Question

The absolute zero is the temperature at which:
A.All substances exist in solid state
B.Molecular motion ceases
C.Water freezes
D.None of these

Answer 1

Hint: Absolute zero is the lowest temperature on the temperature scale. At this temperature the substance comprises the lowest energy which means that it has minimum values of enthalpy and entropy. Thus, at absolute zero temperature the molecules of the substance have no heat energy.

Complete answer:
Temperature is a measure of the hotness or coldness of the body. It defines the average kinetic energy of the molecules in a substance. As temperature increases, the molecules in the substance start moving faster and as the temperature decreases, they slow down. Temperature increases when the substance absorbs heat while it decreases when the substance releases its heat. Since the absolute zero is the lowest temperature on the temperature scale, it is the temperature at which the average kinetic energy of the molecules is zero. This means that the motion of the molecules ceases and they become still. The absolute zero temperature is different on different scales, for example it is $0\text{ K}$ on Kelvin scale, $-273.15\text{ }{}^\circ \text{C}$ on Celsius scale and $-460\text{ F}$ on Fahrenheit scale. Therefore, the correct option is $B$. Although to reach absolute zero temperature is an impossible thing, theoretically, the molecular motion ceases at absolute zero temperature.

Note:
The option $A$ says that at absolute zero temperature, all substances exist in solid state, which is not true because not all but most of the substances exist in solid state, for example, the super fluids. And if we look at option $C$, it’s incorrect because water freezes much before the absolute zero temperature.