Farmers use fertilisers to increase their crop yields. These fertilisers must be dissolved in water before they can be absorbed by the plant roots as only dissolved substances are small enough to be absorbed.
Some fertilisers dissolve easily, but some are designed for slow release giving crops a small amount over a long time.
Fertilisers are needed because the world population is rising and there is a greater demand for food production from the land available.
Ferilisers increase crop yield by;
-replacing essential elements used by the previous crops or providing extra essential elements
-providing nitrogen that is incorporated into plant protein resulting in increased growth.
Fertilsers create problems if they get into ponds,lakes,rivers. They encourage algae growht, leading to eutrophication.
Eutrophication happens when;
-fertilisers are washed off felds; - fertilisers in the water increase the nitrate and phosphate levels in ponds, lakes and rivers; - algae grow quickly on the surface (algal bloom) in the presence of these chemicals; - the algae block off the sunlight to other oxygen-producing plants which die; - aerobic bacteria use up the oxygen in the water and feed on the dead and decaying plants; - most living organisms die.
Excessive use of fertilisers can also pollute water supplies.
Many fertilisers are salts, so they can be made by reacting an acid and an alkali to make salt and water. For example; the fertiliser that would be produced by combining nitric acid and ammonia solution would be ammonium nitrate.
The prcess of producing a fertiliser from the reaction of an acid and an alkali (such as sulfuric acid and ammonia solution) is as follows;
-the alkali is titrated with the acid using an indicator to find out the quantities needed before the main batch is made (this is repeated until the results are consistent); - altjough the acid and alkali have now reacted completely to produce a neutral solution of ammonium sulfate fertiliser, this is contaminated with indicator solution; - the titration results are used to repeat the experiment using the correct quantities; - the dissolved fertiliser is heated to evaporate most of the water off, then left for the remaining solution to crystalise. The crystals are then filtered off.
mining and subsidence
Salt is mined in cheshire in two different ways;
-mining it from the ground as rock salt
-solution mining by pumping in water and extracting saturated salt solution
Mining salt can lead to subsidence. The ground above a mine can sink causing lanslips and destroying homes.
Salt at the surface, particularly brine solution, can escape and affect habitats.
electrolysis of sodium chloride solution
Concentrated sodium chloride solution (brine) can be seperated by electrolysis.
Hydrogen is made at the negative cathode.
Chlorine is made at the positive anode
Sodium hyrdroxide forms in solution
Hydrogen and chlorine are reactive, so it is important to use inert electrodes so that the products dont react before they are collected and the electrodes do not dissolve.
more on electrolysis
During the electrolysis of NaCl solution;
-Na+ and H+ ions migrate to the negative cathode
-Cl- and OH- ions migrate to the positive anode.
-at the cathode, hydrogen is made-electrons are gained so this is reduction as shown in this half-equation-
At the anode, chlorine is made- electrons are lost so this is oxidation.
The ions not discharged make sodium hyrdoxide solution;
The chlor-alkali industry
Sodium hydroxide and chlorine are used to make house-hold bleach.
Chlorine and sodium hydroxide are important raw-materials. They are involved in making about half the chemicals we use on a daily basis including solvents, plastics, paints, soaps, medicines and food additives.