water purity is often assessed by reference to its oxygen content.
(a) Outline the meaning of the term biochemical oxygen demand (BOD). 
The amount of oxygen needed to decompose organic matter (in water sample); in a specified time, at a specified temperature
(b) Describe how the dissolved oxygen concentration in a river would decrease if
(i) a car factory releases warm water into the river after using it cooling. 
Oxygen less soluble in hot water
(ii) a farmer puts large quantities of a fertilizer on a field next to the river. 
Fertilizer causes excessive algae growth so oxygen concentration is reduced
(c) The winkler method uses redox reactions to find the concentration of oxygen in water. 100 cm3 of water was taken from a river and analysed using this method. The reactions taking place are summarized below.
(i) State what happened to the O2 in step 1 in terms of electrons. 
There is gain in number of electrons
(ii) State the change in oxidation number for manganese in step 2. 
From +4 it is +2, so it decreased by 2
(iii) 0.002 moles of I- were formed in step 3. Calculate the amount in moles of oxygen, O2- dissolved in water, 
(d) Due to the shortage of fresh water in many parts of the planet, scientists have designed ways to convert sea water to fresh water. Outline how multi-stage distillation converts sea water to fresh water. 
In order to convert, the sea water is heated then goes to a container, where pressure is lower, where boiling will occur and then only a small part becomes into steam and the remaining water will be sent through other process, as it will have low pressure.
(a) Suggest one way in which a householder could reduce the amount of water used. 
Saving water through not leaving the tap running when not being used, also by not leaving water unattended. Water is usually used way more in a household because of the amount of unattended water we leave running.
(b) Water that allows marine life to flourish needs a high concentration of dissolved oxygen. Several factors can alter the oxygen concentration.
(i) State how an increase in temperature affects the oxygen concentration. 
the water decreases in the temperature affects of oxygen concentration
(ii) Eutrophication is a process that decreases the oxygen concentration of water. Explain how the accidental release of nitrates into a river can cause eutrophication. 
Plant life is important because the algae increases then dies, and also it decays and consumes oxygen.
youtube video based on dissolved oxygen.