Inorganic Salts: Properties, Preparation and Identification of Ions
Senior 2 · Chemistry
Inorganic Salts: Properties, Preparation and Identification of Ions
Lesson 4 of 5
18 min
Walk into an agro-dealer's shop in any district of Rwanda and most of what is on sale is salts: bags of NPK 17-17-17, DAP and potassium chloride, sacks of agricultural lime, and fungicides containing copper. At home, you use salts in cooking, in baking, in medicines and in soap. Rwanda's cement plant at Bugarama, in Rusizi District, turns limestone, a natural salt, into the cement for the country's buildings.
By the end of this lesson you will be able to state where salts come from, link common salts to their uses, and read a fertiliser label.
| Salt | Formula | Main uses |
|---|---|---|
| Sodium chloride | NaCl | Seasoning and preserving food; the raw material for sodium hydroxide and chlorine; salting out soap |
| Ammonium nitrate | NH4NO3 | Nitrogen fertiliser |
| Diammonium hydrogenphosphate (DAP) | (NH4)2HPO4 | Fertiliser supplying nitrogen and phosphorus |
| Potassium chloride | KCl | The potassium (K) part of NPK fertilisers |
| Calcium carbonate | CaCO3 | Agricultural lime for acidic soils; making cement and quicklime |
| Calcium sulfate | CaSO4 | Plaster of Paris for casts on broken bones; plaster for walls |
| Copper(II) sulfate | CuSO4·5H2O | Fungicide (in Bordeaux mixture, with lime, against potato blight); testing for water |
| Sodium hydrogencarbonate | NaHCO3 | Baking powder (releases carbon dioxide so dough rises); antacid for indigestion |
| Sodium carbonate | Na2CO3·10H2O | Washing soda; softening hard water; making glass |
| Magnesium sulfate | MgSO4·7H2O | Epsom salts, used as a laxative; supplying magnesium to crops |
| Potassium iodate | KIO3 | Added to table salt ("iodised salt") to prevent goitre |
Plants need nitrogen (N), phosphorus (P) and potassium (K) in large amounts. A fertiliser bag shows three numbers, the N-P-K ratio. On a bag of NPK 17-17-17, they mean 17% nitrogen (N), 17% phosphorus expressed as P2O5 and 17% potassium expressed as K2O, by mass.
What mass of nitrogen is in a 50 kg bag of NPK 17-17-17?
Solution. 17% of 50 kg = 17 ÷ 100 × 50 = 8.5 kg of nitrogen.
Explain, with an ionic equation, how agricultural lime (calcium carbonate) improves an acidic soil.
Solution. The acid in the soil provides H+ ions. Calcium carbonate reacts with them, removing the acid and raising the soil pH:
CaCO3(s) + 2H+(aq) → Ca2+(aq) + H2O(l) + CO2(g)
Soap is a mixture of the sodium (or potassium) salts of long-chain fatty acids. Soap-making cooperatives boil a fat or oil, such as palm oil, with sodium hydroxide solution: fat + sodium hydroxide → soap + glycerol. Adding common salt makes the soap separate from the water as a solid layer, a step called salting out.
Salts can harm the environment when they are overused. Fertiliser washed off fields by rain can enrich lakes and rivers with nitrates and phosphates, causing a rapid growth of algae that uses up the oxygen in the water. Copper from years of Bordeaux mixture spraying builds up in the soil. Farmers should apply the amount recommended for their soil and crop.
Tip
Exam technique: use + reason. "Copper(II) sulfate is used as a fungicide because copper(II) ions kill fungal spores." "Calcium carbonate is spread on acidic soils because it neutralises the acid." A use without a reason usually earns only half the marks.
Try each item before you read the solution.
What mass of nitrogen is in a 25 kg bag of NPK 17-17-17?
Solution: 17 ÷ 100 × 25 = 4.25 kg.
Give the formula and one use of (a) sodium hydrogencarbonate, (b) calcium sulfate.
Solution: (a) NaHCO3: baking powder, because it gives off carbon dioxide that makes dough rise; also an antacid. (b) CaSO4: plaster of Paris for casts, because it sets hard when mixed with water.
Why is common salt added during soap making?
Solution: Salt makes the soap much less soluble in the watery mixture, so the soap separates out as a solid that can be collected (salting out).
Suggest one harmful effect of using too much nitrogen fertiliser near Lake Muhazi.
Solution: Rain washes the excess nitrate into the lake. It feeds a rapid growth of algae; when the algae die and decay, bacteria use up the dissolved oxygen and fish may die.
Summary
Sources and credits
This lesson adapts CC BY-SA material and is shared under CC BY-SA 4.0.
