3: Solubility

At a fixed gas–liquid system, gas solubility generally increases when pressure or the gas’s partial pressure increases. It usually decreases as temperature rises, because dissolved gas escapes more readily from a warmer liquid. Therefore, low temperature combined with high pressure favours the greatest dissolved amount. Stirring may speed equilibration but is not the principal equilibrium factor in the options.

A saturated solution has already reached the equilibrium limit for dissolving solute at that temperature and solvent amount. Adding more solute cannot increase that limit; the additional material generally remains as solid, although a small transient amount may dissolve before equilibrium is restored. The solution does not become unsaturated, and its solubility does not become infinite.

At constant temperature, Henry’s law gives a direct relationship between the partial pressure of a gas and the amount dissolved in a liquid: increasing pressure increases the dissolved concentration or mole fraction. More gas molecules are driven into the liquid phase until a new equilibrium is established. The relationship is not controlled by colour, and increasing pressure does not make solubility zero.

Oxygen is a gas, and gases generally become less soluble in liquids as temperature rises at approximately constant pressure. Heating gives dissolved oxygen greater thermal energy and makes escape to the gas phase more favourable, so the equilibrium dissolved amount decreases. Oxygen can still dissolve in water; the change is a reduction in solubility, not complete absence. Pressure and gas composition also matter in real systems.

As depth increases, the pressure exerted by the surrounding water and the breathing gas increases. According to Henry’s law, higher partial pressure of a gas promotes a greater amount dissolving in a liquid such as blood. During a rapid ascent, pressure falls and dissolved gases can come out as bubbles, which explains the need for controlled decompression. Colour and high temperature are not the main cause.

A solubility curve normally plots solubility against temperature. A steep positive slope means that a relatively small rise in temperature produces a relatively large increase in the amount that can dissolve. The exact numerical change must be read from the axes and scale, but the trend is clearly strong temperature dependence. The slope does not indicate solvent colour or pressure independence.

Finely grinding a solid produces smaller particles and increases the total surface area exposed to the solvent. Solvent molecules can then contact and remove particles from the surface more rapidly, so the rate of dissolving increases. At the same temperature and pressure, however, the equilibrium solubility is generally not doubled or otherwise changed by particle size. Mass and solvent identity also remain unchanged.

The slope of a solubility curve represents how strongly solubility changes with temperature. An almost horizontal curve has a small slope, so increasing or decreasing temperature produces only a small change in the amount dissolved over the plotted range. It does not mean the solid becomes a gas or that pressure alone determines solubility. The axes and scale should still be checked before making a numerical conclusion.

Water is polar, and salt is ionic; water can stabilize its separated ions through ion–dipole interactions. Oil is mainly non-polar and does not interact favourably enough with water to form a single phase. Thus the example supports the general principle that substances with similar polarity or interaction type tend to dissolve one another better. It is not an absolute rule, but it is the correct qualitative explanation here.

Henry’s law is written as p = K_H x, where p is the partial pressure, K_H is Henry’s constant, and x is the mole fraction of dissolved gas. At fixed pressure, x = p/K_H. Therefore, increasing K_H makes x smaller. Thus, a gas with a larger Henry’s constant is less soluble and has a lower dissolved mole fraction than a gas with a smaller constant under the same conditions.

Henry’s law is written as p = K_H x, where p is the gas’s partial pressure, K_H is Henry’s law constant, and x is its mole fraction dissolved in the liquid. At the same temperature and equal partial pressure, x = p/K_H. Therefore, a smaller K_H gives a larger dissolved mole fraction and hence greater solubility. The higher-constant gas is less soluble under these conditions.

In a sealed carbonated drink, carbon dioxide is kept in contact with the liquid under pressure greater than atmospheric pressure. According to Henry’s law, the solubility of a gas in a liquid increases when its partial pressure above the liquid increases. Therefore, more carbon dioxide remains dissolved in the unopened bottle. Bottle colour and the claim that pressure has no effect are irrelevant.

Oxygen is a gas, and dissolution of gases in liquids is generally less favourable at higher temperature. Heating gives dissolved gas molecules more tendency to escape into the atmosphere. Therefore warm lake water can contain less dissolved oxygen, even though other environmental factors may also contribute.

A solubility curve generally plots solubility on the vertical axis against temperature on the horizontal axis. The steepness of the line represents how quickly solubility changes as temperature changes. A steep upward slope means that even a small rise in temperature produces a large increase in solubility. A nearly horizontal line would instead indicate little temperature dependence.

A hot saturated solution contains the maximum amount of solute that can dissolve at its high temperature. For many solid solutes, solubility decreases on cooling. When the temperature falls, the reduced solvent capacity cannot hold all the previously dissolved solute, so the excess leaves the solution and forms an ordered solid crystal lattice. The process is crystallization; it does not require the solvent to become a new substance.

Gas solubility in a liquid generally increases with pressure because a higher gas partial pressure drives more gas molecules into the liquid. It generally decreases with increasing temperature because dissolved gas molecules escape more readily when heated. Therefore, among the given choices, low temperature combined with high pressure provides the most favourable condition. The open vessel and boiling liquid promote gas escape.

Stirring renews the solvent at the solid surface and reduces concentration gradients, so the solid usually dissolves faster. However, the equilibrium amount that can dissolve at a fixed temperature is governed by solute–solvent interactions and is not necessarily changed by stirring. Rate and capacity are different ideas.

For a given mass, breaking a solid into smaller particles creates a much larger total surface area. More surface is then available for solvent molecules to collide with and remove solute particles. This increases the rate of dissolution, but it does not necessarily change the equilibrium solubility at that temperature.

Fish obtain oxygen for respiration from the water passing over their gills. If the concentration of dissolved oxygen falls, less oxygen is available for diffusion into the blood. Fish may then show respiratory stress or suffocation. The other options do not describe the role of dissolved oxygen in aquatic respiration.

A saturated solution contains the maximum amount of solute that can dissolve in a specified amount of solvent at a particular temperature and, where relevant, pressure. Since solubility often changes with temperature, the same quantities may be saturated at one temperature but unsaturated or capable of crystallizing at another. Thus, the temperature condition is essential for defining saturation meaningfully.

Initially, the solution is saturated at its original temperature, so it cannot dissolve more solute at that temperature. If heating allows additional solute to dissolve without changing the amount of solvent, the maximum dissolving capacity has increased at the new temperature. Therefore, the solute’s solubility increases with temperature in this case. Heating does not mean that the earlier solution contained no solute.

At the lower temperature, the normal solubility limit is smaller than the amount of solute still present in the clear solution. Because cooling occurred without disturbance or nucleation, the excess solute has not yet crystallized. A solution containing more dissolved solute than its equilibrium solubility at that temperature is called supersaturated. It is unstable and may crystallize when seeded or shaken.

A solubility curve gives the amount of a substance that dissolves under specified conditions at each temperature. At the intersection of two curves, both substances have the same vertical-axis value for the same horizontal-axis temperature. Therefore, their solubilities are equal at that temperature. The intersection does not imply that either substance is insoluble, gaseous, or that the solvent has disappeared.

The horizontal axis of a usual solubility curve represents temperature and the vertical axis represents solubility. A downward slope means that, as temperature increases along the horizontal axis, the vertical solubility value becomes smaller. Thus solubility decreases. The graph, not a universal assumption about all solids, determines the answer.

Water is a polar solvent, whereas common oils are largely non-polar. Polar water molecules interact poorly with non-polar oil, while a non-polar solvent can interact more favourably with it. This illustrates the rule “like dissolves like”; temperature also matters, but it is not the main idea here.