Equilibrium in physical and chemical processes

Correct answer: A. It increases

When temperature increases, the average kinetic energy of liquid molecules also increases. Consequently, more molecules acquire enough energy to escape from the liquid surface into the vapour phase. In a closed system, this produces a higher equilibrium vapour pressure. Therefore, vapour pressure generally increases with temperature; it does not become zero or remain constant. Option B gives the opposite trend.

Correct answer: A. Higher

Volatility describes how readily a liquid changes into vapour. In a more volatile liquid, intermolecular attractions are comparatively easier to overcome, so more molecules escape from the surface at the same temperature. At equilibrium this produces a higher vapour pressure. Vapour pressure cannot be negative or necessarily zero.

Correct answer: A. In a closed vessel at equilibrium

The vapour pressure of a liquid is the pressure exerted by its vapour when the liquid and vapour are in dynamic equilibrium at a specified temperature. A closed vessel is needed so that vapour does not continuously escape. In an open vessel or strong wind, vapour is removed and equilibrium pressure is not established. A vacuum without liquid cannot have the vapour pressure of that liquid. Hence A is correct.

Correct answer: A. Lower

A liquid boils when its vapour pressure becomes equal to the external pressure. At the same temperature, a liquid with higher vapour pressure is closer to this condition, so it reaches boiling at a lower temperature. Thus high vapour pressure generally indicates a lower boiling point, as in a more volatile liquid. It is not unrelated to boiling point, and the boiling point is not necessarily zero.

Correct answer: A. Nature of liquid and temperature

Vapour pressure depends mainly on temperature and on the nature of the liquid, especially the intermolecular forces between its molecules. Temperature changes molecular kinetic energy, while molecular attractions affect how easily molecules escape. Container colour, taste, and liquid height alone do not determine the equilibrium vapour pressure.

Correct answer: A. Pascal

Vapour pressure is a pressure, because it is the force per unit area exerted by vapour particles. The SI unit of pressure is the pascal (Pa); other pressure units include atmosphere, bar and millimetre of mercury. Gram measures mass, litre measures volume, and mole measures amount of substance, so they cannot be pressure units.

Correct answer: A. Evaporation and condensation

Liquid molecules continuously escape into the vapour phase; this is evaporation. Vapour molecules simultaneously return to the liquid; this is condensation. At dynamic equilibrium, the forward and reverse rates are equal, so macroscopic amounts remain constant. The other pairs describe different processes and are not the liquid-vapour equilibrium pair.

Correct answer: A. A property dependent on temperature

Vapour pressure is an intensive equilibrium property whose value for a given liquid depends strongly on temperature and liquid composition. It does not simply increase because more liquid is placed in the vessel, provided liquid remains present and equilibrium conditions are reached. Container shape and colour do not determine it.

Correct answer: A. Higher

Intermolecular attractions hold molecules inside a liquid. If these attractions are relatively weak, molecules need less energy to escape from the surface. At the same temperature, more molecules enter the vapour phase and the equilibrium vapour pressure is higher. Hence weak attraction is associated with greater volatility, not zero pressure.

Correct answer: A. Lower

Strong intermolecular attractions hold molecules more firmly within the liquid. At a given temperature, fewer molecules have enough energy to escape into the vapour phase. The equilibrium concentration of vapour is therefore lower, giving a lower vapour pressure and usually lower volatility. Vapour pressure is neither infinite nor negative.

Correct answer: A. Because vapour pressure changes with temperature

Vapour pressure depends strongly on temperature because temperature changes the kinetic-energy distribution of liquid molecules. A warmer liquid generally has more molecules able to escape, producing a higher vapour pressure. If two liquids are measured at different temperatures, the result mixes temperature effects with composition or intermolecular-force effects, so the comparison is not fair.

Correct answer: A. When its vapour pressure becomes equal to the external pressure

Boiling begins when vapour bubbles can form throughout the liquid and their pressure can resist the pressure outside the liquid. This occurs when the liquid’s vapour pressure becomes equal to the external pressure. The boiling temperature therefore depends on both the liquid and the surrounding pressure; lowering external pressure lowers the boiling point. Colour change, zero volume, or absence of solute is not the defining condition.

