO}_3 \xrightarrow{\Delta} 2\text{NaNO}_2 + \text{O}_2\]

(Chemical change)

(iii) Heating of lead nitrate

\[2\text{Pb(NO}_3)_2 \xrightarrow{\Delta} 2\text{PbO} + 4\text{NO}_2 + \text{O}_2\]

(Chemical change)

\[\text{PbO} \xrightarrow{\Delta} \text{PbO}\]

light yellow - reddish

(Physical change)

9. Energy change always takes place during the chemical reaction. The energy is either released or absorbed during the chemical reaction.

10. The chemical reaction which proceeds by the liberation of heat energy is termed as exothermic reaction. During this reaction the heat is produced.

The process of burning of fuels, respiration in human beings are the examples of exothermic reactions.

\[\text{C} + \text{O}_2 \longrightarrow \text{CO}_2 + \text{Heat}\]

11. The chemical reaction which proceeds by the absorption of heat is termed as endothermic reaction. During this reaction heat is absorbed, this type of reaction will only continue in the presence of external source of heat.

Thermal decomposition of metallic carbonate, metallic bicarbonate, metallic hydroxide, metallic nitrate are the examples of endothermic reactions.

\[\text{CaCO}_3 + \text{Heat} \longrightarrow \text{CaO} + \text{CO}_2\]

\[2\text{NaHCO}_3 + \text{Heat} \longrightarrow \text{Na}_2\text{CO}_3 + \text{H}_2\text{O} + \text{CO}_2\]

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