Which one of the following is considered to be the purest source of water?
- (a)River water
- (b)Sea water
- (c)Well water
- (d)Rain water
Correct — D, rain water. Rain water is nature's own distilled water: heat evaporates water from the surface, leaving dissolved salts and impurities behind, and the vapour then condenses and falls as rain. Because the dissolving of ground minerals is bypassed, rain water is the purest of the natural sources listed, even though it can pick up some dissolved gases and dust while falling through the air.
- (a)River water — River water flows over land and rock, dissolving minerals and collecting silt, organic matter and pollutants along the way, so it is far from pure.
- (b)Sea water — Sea water is the least pure of all — it carries about 3.5% dissolved salts, mainly sodium chloride, making it undrinkable without desalination.
- (c)Well water — Well (ground) water seeps through soil and rock and dissolves minerals such as calcium and magnesium salts, which is why it is often hard; it is not the purest.
The natural water cycle acts like a giant distillation unit. When surface water evaporates, dissolved solids are left behind and only pure water vapour rises; this condenses in the atmosphere and returns as rain. That is why rain water, before it interacts with the ground, is regarded as the purest naturally occurring water.
Rank the options by how much contact each has had with soil, rock and salts. Sea water has the most dissolved matter, river and well water pick up minerals from the land, but rain water forms from evaporation and condensation, which strips out those impurities — making it the natural stand-in for distilled water.
- Rain water is naturally distilled — evaporation leaves dissolved salts behind, so the vapour that condenses is nearly pure.
- Sea water contains roughly 3.5% dissolved salts and is the least pure natural source.
- Well and river water dissolve minerals from soil and rock, so they contain more dissolved solids than rain water.
- Falling rain can absorb atmospheric gases such as carbon dioxide, so it is 'purest natural' rather than absolutely pure.
- 'Purest natural source' means before the water touches the ground — rain, not river or well water.
- Rain water is not chemically pure; it dissolves atmospheric gases, which is how acid rain can form.
Water purity is asked as 'which is the purest natural source' — recall that rain water is naturally distilled by the water cycle.
No directly related past PYQ was found.
- practice — not a real PYQ
Rain water is considered the purest natural water because it is
- (a)boiled before falling
- (b)filtered by the soil
- (c)naturally distilled by evaporation
- (d)rich in dissolved minerals
Answer(c) naturally distilled by evaporation — impurities are left behind when the water evaporates.
- practice — not a real PYQ
Which one of the following natural waters contains the most dissolved salts?
- (a)Rain water
- (b)River water
- (c)Sea water
- (d)Melted snow
Answer(c) Sea water — it holds about 3.5% dissolved salts.