What happens to the density of liquid when it is heated?

A gas sample is collected in a balloon. What happens to the density of the gas sample as the sample is heated?

• A) decreases B) increases C) does not change D) could increase or decrease, depending on how much it was heated Please explain also. Thanks!

• Answer:

  The density will decrease. This is because heating the gas will cause its pressure to increase. The rubber (or whatever fabric the balloon is made of) will respond by moving outwards. Therefore the volume will have increased and, since the mass of the gas will not have changed, it density will decrease. The actual amount by which it decreases will depend on the elasticity of the fabric. It's much easier to quantify the decrease if you are dealing with an open-necked balloon (such as a hot-air balloon), as the density of the gas is, to a fairly close approximation, inversely proportional to the absolute temperature. So, if it is heated from 200°C (473K) to 300°C (573K), its density will decrease by a factor of 573 ÷ 473 = 1·211 (or, its density will become 0·825 (82·5%) of what it was before).