O-LevelChemistryChemistry of the Environment5070/22 · May/June 2016 · Q610 Marks

River water contains dissolved minerals and gases.

(a)Carbon dioxide is one of the gases dissolved in river water. Draw the 'dot-and-cross' diagram to show the bonding in a molecule of carbon dioxide. Only draw the outer-shell electrons.[1]
(b)(i)River water often contains dissolved compounds such as ammonium nitrate and calcium phosphate. State one source of both of these compounds.[1]
(b)(ii)Describe and explain the environmental effect of the presence of these dissolved compounds in river water.[3]
(c)River water is often purified for use as drinking water. Describe three processes involved in the purification of river water. process 1 ................................................................................................................ process 2 ................................................................................................................ process 3 ................................................................................................................[3]
(d)(i)Water has a low melting point and is neutral (pH = 7). Explain why water has a low melting point.[1]
(d)(ii)A pH meter can be used to confirm that water is neutral. Describe another way in which a student can confirm that water is neutral.[1]

Mark scheme

Marking points from the official mark scheme (May/June 2016). The mark or mark type is shown in brackets.

[1] Correct 'dot-and-cross' diagram for carbon dioxide
[1] From fields/from farms/fertilisers
[1] Eutrophication
[1] Increased growth of algae/algal bloom/fast growth of algae
[1] This blocks out sunlight/plants can't photosynthesise
[1] Plants (beneath the surface) die AND get decomposed by bacteria that use up oxygen
[1] Other aquatic organisms die because of lack of oxygen
[1] Chlorination – kills bacteria or microbes
[1] Filtration – removes insoluble materials/removes solid/removes named solid
[1] Use of carbon – removes odours/removes (unpleasant) tastes
[1] Weak intermolecular forces/weak forces between molecules/weak intermolecular bonds/weak attractive forces between molecules/not much energy needed to overcome intermolecular forces
[1] Add universal indicator and it will turn green/add full range indicator and it will turn green/add pH paper and it turns green

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