UPSC 2017 CDS I GK Paper

The pH value of a sample of multiple-distilled water is very near to seven. It should be kept in mind that water (distilled, deionised, or tap) is not ‘pure’ (i.e., pH equal to 7). The moment it comes in contact with air, CO2 gas begins dissolving into it, forming carbonic acid. The actual pH, therefore, will often be slightly less than 7.