What water is made of, why it dissolves so much, and how hardness and crystals of salts arise.
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Water is a compound of hydrogen and oxygen. Passing electricity through acidified water splits it: hydrogen collects at the cathode and oxygen at the anode in the volume ratio 2 : 1 (2H₂O → 2H₂ + O₂). By mass the ratio of hydrogen to oxygen is 2 : 16, or 1 : 8. Pure water is colourless, odourless and tasteless, freezes at 0 °C and boils at 100 °C, and is densest at 4 °C, which is why ice floats and lakes freeze from the top.
In electrolysis 20 mL of oxygen collects. What volume of hydrogen collects?
The ratio H₂ : O₂ is 2 : 1, so 20 × 2 = 40 mL.
What mass of hydrogen is present in 18 g of water?
Water is 2 parts H to 16 parts O by mass: 18 × 2 ÷ 18 = 2 g of hydrogen and 16 g of oxygen.
Why is a little acid added in the electrolysis of water?
Pure water is a very poor conductor; the acid supplies ions so that current flows.
Water dissolves more substances than any other liquid, so it is called the universal solvent. Solubility is the mass of solute that dissolves in 100 g of water at a given temperature to give a saturated solution; for most solids it rises with temperature. Soft water lathers easily with soap. Hard water does not, because dissolved calcium and magnesium salts form scum with soap. Temporary hardness is due to hydrogencarbonates and is removed by boiling; permanent hardness is due to chlorides and sulphates and is removed by adding washing soda (sodium carbonate) or by ion exchange (permutit).
Differentiate temporary and permanent hardness.
Temporary: caused by Ca and Mg hydrogencarbonates; removed by boiling. Permanent: caused by Ca and Mg chlorides or sulphates; not removed by boiling.
Why is soft water better for washing clothes?
It lathers easily and does not waste soap in forming scum.
Name two methods of softening hard water.
Adding washing soda; ion exchange (permutit) process; also boiling for temporary hardness.
Some salts hold water inside their crystals, called water of crystallisation. Hydrated copper sulphate, CuSO₄·5H₂O, is blue; on heating it loses the water and becomes white anhydrous CuSO₄, and adding water brings the blue back (a test for water). Hygroscopic substances take moisture from the air without dissolving (quicklime, conc. H₂SO₄); deliquescent ones absorb so much that they dissolve (NaOH, CaCl₂, anhydrous FeCl₃); efflorescent ones lose water of crystallisation to the air and crumble to powder (washing soda crystals, Na₂CO₃·10H₂O).
Find the relative molecular mass of CuSO₄·5H₂O (Cu = 63.5, S = 32, O = 16, H = 1).
CuSO₄ = 63.5 + 32 + 64 = 159.5; 5H₂O = 90; total 249.5.
Find the percentage of water of crystallisation in CuSO₄·5H₂O.
90 ÷ 249.5 × 100 = 36.07%.
Differentiate deliquescent and efflorescent substances.
Deliquescent substances absorb moisture and dissolve in it; efflorescent substances lose water of crystallisation and turn to powder.
1. State the volume ratio of hydrogen to oxygen in electrolysis of water.
2. Why does ice float on water?
3. What is the universal solvent?
4. Define solubility.
5. Name the salts causing hardness of water.
6. How is temporary hardness removed?
7. What happens when blue copper sulphate crystals are heated?
8. Name a drying agent.
9. The volume of hydrogen collected is 60 mL. How much oxygen is collected?
10. Find the percentage of water in washing soda Na₂CO₃·10H₂O (Na = 23, C = 12, O = 16, H = 1).