Ans : Boiling is an excellent method of purifying water for drinking purposes.

How and why?

To be effective, the water (filtered from dust) must be kept boiling (Roll boiling in tamil) for 15 minutes. This kills all pathogens of bacteria, viruses, spores, cysts, ova, etc to give sterilized water. The process also removes temporary hardness by driving out CO2 and precipitating calcium carbonate (responsible for bladder stones). Store the water in the same clean vessel closed with a plate.

A Comparison with other methods.

• For urban supply chlorination of water is common. “Chlorine tablets” are quite good for disinfecting small quantities of water eg. 0.5g tablet is sufficient for 20 litres. Some people don't like chlorine taste and smell.
• 2 drops of 2% solution of iodine in alcohol - water is sufficient to disinfect 1 litre of clear water.
• Filtration thourgh ceramic candle helps remove some pathogens. But it is said that organisms like hepatitis A virus and polio virus can pass through into filtrate. Also the pores may remain clogged with bacteria.
• UV irradiation is effective against most microorganisms, but the process is more expensive and requires an apparatus.
• Water can be effectively sterilized with ozone (O3). Many countries around the world use a combined ozone - chlorination method.
• But boiling of water is very easy and can be done at any place by all sections of people. This boiling process protects us from water - borne diseases like jaundice, typhoid, dysentery, cholera, amoebiasis, polio, etc.