Page 43 - Curriculum Visions Dynamic Book
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Superphosphate
This is the most important salt derived from phosphoric acid. In the most common form of preparation, phosphate rock is treated with (inexpensive) sulphuric acid. This produces the products superphosphate and gypsum (calcium sulphate), although as the gypsum has no harmful effects it is not removed.
If calcium phosphate is treated with phosphoric acid instead of sulphuric acid, only superphosphate is produced. This more concentrated form of fertiliser is called double
or triple phosphate.
EQUATION: Preparation of superphosphate
detergent: a petroleum-based chemical that removes dirt.
ion: an atom, or group of atoms, that has gained or lost one or more electrons and so developed an electrical charge.
nutrients: soluble ions that are essential to life.
precipitate: tiny solid particles formed as a result of a chemical reaction between two liquids or gases.
Calcium phosphate + sulphuric acid ➪ calcium phosphate + calcium sulphate Ca3(PO4)2(s) + 2H2SO4(l) ➪ Ca(H2PO4)2(s) + 2Ca(SO)4(s)
 These tablets contain phosphate fertilisers. Superphosphates are used widely in agriculture, but although they are excellent fertilisers, when phosphate-rich waters seep through soils to rivers, they
can cause just the same environmental damage that caused the removal of phosphates from detergents.
 Phosphates formed the basis of detergents for many years, but they are now less frequently used because of the environmental damage they can do.
superphosphate
Phosphates and pollution
Phosphates are not stored in the soil, so they have to be applied quite frequently. Not all the fertiliser applied is used by plants; some drains away with rainwater and reaches rivers. Here water life, especially algae, make use of the increased supply of phosphate fertiliser by growing vigorously.
Sodium triphosphate has been used
in detergents because the phosphate ion acts to stop dirt particles that have been removed by detergents from sticking back on to clothes. As the phosphates are washed away through the drains, they eventually reach water supplies where they, too, act as nutrients for algae.
The combined effects of extra sources of phosphates in the world’s natural drainage basins has caused widespread alarm because it causes excessive algal growth (called algal blooms) that take the oxygen from the water they have been growing
in and so make life almost impossible for
all other water creatures. For this reason, few detergents now contain phosphates. Farm fertilisers seeping to waterways, however, remain a serious pollution problem.
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