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The Amazing World of Solutes, Solvents, and Solutions - Physical and Chemical Changes

Grade 7CBSE

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

🔑Concepts

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A Solution is a homogeneous mixture of two or more substances. It consists of a Solute (the substance that is dissolved) and a Solvent (the medium in which the solute is dissolved). For example, in a salt solution, salt is the solute and H2OH_2O is the solvent.

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A Physical Change is a change in the physical properties of a substance such as shape, size, or state. No new substance is formed, and these changes are generally reversible. Examples include the melting of ice (H2O(s)→H2O(l)H_2O(s) \rightarrow H_2O(l)) and the dissolution of sugar in water.

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A Chemical Change occurs when one or more new substances with different chemical properties are formed. These are usually irreversible and accompanied by heat/light evolution or gas production. An example is the burning of magnesium ribbon: 2Mg+O2→2MgO2Mg + O_2 \rightarrow 2MgO.

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Rusting of Iron is a chemical change that occurs when iron (FeFe) reacts with oxygen (O2O_2) from the air and moisture (H2OH_2O). The resulting flaky brown substance is Iron Oxide: Fe2O3⋅xH2OFe_2O_3 \cdot xH_2O.

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Galvanisation is the process of depositing a layer of zinc (ZnZn) on iron to prevent rusting, as zinc is more reactive and forms a protective layer.

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Crystallisation is a physical process used to obtain a solid substance in its pure crystal form from its solution. For example, obtaining pure CuSO4CuSO_4 (Copper Sulphate) crystals from an impure sample.

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When CO2CO_2 (Carbon Dioxide) is passed through lime water (Ca(OH)2Ca(OH)_2), it turns milky due to the formation of CaCO3CaCO_3 (Calcium Carbonate).

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Displacement Reaction: When an iron nail (FeFe) is placed in a blue copper sulphate (CuSO4CuSO_4) solution, the color changes to green because of the formation of iron sulphate (FeSO4FeSO_4) and the deposition of brownish copper (CuCu).

📐Formulae

Solution=Solute+Solvent\text{Solution} = \text{Solute} + \text{Solvent}

Concentration (mass/mass %)=Mass of SoluteMass of Solution×100\text{Concentration (mass/mass \%)} = \frac{\text{Mass of Solute}}{\text{Mass of Solution}} \times 100

2Mg+O2→2MgO (Magnesium Oxide)2Mg + O_2 \rightarrow 2MgO \text{ (Magnesium Oxide)}

MgO+H2O→Mg(OH)2 (Magnesium Hydroxide - Basic)MgO + H_2O \rightarrow Mg(OH)_2 \text{ (Magnesium Hydroxide - Basic)}

Fe+CuSO4→FeSO4+CuFe + CuSO_4 \rightarrow FeSO_4 + Cu

CO2+Ca(OH)2→CaCO3+H2OCO_2 + Ca(OH)_2 \rightarrow CaCO_3 + H_2O

4Fe+3O2+2xH2O→2Fe2O3⋅xH2O (Rust)4Fe + 3O_2 + 2xH_2O \rightarrow 2Fe_2O_3 \cdot xH_2O \text{ (Rust)}

💡Examples

Problem 1:

A solution is prepared by dissolving 40 g40\text{ g} of common salt in 320 g320\text{ g} of water. Calculate the concentration in terms of mass by mass percentage of the solution.

Solution:

Mass of Solute (Salt)=40 g\text{Mass of Solute (Salt)} = 40\text{ g} Mass of Solvent (Water)=320 g\text{Mass of Solvent (Water)} = 320\text{ g} Mass of Solution=40 g+320 g=360 g\text{Mass of Solution} = 40\text{ g} + 320\text{ g} = 360\text{ g} Concentration=(40360)×100=11.11%\text{Concentration} = \left( \frac{40}{360} \right) \times 100 = 11.11\%

Explanation:

To find the concentration, we first find the total mass of the solution by adding the solute and solvent. Then we divide the mass of the solute by the total mass of the solution and multiply by 100.

Problem 2:

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

Solution:

Fe(s)+CuSO4(aq)→FeSO4(aq)+Cu(s)Fe(s) + CuSO_4(aq) \rightarrow FeSO_4(aq) + Cu(s) Blue→Green+Brown deposit\text{Blue} \rightarrow \text{Green} + \text{Brown deposit}

Explanation:

This is a Chemical Change known as a displacement reaction. The iron displaces copper from its solution, forming Iron Sulphate (which is green) and depositing reddish-brown copper on the nail.

Problem 3:

Explain why the burning of magnesium ribbon is considered a chemical change while the melting of wax is a physical change.

Solution:

  1. Burning Magnesium: 2Mg+O2→2MgO2Mg + O_2 \rightarrow 2MgO. A new substance (Magnesium Oxide) is formed. It cannot be reversed easily.
  2. Melting Wax: Only the state changes from solid to liquid. No new substance is formed, and it can be reversed by cooling.

Explanation:

Chemical changes result in the formation of new products with different chemical identities, whereas physical changes only alter the appearance or state without changing the molecular structure.