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    When animal waste is broken down by bacteria, the bacteria use oxygen for respiration as they grow. Describe an investigation to find whether the release of slurry (animal waste) from a farm into a nearby river lowers the oxygen concentration of the water in the river. (6mks)

    Hi, Let us work it out step by step, yeah?

    1. To check if the O2 levels are reduced by animal waste, the 1st point is we need a way to estimate/measure O2 in the water.

    2. To work out whether the animal waste is causing a drop in O2 levels, we need to compare the O2 levels in water with and without the waste - does that sound reasonable?

    3. We need to design a method that is (in modern baby language haha!) a "fair test" - really we need to use an experiment that does not let other factors invalidate our result [such other influences are called confounding factors] - we can use controls.

    4. Lastly we need to analyse the results [e.g. by drawing tables/graphs] and interpret the results, then evaluate our experiment.

    Taking each step in order:

    1. O2 levels could be measured using an oximeter (if your school can afford one!) OR we could try and use a chemical or element that reacts with oxygen, and then measure the mass of product produced [from which you can work out the original oxygen conc-n by a stoichiometry calculation [this is when you add up the mass numbers of each chemical in a reaction, then divide the masses in grams by these mass numbers to get number of moles, and calculate the mass of the unknown chemical] e.g. try and use the oxygen as a reactant with carbon to produce CO2, then pass this through lime water and measure the amount of precipitate (CaCO3) OR bubble some sulphur dioxide (SO2) through the water and measure the amount of sulphur trioxide (SO3) produced by reacting the sulphuric acid produced with an alkali and titrate the salt produced.

    C + O2 -----> CO2

    2SO2 + O2 -----> 2SO3

    SO3 + H2O = H2SO4

    2. You could to this for water samples taken from upstream [before the waste enters the river] and downstream [after] and then compare the results. So we know the difference between water before the waste was added and after.

    3. How can we make a valid test? What do you think?.......................... ................................ ............................... Yes well done!, use the same volume of water samples for both - use the same temperature, pressure, pH, etc. AND take several samples then take averages, so errors are less likely.

    4. If the average O2 levels are lower in the downstream samples, we know that something has reduced its levels, probably the waste - EVALUATION: say what was good about your plan, and any shortcomings e.g. the oxygen could have been reduced simply because there were more fish downstream using up O2, and we have not taken that into account - any criticism of your own experiment that makes sense will earn marks.

    See, "Easy peasy, lemon squeezy!" right? Best of luck!

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