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What is Solubility?

solution example

Core Concepts

In this tutorial, you will be introduced to the properties of solubility and the varying factors that effect it. You will also be introduced to solutes, solvent, and solutions.

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Vocabulary

  • Solute – is the substance that dissolves in a solvent to form a homogeneous mixture.
  • Solvent – is the substance that is used to dissolve the solutes. This is usually the substance in higher quantity.
  • Solution – a homogeneous mixture of one or more solutes.

Understanding Solutions

To understand solubility first we must understand what makes up a solution. Solutions are made up of a solute and a solvent. To understand the difference between the two refer to the example of saltwater. Water is the solvent because that is the substance being used to dissolve the solute. The salt is the solute because it is the substance being dissolved into the water. Solvents are usually more abundant in the solution. Solutions can come in any forms of the three states of matter, that means that not all solutions are liquids, steel is a solution.

Example of what makes a solution

For something to be considered a solution the proportions of the different components (the solute and solvent) must be considered. In the photo below one cup is filled with sugar, another with salt, and the last with mud. The cup filled with mud is very distinguishable from the cups with salt and sugar. This is because the cup filled with mud is a heterogeneous mixture and not a solution. All solutions are homogeneous, meaning that the compositions are uniform throughout.

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Example of homogeneous solutions

Solubility

Solubility is the ability of a solute to dissolve in a solvent to form a solution. This is the property that allows things like sugar molecules to dissolve in a cup of coffee. Water is known as a “universal solvent” because it can dissolve most substances but there are a few exceptions.

Solubility forms new bonds between solute molecules and solvent molecules. This is the maximum concentration of solute that dissolves in a known concentration of solvent at a given temperature. Solutes are categorized into three categories: highly soluble, sparingly soluble, or insoluble. To be considered soluble a concentration of 0.1 g or more of a solute can be dissolved in a 100ml solvent. If a concentration below 0.1 g is dissolved in the solvent it is said to be sparingly soluble. Solubility can be expressed as the number of grams of solute in one liter of a saturated solution. For example, the solubility in water might be shown as 12 g/L at 25 oC. Molar solubility is the number of moles of solute per one liter of saturated solution. For example, 0.115 mol/L at 25 oC.

Factors That Affect Solubility

Temperature

One of the factors that may affect a solutes solubility is the variation in temperature. If the temperature is changed we can increase the solubility of a substance. A sparingly solid-liquid can be dissolved completely by increasing the temperature making that substance soluble at the given temperature. For example, sugar may be more soluble in water at higher temperatures. In the case of a substance in a gaseous state temperature will have the inverse effect. So, as the temperature increases the solubility of a gas in a liquid decreases.

solubility curve of copper sulfate
As temperature increases as does solubility

Bonds and Varying Forces

Forces and bonds may also affect a substances solubility. The type of bonds and intermolecular forces differ among each substance and its molecules. The term, “like dissolves like”, can be used to describe polar solvents dissolving polar solutes and non-polar solvents dissolving non-polar solutes. For example, since water is a polar solvent it can be used to dissolve hydrochloric acid since it is a polar molecule. Therefore, the stronger the intermolecular forces between solute molecules and solvent molecules the greater the solubility of the solute in the solvent.

Pressure

Pressure can influence solubility in various ways. When it comes to pressure, gasses are influenced a lot more than liquids. Pressure and solubility are directly related, so when partial pressure increases so does solubility. When pressure decreases the same happens for solubility. For example, water at room temperature has already dissolved gas molecules. When water gets boiled gas escapes in the form of bubbles thus decreasing the solubility of the gas.

Solubility of Solids in Liquids

It can be inferred that the solubility of a solid depends on the factors of the solute and the solvent. Because of the varying intermolecular forces in a substance, it has been seen that only polar solutes tend to dissolve in the polar solvent and non-polar solvents dissolve only non-polar solutes.

After a solid solute is placed into a solvent, the particles of the solute dissolve in the solvent, a.k.a dissolution. At this point, dynamic equilibrium is reached. The number of solute molecules in the solution will equal the number of particles that are flushed out of the solution. The concentration of the solute in the solution will remain constant at a given temperature and pressure. When no more solute can be dissolved into a solvent the solution is considered saturated. This is when molarity (M) and solubility become directly related. The molarity of the solute is equal to its solubility. The concentration of solute in such a solution is called its solubility at that given temperature and pressure. When no more solute can be added to the solution then the solution is unsaturated.