Which one of the following is NOT the common method of disinfection of water?
- ABy boiling of water
- BBy ultra-violet rays
- CBy use of ozone
- DBy use of sodium chlorideCorrect
Solution
Concept
Disinfection kills disease-causing germs. Common methods are boiling, UV rays, ozone and chlorine compounds. Sodium chloride (common salt) is not a disinfectant.
Step-by-step solution
1. List the common disinfection methods: boiling, chlorination, UV rays, ozone, excess lime, iodine/bromine. 2. Check each option. Boiling, UV and ozone are all in this list. 3. Sodium chloride (NaCl) is common salt. It does not kill pathogens at normal doses. 4. So option D is NOT a method of disinfection.
Explanation
Boiling, UV rays and ozone all kill bacteria and viruses in water. Sodium chloride is only common salt and has no disinfecting action. Do not confuse it with chlorine or sodium hypochlorite (NaOCl), which are disinfectants.
Why the other options are wrong
A: Boiling kills germs, so it is a valid method (used at home). B: UV rays destroy the DNA of microbes, so it is a valid method. C: Ozone is a strong oxidant and a valid disinfectant.
Shortcut
Chlorine and hypochlorite disinfect; chloride (NaCl, common salt) does not.
Common mistake
Students confuse sodium chloride (NaCl) with sodium hypochlorite (NaOCl).
Correct answer: D) By use of sodium chloride