Correct answer: A. Boiling point decreases

A liquid boils when its vapour pressure equals the external pressure. At high altitude, atmospheric pressure is lower, so the liquid reaches this required vapour pressure at a lower temperature. Consequently, the boiling point decreases. This is why water can boil below 100°C at high altitude, although boiling does not mean that heating becomes unnecessary.

Correct answer: A. Because the external pressure inside increases

A pressure cooker confines steam and raises the pressure above the water. Since boiling requires the water vapour pressure to equal the external pressure, water must reach a higher temperature before that condition is met. The increase is caused by pressure, not sweetness, colour, or a zero vapour pressure.

Correct answer: A. In a liquid with higher vapour pressure

Molecules leave the surface during evaporation. At the same temperature, a liquid with higher vapour pressure has a greater equilibrium tendency to enter the vapour phase, indicating that its molecules escape more readily. A low-vapour-pressure or non-volatile liquid has stronger retention in the liquid phase; a frozen substance is not the relevant liquid comparison.

Correct answer: A. Direct relation

At a fixed temperature, vapour pressure reflects the escaping tendency of molecules from a liquid. If molecules evaporate more readily, more vapour accumulates before equilibrium is established, producing a higher vapour pressure. Thus the relationship is direct. The exact pressure still depends on temperature and the nature of the liquid.

Correct answer: A. Low

Volatility is the tendency of a liquid to produce vapour. At the same temperature, low vapour pressure means that relatively few molecules escape and remain in the vapour phase at equilibrium. Therefore the liquid is less volatile. High volatility would instead be associated with a higher vapour pressure, not an infinite or necessarily constant value.

Correct answer: A. Because molecular kinetic energy is low

At lower temperature, the average kinetic energy of liquid molecules is lower. Consequently, fewer molecules have enough energy to overcome intermolecular attractions and escape through the surface. The equilibrium vapour contains fewer particles and exerts lower pressure. The effect is not caused by a changing name, a smaller container, or vapour becoming heavier.

Correct answer: A. Vapour state

Vapour pressure is the pressure exerted by vapour particles present above a liquid or solid in a closed system at equilibrium. In the context of a liquid, it is specifically associated with particles that have entered the vapour phase. The liquid supplies those particles, but the pressure itself is exerted by the vapour, not exclusively by solids, plasma or metals.

Correct answer: A. Because vapour keeps escaping

In an open vessel, molecules that evaporate disperse into the surrounding atmosphere instead of accumulating above the liquid. A stable liquid-vapour equilibrium and a definite equilibrium pressure therefore cannot be maintained in the usual sense. A closed vessel allows evaporation and condensation to balance. Vapour does form in an open vessel; it simply escapes continuously.

Correct answer: A. Temperature

Increasing temperature raises the average kinetic energy of liquid molecules and increases the fraction that can escape into the vapour phase. The equilibrium vapour pressure therefore rises for a given liquid. The age of the liquid, the appearance of the container and the length of its name do not define this thermodynamic relationship under ordinary comparable conditions.

Correct answer: A. Rates of evaporation and condensation becoming equal

Liquid-vapour equilibrium is dynamic, not static. Molecules continue to evaporate from the liquid and condense from the vapour, but at equilibrium the two rates are equal. As a result, the macroscopic amounts and pressure remain constant. The liquid does not need to disappear, and evaporation does not stop completely.

Correct answer: A. Increase temperature

Increasing temperature raises molecular kinetic energy and allows more liquid molecules to escape into the vapour phase, so vapour pressure increases. Adding a non-volatile solute or decreasing solvent mole fraction generally lowers the solvent's vapour pressure according to Raoult's law. The vague phrase 'make concentrated' does not provide a valid increase mechanism here.

Correct answer: A. Vapour pressure is pressure exerted by vapour at equilibrium in a closed vessel

The equilibrium vapour pressure is the pressure exerted by vapour in contact with its liquid in a closed system at a specified temperature. It reflects a dynamic balance between evaporation and condensation. It is not the weight of the liquid, is not always zero, and changes with temperature, generally increasing as temperature rises.

Correct answer: A. Vapour pressure

When a liquid and its vapour coexist in a closed vessel at a fixed temperature, evaporation and condensation eventually occur at equal rates. The pressure exerted by the vapour at this dynamic equilibrium is called vapour pressure. Osmotic pressure concerns solvent flow through a semipermeable membrane, while atmospheric pressure is the pressure of the surrounding atmosphere.