3: Solubility

A substance that dissolves only to a small, non-zero extent is described as sparingly soluble or slightly soluble. The term insoluble is often used informally for negligible dissolution, but it is less precise when some measurable solubility exists. Infinite solubility and saturated solvent are unrelated descriptions.

A hot saturated solution can hold a larger amount of many solids. When it is cooled, the solubility of the desired solid often decreases, making the excess solute separate as crystals. Slow, controlled cooling can improve crystal formation and purity. Cooling does not transform the solvent into a metal or make solubility infinite.

At a fixed temperature, an unsaturated solution contains less solute than the maximum amount that can dissolve, so additional solute can still dissolve. A saturated solution has reached that equilibrium limit; extra solute remains undissolved unless conditions change. Both types contain solvent and dissolved solute, so the other statements are incorrect.

The rate of dissolving describes how quickly a substance reaches the solution, whereas solubility describes the maximum equilibrium amount that can dissolve under specified conditions. Stirring, smaller particles, and heating may change the rate without producing the same change in equilibrium solubility. Therefore fast dissolution alone does not prove higher solubility.

The nature of the solute and solvent determines the types of intermolecular attractions involved. Temperature affects most solid solubilities and generally affects gas solubility oppositely, while pressure is especially important for gases. Identifying these variables first helps select the correct law, graph, or proportional calculation.

At constant temperature, Henry’s law states that the concentration or mole fraction of a gas dissolved in a liquid is proportional to the partial pressure of that gas above the liquid. In simplified form, p = KHx, so increasing p increases x when KH is fixed. Therefore, higher gas pressure leads to more dissolved gas, within the range where the law applies.

Henry’s law in mole-fraction form is p = KHx. At constant temperature, KH remains constant, so x = p/KH and the dissolved-gas mole fraction is directly proportional to partial pressure. If p changes from p to 2p, x changes from x to approximately 2x, provided the solution remains dilute and the law’s applicable range is not exceeded. Thus the mole fraction nearly doubles.

Dissolving a gas in a liquid is generally accompanied by release of heat. According to Le Chatelier's principle, lowering temperature favors the exothermic direction, so more gas remains dissolved. Heating promotes escape of the gas. The trend is not caused by a colour change, loss of liquid mass, or automatic ionization of the solvent.

A sealed soft drink contains carbon dioxide under pressure. Lower temperature generally increases the solubility of this gas in the liquid, while pressure also helps retain it. Therefore cooling reduces the tendency of carbon dioxide to escape, although opening the container still lowers pressure and can produce bubbles.

Before opening, carbon dioxide is dissolved under a relatively high pressure, so its equilibrium solubility is high. Opening the bottle sharply lowers the gas partial pressure above the liquid. The new equilibrium supports less dissolved gas, so excess carbon dioxide leaves the solution as bubbles until a lower-pressure balance is reached.

Oxygen is a gas, and its solubility in water generally decreases as temperature rises. Consequently, warm water may contain less dissolved oxygen available for respiration by fish and other aquatic organisms. If oxygen falls too low, biological stress can result. The change is not due to oxygen becoming a metal or pressure becoming infinite.

Atmospheric pressure is lower at high altitude. For an open drink, the gas above the liquid is therefore at a lower partial pressure, and Henry's law predicts a lower equilibrium solubility. The liquid then contains more gas than the new equilibrium permits, so gas escapes until balance is restored. Temperature is not necessarily zero.

Gas solubility in a liquid generally increases with pressure and decreases with temperature. A high pressure raises the gas concentration at the liquid surface, while a low temperature reduces the tendency of dissolved gas to escape. Thus high pressure combined with low temperature is the most favorable choice among the given conditions.

Saturation means that the solution contains the maximum equilibrium amount of solute possible under specified conditions. Since solubility commonly changes with temperature, changing the temperature can permit more solute to dissolve or cause some solute to crystallize. Thus a solution saturated at one temperature may not be saturated at another.

Many solids are more soluble in a hot solvent than in a cold one. A hot saturated solution therefore contains more solute than can remain dissolved after cooling. The excess solute separates from the solution and may form crystals, especially during slow cooling. The exact trend is substance-dependent, so ‘may’ is more accurate than ‘must’ for all solids.

At a given temperature, a normal saturated solution contains the equilibrium maximum amount of dissolved solute. A supersaturated solution temporarily contains more than that amount, usually because of careful cooling or evaporation without crystallization. It is metastable and can crystallize after disturbance, so it is not necessarily more stable.

A supersaturated solution contains excess dissolved solute in a metastable state. The added crystal supplies a nucleation surface, allowing solute particles to arrange into the solid lattice. Crystallization can then propagate and remove the excess solute from the liquid. The seed does not make the solvent a gas or increase solubility to infinity.

On a standard solubility graph, temperature is on the horizontal axis and solubility is on the vertical axis. If the solubility value becomes smaller as temperature increases, the plotted line or curve moves downward from left to right and has a negative slope. The graph remains meaningful; its direction simply represents the inverse temperature trend.

First read the labels and units on both axes. Usually, temperature is shown on the horizontal axis and solubility on the vertical axis, often as grams of solute per 100 grams of solvent. Only after identifying these quantities should the curve be used to obtain a value. Colour, paper thickness, and question number do not provide scientific information.

Solubility is given as 40 g of salt for every 100 g of water at a fixed temperature. The amount of water changes from 100 g to 50 g, which is one-half. Therefore, the maximum salt that can dissolve also becomes one-half: (40/100) × 50 = 20 g. Thus, option A is correct. The other values do not follow the required direct proportion.

Oil is mainly non-polar, whereas water is polar, so the interaction between oil and water is not favorable enough for good mixing. Petrol contains non-polar hydrocarbons and therefore interacts more suitably with oil. This illustrates the useful rule “like dissolves like,” although it is a general guideline rather than an absolute law. Mass, colour, and pressure are not the explanation here.

Stirring moves fresh solvent into contact with the sugar surface and carries dissolved sugar away from that surface. This reduces local concentration gradients and increases the rate at which sugar dissolves. Once equilibrium is reached, however, the maximum amount dissolved at a fixed temperature is generally unchanged. Stirring does not alter sugar’s identity or water’s polarity.

The absolute mass of solute that dissolves depends on how much solvent is present. For example, twice as much solvent can generally dissolve twice the amount at the same temperature. Therefore, a fair comparison uses the same solvent mass, commonly 100 grams, and also keeps temperature and other conditions fixed. Colour, names, and zero pressure are unrelated to this requirement.

At a specified temperature, solubility represents the maximum amount of solute that can remain dissolved in a given amount of solvent at equilibrium. If more solute is added than this limit, the solvent cannot dissolve all of it. The excess normally remains as solid, while the liquid portion becomes saturated. Adding more material does not make solubility infinite.

A saturated solution contains the maximum amount of solute that can dissolve under the specified conditions, especially at a given temperature and solvent amount. If a small additional amount of solute is added and remains undissolved, the solution has reached saturation. Saturation is not identified by colour, by being dilute or concentrated alone, or by the physical state of the solute.