General Science Quiz: Chemistry of Water: Hard Water, Heavy Water and Purification

In an ion exchange plant the water passes through a cation resin, which takes up the calcium and magnesium ions, and then through an anion resin, which takes up the sulphate, chloride and bicarbonate ions; because the hardness-causing ions are actually taken out of the water, the hardness can be…

Heavy water is D2O, in which both hydrogen atoms are deuterium, the isotope whose nucleus has one proton and one neutron, so its molecular mass is twenty instead of eighteen.

These bicarbonates are formed when water containing dissolved carbon dioxide passes over limestone, and they are unstable; on boiling they lose carbon dioxide and settle as insoluble carbonates, which is why this kind of hardness is called temporary and can be removed by boiling alone, or by the Clark method…

Water contracts as it is cooled to four degrees, is densest at that temperature, and then expands again as it cools further to zero, which is called the anomalous expansion of water.

Alum, that is aluminium sulphate or potash alum, is a coagulant: in water it forms a gelatinous hydroxide that gathers the fine clay and colloidal particles into heavier flocs, which then settle in the sedimentation tank and are removed, leaving clear water for filtration.

Bleaching powder is prepared by passing chlorine over dry slaked lime and is written as calcium oxychloride; in water it releases chlorine, which kills bacteria and other pathogens, and that is why it is used both for disinfecting water supplies and for bleaching cloth and paper.

In reverse osmosis the water is pushed through a semi-permeable membrane at a pressure greater than its osmotic pressure, so the flow runs opposite to natural osmosis; the membrane lets water molecules through and holds back dissolved salts, heavy metals and most microbes, which is why reverse osmosis is also…

A moderator slows down the fast neutrons released by fission so that they can cause further fission, and heavy water is the best of them because it slows neutrons well while absorbing very few; in Indian pressurised heavy water reactors it also serves as the coolant that carries heat away…

Soap is the sodium or potassium salt of a long chain fatty acid.

Cholera is caused by the bacterium Vibrio cholerae and spreads through water and food contaminated with faecal matter, which is why it breaks out after floods and wherever sewage mixes with the water supply; typhoid, hepatitis A, dysentery and polio spread the same way.

A water molecule has two hydrogen atoms of atomic mass one and one oxygen atom of atomic mass sixteen, so the masses are two and sixteen, which is a ratio of one to eight; this fixed ratio in every sample of water, whatever its source, is the classic illustration of…