4: Vapour Pressure

At the same temperature, a non-volatile solute lowers the solvent mole fraction and does not contribute appreciably to the vapour. Raoult's law therefore predicts a solution pressure below the pure-solvent pressure. The solution still contains molecules and liquid; an infinite temperature would not explain a lower pressure in this context.

On heating, the average kinetic energy of liquid particles increases. Consequently, a larger number of particles can overcome the attractive forces at the surface and enter the vapour phase. This increases the number of vapour particles in equilibrium and therefore raises vapour pressure. The other options contradict particle motion or conservation of matter.

A non-volatile solute contributes negligibly to the vapour phase. When it is mixed with the solvent, the mole fraction and escaping tendency of solvent molecules decrease; in a simple particle picture, fewer solvent molecules are available at the surface. Hence the equilibrium vapour pressure of the solvent falls. The other choices describe processes that do not occur.

For an ideal solution containing a non-volatile solute, Raoult’s law is written as P_solvent = X_solvent P°_solvent. At a fixed temperature, the pure-solvent vapour pressure P° is constant, so the solution vapour pressure is directly proportional to the solvent mole fraction X. Colour, container shape, and smell do not occur in this relation.

In a pure solvent, the solvent mole fraction is one and the maximum number of surface molecules can escape. Adding a non-volatile solute lowers the solvent mole fraction, while the solute contributes almost no vapour. By Raoult's law, P_solution = X_solvent P°; hence the pressure is lower than P°.

Vapour pressure depends on how readily particles escape from the liquid surface at a given temperature. Weaker intermolecular attractions hold particles less strongly, so more particles can enter the vapour phase and establish a higher equilibrium pressure. Strong attractions produce the opposite effect. The statements that a liquid never vapourises or has no temperature are not physically suitable comparisons.

Strong intermolecular attraction keeps liquid particles together and makes it more difficult for them to escape into the vapour phase. Therefore, at the same temperature, fewer particles are present in the vapour and the equilibrium vapour pressure is lower. Weak attraction and rapid evaporation instead indicate higher vapour pressure. Liquid particles always possess some thermal motion above absolute zero.

Boiling begins when vapour bubbles can form throughout the liquid and their pressure can balance the pressure outside the liquid. This occurs when the liquid’s vapour pressure becomes equal to the external pressure. A colour change or disappearance of mass is not required, and vapour formation is essential rather than absent. Lower external pressure therefore gives a lower boiling temperature.

At a specified external pressure, boiling occurs when vapour pressure reaches that pressure. A liquid with higher vapour pressure at the same temperature reaches this condition sooner, so it boils at a lower temperature. Thus high volatility generally corresponds to a lower boiling point. The boiling point is not always zero, and the statement about mole fraction does not answer the comparison.

Boiling requires the liquid vapour pressure to equal the external pressure. If a liquid has low vapour pressure at a given temperature, its particles need additional heating to escape sufficiently and raise the vapour pressure to that level. Therefore its boiling point is generally higher. The relation is not zero or absent; it follows directly from the boiling condition.

In the expression P = X_solvent P°, the superscript ° denotes the standard or pure-component value at the specified temperature. For the solvent term, P° is therefore the vapour pressure of the pure solvent, before the solute is added. It is not a property of the container, vapour colour, or automatically the pure solute. Adding a non-volatile solute usually makes P smaller than P°.

For an ideal solution with a non-volatile solute, Raoult’s law gives P_solvent = X_solvent P°_solvent. At constant temperature, P°_solvent is fixed. Thus decreasing X_solvent multiplies the fixed pure-solvent pressure by a smaller number, so the solvent vapour pressure decreases. It cannot become infinite, and colour is irrelevant to this quantitative relationship.

The mole fraction of solvent is defined as X_solvent = n_solvent/(n_solvent + n_solute). When solute is added, the solvent amount may remain unchanged, but the total number of moles in the denominator increases. Therefore X_solvent decreases from its pure-solvent value of one. It does not increase, remain one, or become infinite after adding solute.

Dissolved sugar is treated as a non-volatile solute under ordinary conditions, so it contributes essentially no vapour of its own. Its presence lowers the mole fraction and escaping tendency of water molecules. By Raoult’s law, the water vapour pressure becomes X_water P°_water, which is less than P°_water when X_water is below one. Sugar does not convert water into a gas.

A measurable equilibrium vapour pressure requires a closed space in which vapour can accumulate above the liquid. In an open container, molecules that evaporate continually diffuse away, so the vapour concentration cannot normally build up to the reversible evaporation–condensation equilibrium value. The liquid still evaporates, but a stable equilibrium pressure is not established in the same way.

Volatility is the tendency of a substance to pass from the liquid state into the vapour state. At the same temperature and external conditions, a high vapour pressure means that many particles can escape the liquid and remain in the vapour phase at equilibrium. Therefore the liquid is more volatile. Low volatility would be associated with stronger retention of particles and lower vapour pressure, not zero temperature dependence.

Low volatility means that liquid particles are relatively difficult to remove from the surface, usually because intermolecular attractions are stronger. At a fixed temperature, fewer particles enter the vapour phase, giving a lower equilibrium vapour pressure. Thus low volatility and low vapour pressure generally go together; infinite pressure is impossible in this context.

For a non-volatile solute, Raoult’s law is P = X_solvent P°_solvent. Substituting X_solvent = 0.9 and P°_solvent = 100 gives P = 0.9 × 100 = 90. The answer is therefore 90 in the same pressure units as the given value. It is lower than 100 because the solvent mole fraction is less than one; 110 would contradict the lowering effect.

Raoult’s law at constant temperature is P_solvent = X_solvent P°_solvent. Since P°_solvent does not change in this comparison, pressure has the same proportional dependence as X_solvent. If X_solvent changes from X to X/2, the new pressure is (X/2)P° = P/2. It therefore becomes half, not double or four times, and it is not unchanged.

By definition, a non-volatile solute has very low vapour pressure at the temperature under consideration. Its partial pressure in the vapour phase is therefore taken as negligible in the usual school-level treatment. The total pressure is mainly the solvent's partial pressure. This does not mean the solute disappears; it remains dissolved in the liquid.

A volatile solute can escape from the liquid and establish its own partial pressure above the solution, just as the solvent does. Therefore total vapour pressure is the sum of the partial pressures of both volatile components, subject to the solution model used. A container or colour does not provide a vapour contribution, and vapour is not absent.

Dalton's law states that the total pressure of a mixture of non-reacting gases is the sum of the partial pressures exerted by its individual components: P_total = p_1 + p_2 + p_3 and so on. A vapour mixture follows this additive pressure rule under the usual ideal-gas approximation. Colour and particle stoppage are irrelevant.

The liquid's nature determines the strength of intermolecular attractions and therefore how easily particles escape. Temperature determines the particles' kinetic energy and the fraction able to escape. These are the principal factors controlling vapour pressure for a fixed phase and composition. Container colour, word length and observer identity have no physical role in the equilibrium value.

A higher temperature increases the average kinetic energy of liquid particles. More particles then have enough energy to overcome surface attraction and escape, so the equilibrium vapour pressure and the tendency for evaporation increase. Evaporation can occur at the surface below the boiling point, whereas boiling requires equality with external pressure. The other choices reverse or deny the effect of heating.

Faster evaporation indicates that a larger fraction of the liquid's molecules can escape from the surface at that temperature. This corresponds to greater volatility and, at equilibrium, a higher vapour pressure. The comparison assumes the same temperature and comparable conditions. A rapidly evaporating liquid therefore cannot normally be assigned a lower or zero vapour pressure.