Question

What happens when an astronaut heats ice at $- {5^0}C$ on the moon?
A. It melts into water $- {5^0}C$.
B. Its temperature rises and then melts at ${0^0}C$.
C. It sublimates into vapours.
D. It melts at temperature greater than ${0^0}C$.

Answer 1

In order to answer this question, you must be aware about the concept of vapour pressure. It is defined as a measure of the tendency of a material to change into the gaseous or vapour state. The vapour pressure of a material increases with temperature.

Complete answer:
If the vapour pressure of a liquid is greater than the external pressure, the liquid will begin to boil. Since, the moon does not have an atmosphere, therefore the pressure on the moon is zero. When ice at $- {5^0}C$ is heated, the liquid water formed evaporates instantly because water has a vapour pressure greater than zero. As a result, ice sublimes directly into vapour.

Hence, option C is correct.

Note: The phenomenon in which solid directly changes into vapour is called sublimation. It is caused by the absorption of heat which provides enough energy for some molecules to overcome the attractive forces of the surrounding molecules and convert into the vapour phase.