What is Solubility?

Solubility is the property of a substance, known as the solute, that allows it to be dissolved in a solvent. Virtually all substances are either partially or completely soluble in a particular solvent. 

In many cases, a solute can also be a solvent, based on its proportion with a solute. For example, when there is a higher concentration of alcohol in a solution than water, the water becomes the solute rather than the solvent. 

What Factors Affect Solubility?

Several factors affect the solubility of a solute. Primarily, the type of solvent is the main factor.

  1. Polarity of the solvent or solute: For example, gasoline is a nonpolar solvent, meaning it cannot dissolve polar substances like table salt (sodium chloride). However, it can easily dissolve nonpolar substances such as styrofoams.
  1. Concentration of solute: A solute is typically a solid that dissolves in a liquid. However, it can also be a gas that dissolves in liquid or a solid that dissolves in another solid. Regardless of the type of solute, it generally has a limit, or saturation point. When this limit is reached, less or no additional solute can be dissolved at normal temperature and pressure.
  1. Temperature: Why do you need to use hot water to brew coffee? Well, the simple answer is solubility. More of the essential ingredients of coffee beans can be dissolved in water if the water is boiling. This is because an increase in temperature can make a solution supersaturated. Higher temperatures simply mean that particles move faster. The faster the particles, the higher the probability that the particles will bump into each other.
  1. Pressure: The solubility of gas in liquid is affected by pressure. In fact, gas solubility is directly proportional to the pressure. For instance, carbon dioxide is easily dissolved in soda water if some pressure is exerted. This is the reason why you should be careful when opening a bottle or a can of soda, which can overflow as the carbon dioxide gas escapes the container.
  1. Molecule or particle size: Substances with larger molecules are more difficult to dissolve, even in powder form. For example, table sugar, or sucrose, is relatively easy to dissolve in water even without applying heat. Cornstarch, on the other hand, is difficult to dissolve. Technically, both are carbohydrates, but the latter has larger molecules. If you try to dissolve cornstarch in water, instead of turning into a solution, it will eventually precipitate if left undisturbed. 
  1. Agitation or stirring of the mixture: Stirring the solvent as you add the solute will increase solubility. It has a similar but weaker effect as heating the solution. Basically, a solute dissipates more quickly in a solvent when stirred because the action of stirring distributes the solute particles more effectively.
Substances with larger molecules, like cornstarch, are more difficult to dissolve. If left undisturbed, they precipitate rather than forming a solution.

The Solubility of Liquid in Liquid

Many liquids are miscible, or highly soluble, in a particular solvent regardless of the concentration of the liquid solute. A good example of this is ethanol or ethyl alcohol, which is highly miscible with water.

Ethanol can proportionally exceed water in a solution, which technically reverses the solute-solvent interaction. When ethanol exceeds water in a solution, such as in the case of Spirytus Vodka, which is 192 proof or 96% alcohol by volume, it means that water is just 4% by volume, making water the solute rather than the solvent.

In highly miscible liquid-liquid solutions, the only way of separating the liquids is by distillation. In a nutshell, this is where the liquid in the solution with the lowest boiling point is made to evaporate and condensate so that it can be collected in a separate container.

Polarity is central in the solubility of liquids in other liquids. Gasoline, for example, is a homogeneous mixture of other petroleum products. The components of gasoline can be separated through distillation. When you know the boiling point of each component, specific liquids can be separated.

The Solubility of a Solid in Liquid

The solubility of solids in liquid is mainly determined by the polarity of the substances involved. Other factors, such as particle size, temperature, and pressure, also play important roles. Virtually all solids are soluble in varying degrees depending on the type of solvent. Water is generally considered a universal solvent because it can dissolve almost all polar and ionic substances.

However, you shouldn’t confuse solubility with the property of a substance to simply be dissolved in a solvent. Solubility doesn’t involve chemical reactions. For example, when sugar is dissolved in water, the sugar remains chemically the same. This is solubility. On the other hand, when a metal like aluminium is dissolved in hydrochloric acid, an exothermic chemical reaction occurs. The reaction can be summarised as follows:

HCl + Al → AlCl3 + H2

As you can see in the balanced chemical equation, hydrogen gas is liberated and aluminium chloride is formed when metal aluminium reacts with hydrochloric acid. The metal, however, isn’t only physically dissolved, but also chemically changed into a compound. 

Aluminium is also minimally soluble in water between pH 5.5 and pH 6.0. The concentration of dissolved aluminium is directly proportional to the change in pH above or below this range. The chemical reaction between water and aluminium can be summarised as follows:

2Al + 3H2O → 3H2 + Al2O3

In this example, the solid solute reacts with a solute that is amphoteric. At neutral pH, water is neither an acid nor a base, but it can either donate or accept hydronium ions depending on the solute.

The Solubility of Gases in Liquid

Gases are easily soluble in water and other liquid solvents if sufficient pressure is applied. At normal room temperature and sea-level atmospheric pressure, air easily dissolves in water if the water is simply stirred. This is the reason why gas bubbles are formed before water reaches its boiling point. Similarly, if you just allow water to sit undisturbed in a container, you’ll notice that some bubbles form on the inner surface of the container that’s in contact with the water.

When it comes to liquid and solid solutes, the polarity of the solvent matters a lot. However, this isn’t the case with the solubility of gases in liquids. For example, methane is a nonpolar gas that isn’t only soluble, but miscible in water at 3.5 GPa of pressure at 100°C of temperature.

The solubility of substances is determined by various factors which are essential in determining various physical and biological processes.

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