Question

4(b). (i) State what is achieved at each of the following stages in the purification of town water supply: (I) aeration; (II) screening; (III) sedimentation. (ii) Name two substances responsible for hardness in water. (iii) State two methods for the removal of hardness in water. (v) Give one disadvantage of hard water.

Accepted Answer

(i) I. Aeration removes odour/unpleasant taste and kills some bacteria (adds oxygen). II. Screening removes large debris/floating matter. III. Sedimentation removes suspended solid particles that settle out.
(ii) Any two of: calcium/magnesium hydrogen trioxocarbonate(IV); magnesium chloride; magnesium or calcium tetraoxosulphate(VI); iron(II) hydrogen trioxocarbonate(IV) or tetraoxosulphate(VI); calcium chloride.
(iii) Any two of: addition of washing soda (Na2CO3); addition of a calculated amount of slaked lime Ca(OH)2; use of ion-exchange resin/permutit/zeolite; boiling/distillation.
(v) Any one of: furring of kettles/pipes causing blockages and wasted energy; wastes soap by forming scum; unsuitable for laundry, dyeing, textile or paper industries.

WAEC Chemistry 2021 Theory — Question 24

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