Observation

Majority of the candidates responded to this question and their performance was above average.

In part (a), majority of the candidates stated the purpose of carbon and chlorine in the treatment of water.

In part (b), majority of the candidates could not describe how the combustion of fossil fuel leads to the formation of acid rain but they could not describe the effect of acid rain on buildings.

In part (c), majority of the candidates defined allotropy and explained why graphite as a good conductor of electricity while diamond is not.

In part (d), majority of the candidates described why water is a universal solvent and were able to give two disadvantages of hard water. However, they could not explain why the boiling point of water is higher than that of hydrogen sulphide.

The expected answers include:

(a)(i)   

I.          Carbon - for removing odour
- for removing tastes
- removing poisonous substances
II.        Chlorine to kill  - bacteria
-  kill microbes
-  kill micro-organisms                                             
(ii)        

• coagulation/flocculation: addition of chemicals to the water to cause   
  formation of larger particles of dirt
• sedimentation:  larger particles of dirt settle to bottom of water supply

• disinfection:  after filtration, a disinfectant (chorine) is added to kill any remaining parasite/micro-organisms to protect the water from germs when it is piped to homes or aeration: exposure of water to air and sunlight to kill bacteria

• filtration: the clear water is passed through filters of various sizes of pores to remove undissolved particles.                                                                                                 

(b)(i)     Fossil fuels burn to produce oxides of Sulphur and nitrogen. These oxide reacts
with rain water in the atmosphere to form acid.
(ii)    Acids corrodes/damages buildings of carbonate rocks/reacts with building (made of carbonate rocks)/ corrodes mortar/reacts with mortar causes points to peel                                                                
(c)(i)   existence of different forms of the same element in the same physical state                                                                                                            
(ii)     graphite contains free electrons whereas all the electron in diamond are bonded
and the free/mobile electrons in graphite are used in the generation of electricity.
(iii)     activated charcoal/animal charcoal                                                                                                                   
(d) (i)    because it dissolves most substances
(ii)     

• wastes soap (do not accept “does not readily lather with soap”)
• graying of white fabrics/not suitable for tanning/laundering/paper manufacturing
• scale formation in boilers
• causes dry itching skin
• causes appliances to wear out easily

                                                                                                                       
(iii)       Water molecules are held by hydrogen bonding which is stronger than weak
van der Waal’s (dipole-dipole) forces between molecules of hydrogen sulphide                                                                                                          
(iv)  

- industrial manufacture of hydrogen
- fuel (for the manufacture of steel)