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Is Matter Around Us Pure - Physical and Chemical Changes

Grade 9CBSE

Review the key concepts, formulae, and examples before starting your quiz.

🔑Concepts

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A Physical Change is a temporary change where no new substance is formed. The chemical composition of the matter remains the same, although physical properties like state, shape, or size may change. Most physical changes are reversible, such as the interconversion of states: H2O(s)ightleftharpoonsH2O(l)H_{2}O(s) ightleftharpoons H_{2}O(l).

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A Chemical Change is a permanent change in which one or more new substances with entirely different properties are formed. This involves a chemical reaction where old bonds are broken and new bonds are formed. For example, the burning of magnesium ribbon: 2Mg+O2ightarrow2MgO2Mg + O_{2} ightarrow 2MgO.

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Key characteristics of Chemical Changes include the evolution of gas, change in color, formation of a precipitate, or change in temperature (absorption or release of energy in the form of heat or light).

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The Burning of a Candle is a unique example involving both physical and chemical changes. The melting of wax is a physical change (Wax(s)ightarrowWax(l)Wax(s) ightarrow Wax(l)), while the burning of wax vapor to produce CO2CO_{2} and H2OH_{2}O is a chemical change.

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Physical properties include color, hardness, density, melting point (M.P.M.P.), and boiling point (B.P.B.P.). Chemical properties include reactivity, inflammability, and toxicity.

📐Formulae

Physical Change (Interconversion of States): Solid⇌freezemeltLiquid⇌condenseevaporateGas\text{Physical Change (Interconversion of States): } \text{Solid} \xrightleftharpoons[\text{freeze}]{\text{melt}} \text{Liquid} \xrightleftharpoons[\text{condense}]{\text{evaporate}} \text{Gas}

Rusting of Iron: 4Fe+3O2+xH2O→2Fe2O3⋅xH2O (Hydrated Iron (III) Oxide)\text{Rusting of Iron: } 4Fe + 3O_{2} + xH_{2}O \rightarrow 2Fe_{2}O_{3} \cdot xH_{2}O \text{ (Hydrated Iron (III) Oxide)}

Photosynthesis (Chemical Change): 6CO2+6H2O→Chlorophyll/SunlightC6H12O6+6O2\text{Photosynthesis (Chemical Change): } 6CO_{2} + 6H_{2}O \xrightarrow{\text{Chlorophyll/Sunlight}} C_{6}H_{12}O_{6} + 6O_{2}

Burning of Methane: CH4+2O2→CO2+2H2O+Heat\text{Burning of Methane: } CH_{4} + 2O_{2} \rightarrow CO_{2} + 2H_{2}O + \text{Heat}

💡Examples

Problem 1:

Classify the following as physical or chemical changes: (a) Cutting of trees, (b) Melting of butter in a pan, (c) Rusting of almirah, (d) Passing electric current through water.

Solution:

(a) Physical Change, (b) Physical Change, (c) Chemical Change, (d) Chemical Change.

Explanation:

Cutting trees is physical because the wood's composition doesn't change. Melting butter is a change of state (solid to liquid). Rusting forms a new chemical compound (Fe2O3Fe_{2}O_{3}). Passing electricity through H2OH_{2}O decomposes it into H2H_{2} and O2O_{2} gases, which is a chemical reaction.

Problem 2:

During the burning of a candle, both physical and chemical changes take place. Identify these changes.

Solution:

Physical Change: Melting of wax; Chemical Change: Combustion of wax.

Explanation:

When the candle burns, some wax melts and solidifies again without changing its chemical nature (Waxsolid→WaxliquidWax_{solid} \rightarrow Wax_{liquid}). Simultaneously, the wax vapor reacts with O2O_{2} to form CO2CO_{2} and H2OH_{2}O vapor, releasing heat and light, which cannot be reversed.