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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 (→\rightarrow).

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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+B→C+DA + B \rightarrow 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.