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Chemical Changes and Reactions - Physical vs Chemical Changes

Grade 7ICSE

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 remains the same, and it is usually reversible. Examples include the melting of ice (H2OH_2O) or the dissolving of salt (NaClNaCl) in water.

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A Chemical Change is a permanent change where one or more new substances with different properties are formed. It is usually irreversible. Example: Burning of Magnesium (MgMg) in air.

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Characteristics of Chemical Changes: These involve the breaking and making of chemical bonds, change in energy (evolution or absorption of heat/light), change in color, and evolution of gas (like CO2CO_2 or H2H_2).

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Reactants and Products: In a chemical reaction, the substances that take part are called reactants (written on the left), and the new substances formed are called products (written on the right), separated by an arrow (ightarrow ightarrow).

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Law of Conservation of Mass: During a chemical reaction, the total mass of the reactants is always equal to the total mass of the products. For example, in the reaction A+BightarrowC+DA + B ightarrow C + D, Mass(A)+Mass(B)=Mass(C)+Mass(D)Mass(A) + Mass(B) = Mass(C) + Mass(D).

📐Formulae

2Mg+O2→heat2MgO2Mg + O_2 \xrightarrow{\text{heat}} 2MgO

Fe+S→ΔFeSFe + S \xrightarrow{\Delta} FeS

CuSO4+Fe→FeSO4+CuCuSO_4 + Fe \rightarrow FeSO_4 + Cu

Zn+H2SO4→ZnSO4+H2↑Zn + H_2SO_4 \rightarrow ZnSO_4 + H_2 \uparrow

CaCO3→heatCaO+CO2↑CaCO_3 \xrightarrow{\text{heat}} CaO + CO_2 \uparrow

💡Examples

Problem 1:

Classify the process of 'Heating a Platinum wire' and 'Heating a Magnesium wire' as physical or chemical changes.

Solution:

Heating Platinum wire is a Physical Change; Heating Magnesium wire is a Chemical Change.

Explanation:

Platinum is a noble metal and does not react with oxygen on heating; it simply glows and returns to its original state on cooling. However, Magnesium reacts with oxygen to form a new substance, Magnesium Oxide (2Mg+O2→2MgO2Mg + O_2 \rightarrow 2MgO), which is a white powder.

Problem 2:

What happens when an iron nail is placed in a solution of Copper Sulphate (CuSO4CuSO_4)? State the type of change.

Solution:

Fe+CuSO4→FeSO4+CuFe + CuSO_4 \rightarrow FeSO_4 + Cu. This is a Chemical Change.

Explanation:

Iron (FeFe) is more reactive than Copper (CuCu). It displaces copper from the blue CuSO4CuSO_4 solution to form a green solution of Ferrous Sulphate (FeSO4FeSO_4). A reddish-brown coating of copper is deposited on the nail.

Problem 3:

Is the 'Souring of Milk' a physical or chemical change? Justify.

Solution:

Chemical Change.

Explanation:

Souring of milk results in the formation of a new substance (Lactic acid) due to bacterial action. The process is irreversible, and the original milk cannot be recovered.