3: Solubility

Solubility is the maximum amount of a solute that dissolves in a specified amount of solvent at a specified temperature, usually when the solution is saturated. The exact numerical expression may use mass, moles, or another stated basis. Molarity is concentration per litre of solution, pressure is force per area, and vapour pressure concerns evaporation. Thus A is correct.

For many solids, dissolution absorbs heat or is otherwise favored by heating, so increasing temperature generally permits more solute to dissolve in the liquid. This is a general trend rather than an exception-free law; a few substances show little change or decreased solubility. Therefore A is the correct general answer, while B, C, and D are too absolute or false.

At constant temperature, increasing the partial pressure of a gas above a liquid increases the amount of that gas dissolved in the liquid. This direct relationship is expressed by Henry’s law and is used when carbon dioxide is dissolved in beverages. Pressure does not normally make the solvent solid, and the solubility does not remain unaffected. Hence A is correct.

A solubility curve is a graph showing how the solubility of a substance changes with temperature, usually for a fixed amount of solvent. By reading a point on the graph, we can find the maximum amount that dissolves at that temperature. The other options are unrelated to solubility.

A solubility curve shows how much solute dissolves at different temperatures. If the curve rises from left to right, the plotted solubility value becomes larger as temperature increases. Therefore, more solute can dissolve at the higher temperature, under the conditions represented by the graph. The other choices describe effects that are not represented by an upward solubility trend.

A supersaturated solution contains more dissolved solute than the equilibrium solubility at that temperature. It can exist temporarily, but it is not in a stable equilibrium state. A small disturbance, scratching the container, or adding a seed crystal may cause the excess solute to crystallize. Thus it is generally unstable, not permanently stable and not restricted to gases.

A supersaturated solution holds more solute than its normal equilibrium limit, but the excess remains dissolved in a metastable condition. A seed crystal supplies a surface and an arrangement that helps the excess solute leave the solution and form crystals. An unsaturated solution would instead be able to dissolve more solute, while pure solvent or an insoluble mixture does not explain this crystallization.

At the same temperature, solubility per unit mass of water remains the same. The given ratio is 36 g solute per 100 g water. For 50 g water, calculate 36 × 50 ÷ 100 = 18 g. Therefore, 18 g is the saturated amount under these conditions. The other values do not follow the required proportional scaling of solute with solvent mass.

The solubility ratio is 25 g solute for every 100 g water. Since 300 g water is three times 100 g, the saturated solute amount is also three times 25 g: 25 × 300 ÷ 100 = 75 g. This answer refers to the solute dissolved, not the total mass of the solution. Hence 75 g is correct.

At this temperature, 100 g of water can dissolve up to 35 g of the solute. Only 28 g is present in solution, so the amount is below the maximum capacity by 7 g. More solute could still dissolve; therefore the solution is unsaturated. A saturated solution would contain 35 g, while a supersaturated solution would contain more than 35 g under the stated basis.

Solubility is the maximum amount of solute that can dissolve in a specified mass of solvent at a particular temperature. Here, the dissolved amount, 35 g, exactly equals the solubility for 100 g water. Thus the solution has reached its limit and is saturated; it is neither below nor above that limit.

For 100 g water at the given temperature, 35 g is the normal saturation limit. The stated dissolved amount, 42 g, is greater by 7 g. If that excess remains dissolved, the solution is supersaturated and usually unstable. It cannot be called unsaturated because it already exceeds the equilibrium limit.

Solubility is conventionally expressed as the mass of solute that dissolves in 100 g of solvent at a fixed temperature. The given ratio is 24 g in 80 g water, so the equivalent amount is (24/80) × 100 = 30 g. Thus 24 g is not the required value because it refers only to 80 g water.

A meaningful solubility comparison requires the relevant conditions to be controlled or stated. Temperature can change solubility, and the mass of solvent provides the basis for expressing the amount dissolved. Container shape and colour are irrelevant, while time may matter for rate but does not replace the required conditions.

Solubility depends strongly on the interactions between solute and solvent particles. Polar solvents can stabilize ions through ion-dipole attractions, so many ionic substances dissolve appreciably in them. Oil is non-polar, sand does not normally ionize and dissolve in water, and wax is not more water-soluble than salt.

Common salts such as sodium chloride contain oppositely charged ions. Water molecules are polar: their partially charged ends attract and surround the ions, helping separate them from the crystal lattice and stabilize them in solution. The process is not due to salt being a gas or water becoming a metal.

Water molecules are polar and form strong hydrogen-bond interactions with one another. Most oils are largely non-polar, so they cannot form comparably favorable interactions with water. Separating into layers reduces the unfavorable contact between unlike substances. The immiscibility is not because water lacks particles or because both liquids are ionic.

Atmospheric pressure decreases with altitude. When a carbonated bottle is opened, the gas pressure above the liquid falls toward the lower surrounding pressure. The reduced pressure lowers the equilibrium solubility of carbon dioxide, so dissolved gas escapes as bubbles. The effect is not caused by the drink lacking water or by solid carbon dioxide formation.

Oxygen is a gas that dissolves in water and supplies aquatic organisms. For gases, solubility generally decreases as temperature rises, because faster-moving molecules escape from the liquid more easily. Thus warm water can hold less dissolved oxygen, although the exact amount also depends on pressure and other conditions. Heating does not turn oxygen into salt or require solid oxygen for fish.

Henry's law states that, at constant temperature and for the same gas-solvent system, the dissolved amount or mole fraction of a gas is directly proportional to its partial pressure. Thus multiplying the pressure by three multiplies the ideal solubility by three. It does not produce an inverse or squared relationship.

Partial pressure is the pressure contributed by one gas in a mixture, so it is directly relevant when a gaseous solute dissolves in a liquid. Henry's law relates that partial pressure to the dissolved mole fraction. It is not the primary pressure variable for ordinary solid solutes in the same way.

Breaking a solid into smaller particles increases its total surface area for the same mass. More solute particles can then contact the solvent at once, so the rate of dissolution increases. However, at a fixed temperature the equilibrium solubility is determined by the system and does not necessarily increase merely because particle size is reduced.

Stirring continually brings fresh solvent into contact with the sugar surface and removes solution that is already concentrated near that surface. This improves mass transfer and makes the sugar dissolve faster. Once equilibrium is reached at a fixed temperature, stirring does not necessarily increase the maximum amount that can dissolve.

Many common salts dissolve appreciably in water because water is polar and stabilizes their ions. Ordinary sand, which is largely composed of insoluble silicate minerals, dissolves only negligibly and usually settles as a solid. Oil and wax are mostly non-polar, so the alternatives do not represent their usual water solubility.

If undissolved solute remains after thorough mixing and enough time, the solvent has reached its dissolving capacity at that temperature. The liquid phase is then saturated, while the excess solid forms a separate phase at the bottom. This observation does not mean that there is no solvent or that the solution must be non-polar.