Which substance has the greatest distance between its particles at 20°C? Explain your answer.
Why is not boiling point for some substance C?
A table showing the melting point and boiling point of some substance at °C
substance : M.P.(°C) :B.P.(°C)
A : -259 :-253
B: 0 :100
D : -116: 34.5
E: 801: 1413
In a gas the distance between its molecules (or atoms) is the highest from all three states of matter because the interaction forces between them are the weakest . In a solid, the distance between its molecules or atoms is the lowest from all three states of matter because the interaction forces between them are the strongest.
For the same state of matter (be it solid, liquid or gas) the distance between molecules increases with temperature (because the thermal agitation of the molecules or atoms is higher). This is the dilatation phenomena.
The biggest distance between its particles at 20 degree C will have the substance that is closest to the gaseous state or is already gas. From the table this is the substance A) which boils at -253 degree Celsius.
Therefore the correct answer is substance A) will have the biggest distance between its particles.
A substance in a given state (solid, liquid or gas) can be in equilibrium with another state (for example solid is in equilibrium with gas, or liquid is in equilibrium with gas) for some domains of pressure and temperature. It exist a fixed pressure and temperature where all 3 states (solid, liquid and gas) of the substance can exist at the same time. This is named the triple point. See the figure below.
If the pressure of the triple point is too big (and correspondingly the temperature of the triple point is too big) at normal pressures the substance will only be found as solid or vapour. This is why for substance C there is no boiling point - the pressure of the triple point is much bigger than the normal pressure.