The question was attempted by most candidates and the performance was good.
In (a)(i), candidates correctly defined hard water as water which will not readily form lather with soap.
In (a)(ii) candidates correctly named two substances responsible for hardness in water from among the following:
calcium tetraoxosulphate (VI)
calcium hydrogentetraoxosulphate (IV)
magnesium tetraoxosulphate (VI)
magnesium hydrogentetraoxosulphate (IV)
In (a) (iii), candidates gave two methods for the removal of hardness in water from among the following:
heating the water to boiling/distillation
addition of calculated amount of calcium hydroxide [Ca(OH) 2]
addition of washing soda crystals to the water [Na2CO3.10H2O]
ion exchange resin/permutit method.
In (b)(i), candidates correctly gave the raw materials for the manufacture of tetraoxosulphate (VI) acid by the contact process as sulphur and air/oxygen.
In (b)(ii) and (iii), candidates correctly wrote an equation for the reaction that requires a catalyst in the contact process and stated the catalyst used.
The equation is
2SO2 + O2   » 2SO3(g)
and the catalyst employed is V2O5/ vanadium (V) oxide.
In (c)(i) most candidates were able to correctly give two uses of sodium tetraoxosulphate (VI) from among the following:
in glass/paper manufacture
as analytical reagent
in the manufacture of detergents
In (c)(ii) most candidates correctly gave the name of the type of reaction that was represented by the given equation.
In (c)(iii) candidates correctly calculated the solubility of Na2SO3 at 25°C.
In (d)(i), candidates did not know that activated complex is the intermediate (complex) that reactants must attain in order to form a product or a temporary species formed by reactant molecules as a result collision before the products are formed, instead they gave the definition of activation energy.
In (d)(ii), most candidates could not state one reason why collision may not produce a chemical reaction. They did not know that if the energy of the colliding particles is less than the activation energy or the reactants are not properly aligned, chemical reaction may not occur inspite of collision.
In (d)(iii), candidates drew the energy profile diagram for the endothermic reaction but most of them lost marks because they indicated the enthalpy of reaction instead of the enthalpy of reactants. The correct energy profile diagram is as drawn below.