(a) (i) What is meant by hardness of water? (ii)... | WAEC SSCE - Chemistry - 2019 (Essay)

Question 1 Report

(a) (i) What is meant by hardness of water?

(ii) Describe briefly how you would determine what proportion of hardness in a given sample of water is due to permanent hardness.

(iii) Give two reasons why hardness of water is an undesirable property.

(b) State the:

(i) reagents;

(ii) conditions for the laboratory preparation of trioxonitrate (V) acid.

(iii) How does concentrated trioxonitrate (V) acid reacts with:

(I) sulphur;

(II) aluminium.

Answer

(a)(i) Hardness of water is the property of water that prevents it from readily forming lather (foam) with soap, caused by dissolved calcium and magnesium salts.

(ii) Boil a measured sample of the water to remove temporary hardness, then filter. Determine (by soap-lather titration) the soap needed to give a lasting lather with the unboiled sample (total hardness) and with the boiled sample (permanent hardness). The permanent hardness is the value obtained with the boiled sample; the difference gives the temporary hardness.

(iii) Any two: it wastes soap and forms scum; it deposits scale (fur) in kettles, boilers and pipes, which wastes fuel and can block pipes.

(b)(i) Reagents: potassium (or sodium) trioxonitrate(V), KNO₃, and concentrated tetraoxosulphate(VI) acid.

(ii) Conditions: gentle heating, using all-glass apparatus (the acid attacks cork and rubber).

(iii) Concentrated HNO₃ with:

Sulphur: it oxidizes sulphur to tetraoxosulphate(VI) acid. \(S + 6HNO_3 \rightarrow H_2SO_4 + 6NO_2 + 2H_2O\)
Aluminium: no reaction; aluminium is rendered passive because a protective oxide layer forms on its surface.

(c) One amphoteric oxide: aluminium oxide, Al₂O₃ (also ZnO or PbO).

Read lesson note on Oxidation And Reduction (JAMB)

Answer Details

(a)(i) Hardness of water is the property of water that prevents it from readily forming lather (foam) with soap, caused by dissolved calcium and magnesium salts.

(iii) Any two: it wastes soap and forms scum; it deposits scale (fur) in kettles, boilers and pipes, which wastes fuel and can block pipes.

(b)(i) Reagents: potassium (or sodium) trioxonitrate(V), KNO₃, and concentrated tetraoxosulphate(VI) acid.

(ii) Conditions: gentle heating, using all-glass apparatus (the acid attacks cork and rubber).

(iii) Concentrated HNO₃ with:

Sulphur: it oxidizes sulphur to tetraoxosulphate(VI) acid. \(S + 6HNO_3 \rightarrow H_2SO_4 + 6NO_2 + 2H_2O\)
Aluminium: no reaction; aluminium is rendered passive because a protective oxide layer forms on its surface.

(c) One amphoteric oxide: aluminium oxide, Al₂O₃ (also ZnO or PbO).

Read lesson note on Oxidation And Reduction (JAMB)

Oxidation And Reduction

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(a) (i) What is meant by hardness of water? (ii) Describe briefly how you would determine what proportion of hardness in a given sample of water is due to p...

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(a) (i) What is meant by hardness of water? (ii) Describe briefly how you would determine what proportion of hardness in a given sample of water is due to p...

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