Chemistry

Factors that Affect Solubility



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The maximum amount of solute that can dissolve in a certain amount of solvent is know as solubility. The solubility of a solute is measured by the concentration of the solution - as more solute is being added, the solvent's concentration will increase, eventually becoming saturated (the point where no more solute can be added.)

Several factors have an influence on the solubility of a solute, including temperature, pressure, polarity, and the nature of the solute or solvent.

TEMPERATURE

The solubility of a solute will increase as the temperature increases, in the case of most liquid solvents. Heat is required to break the bonds that hold the molecules of a solute together. The faster and more often the bonds break, the more of a solute that can dissolve. A temperature increase means a solubility increase for all substances that absorb energy when dissolving. But, for substances that release energy, as is the case for most gases, solubility will decrease as temperature increases.

PRESSURE

The solubility of solid and liquid solutes is not affected by changes in pressure. For gaseous solutes, however, an increase in pressure causes an increase in solubility. This is shown by Henry's Law, which states that the solubility of a gas is directly proportional to the pressure exerted on the gas. The equation P = KC (where P is pressure, K is a temperature-dependent constant, and C is the concentration) represents this.

The influence of pressure on a gas can be seen by the bubbles that form when a soda bottle cap is removed. Pressure is released, and gas (the bubbles) escape out of the solution in a process known as effervescence.

POLARITY

"Like dissolves like." As chemists often say, a solute will easily dissolve in a solvent with similar polarity. Polar solutes will dissolve in polar solvents; non-polar solutes will dissolve in non-polar solvents. Non-polar will not dissolve in polar, as is shown by the simple laws of polarity.

The particles of the solvent must be able to dissolve the particles of the solute. For two polar substances, the positive end of the solvent molecule will attract the negative end of the solute molecule. When the forces of attraction between them become stronger than the forces between surrounding solute molecules, the solvent pulls the solute in, and the solute becomes dissolved.

NATURE OF THE SOLUTE/SOLVENT

Some solutes more readily dissolve in solvents than others, while some solvents more readily dissolve solutes than others. The natures of these molecules affect the solubility in terms of amount that can be dissolved. For example, while just 1 gram of lead (II) chloride can dissolve in 100 grams of water, 200 grams zinc chloride can be dissolved in that same water.

It should be noted that solubility and rate of solution are two different things. Solubility refers to the total amount of a solute that can be dissolved. The rate of a solution refers to how fast a solute dissolves. Speed has no influence on solubility. Stirring (if a solution is stirred, the solute will dissolve more quickly), size of the particles (smaller particles dissolve more quickly, require less energy input), amount of solute already dissolved, and temperature are all factors that affect the rate of a solution.

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