When water is heated from 0°C to 4°C, its density
- (a)remains constant.
- (b)increases.
- (c)decreases.
- (d)first increases then decreases to its original value.
Correct — B, increases. Water does not behave like an ordinary liquid in this narrow band. Warm it from 0 degrees Celsius and it contracts instead of expanding, so the same mass takes up less room and the density climbs. The climb stops at 4 degrees Celsius, where water reaches its maximum density of 1000 kilograms per cubic metre; heat it further and it expands like anything else and the density falls again. The reason lies in hydrogen bonding. In ice each molecule is held in a bulky open cage, and just above melting a good deal of that cage structure survives in clusters. Warming breaks the clusters and lets the molecules pack closer, which pulls the density up, while ordinary thermal agitation is pushing it down. Up to 4 degrees the collapse of the cages wins.
- (a)remains constant. — This is the one interval in which water's density is changing in a way worth examining. Density does not hold still anywhere on the liquid range; it merely turns around at 4 degrees Celsius.
- (c)decreases. — The right answer for almost every other liquid, and for water itself above 4 degrees Celsius. Between 0 and 4 the anomaly reverses it, and the whole point of the question is that band.
- (d)first increases then decreases to its original value. — This describes a round trip that carries the water past the density maximum and back down again, so it needs a heating range that goes well beyond 4 degrees Celsius. The stem stops at 4, which is the peak — the density has risen and has not yet begun to fall.
The anomalous expansion of water is the reason a pond freezes from the top down. Surface water cooling towards 4 degrees Celsius grows denser and sinks, setting up a circulation that keeps mixing the pond. Below 4 degrees the surface water becomes lighter than the water beneath, so it stops sinking, stays on top, and eventually freezes. Ice itself is less dense still and floats, and it then insulates what lies below, which is why deep lakes hold liquid water at about 4 degrees at the bottom through a hard winter and fish survive there.
Read the range in the stem carefully, because the whole item lives in it. Heating from 0 to 4 degrees is the contracting half of the curve; heating from 4 degrees upward is the ordinary expanding half; and a question that spanned both halves would have a different answer. The wording as printed is exact — water heated from 0 to 4 degrees Celsius does gain density — so there is nothing in this stem to correct, and the anomaly is precisely what is being tested. A neighbouring UPSC item makes the pairing plain by asking about volume rather than density over 0 to 10 degrees, where the answer has to be that the volume first falls and then rises.
- Water reaches its maximum density at 4 degrees Celsius, where the density is 1000 kilograms per cubic metre.
- Between 0 and 4 degrees Celsius, heating water contracts it and so raises its density.
- Above 4 degrees Celsius water expands on heating like an ordinary liquid.
- The behaviour comes from the open hydrogen-bonded cage structure inherited from ice, which collapses as the water warms.
- Because the densest water sinks, lakes freeze from the surface downward and stay liquid at depth.
- Applying the ordinary rule that heating expands a liquid without checking the temperature range.
- Confusing a statement about volume with one about density; they move in opposite directions.
- Placing the density maximum at 0 degrees Celsius rather than at 4.
As a density-or-volume item over a stated temperature range, or through its consequence — why a lake freezes from the top and stays liquid below.
When water is heated from 0 °C to 10 °C, its volume
- (a) increases
- (b) decreases
- (c) does not change
- (d) first decreases and then increases
Answer(d) first decreases and then increases
The same curve, read over a wider range and in terms of volume. Because the range runs past 4 degrees Celsius the volume falls and then rises — which is the mirror image of the density rising and then falling, and shows how much the stated range decides the answer.
The surface of a lake is frozen in severe winter, but the water at its bottom is still liquid. What is the reason ?
- (a) Ice is a bad conductor of heat
- (b) Since the surface of the lake is at the same temperature as the air, no heat is lost
- (c) The density of water is maximum at 4°C
- (d) None of the statements (a), (b) and (c) given above is correct
Answer(c) The density of water is maximum at 4°C
The consequence of the same anomaly, asked as a real-world puzzle. Water colder than 4 degrees Celsius is lighter than water at 4, so it will not sink — which is why the freezing happens at the surface and the depths stay liquid.
Which one of the following regarding density of water at atmospheric pressure is correct?
- (a) Density of water at 4°C is 1000 kg/m³.
- (b) Density of water at 0°C is 1000 kg/m³.
- (c) Density of water at 0°C is 100 kg/m³.
- (d) Density of water at 4°C is 10 kg/m³.
Answer(a) Density of water at 4°C is 1000 kg/m³.
The number behind this answer. Fixing 1000 kilograms per cubic metre to 4 degrees Celsius rather than to 0 settles both questions at once, because it locates the peak of the curve.
- practice — not a real PYQ
Liquid water attains its maximum density at which one of the following temperatures?
- (a)0 °C
- (b)4 °C
- (c)10 °C
- (d)100 °C
Answer(b) 4 °C — where the density is 1000 kilograms per cubic metre, above and below which it falls.
- practice — not a real PYQ
In a severe winter the surface of a lake freezes while the water at its bottom stays liquid. The decisive reason is
- (a)ice is a good conductor of heat
- (b)the density of water is maximum at 4 °C
- (c)the lake bed is warmed by the Earth
- (d)salt dissolved in the water lowers its freezing point
Answer(b) the density of water is maximum at 4 °C — so water colder than that floats instead of sinking, and freezes at the surface.