Solubility

In chemistry, solubility is how well a substance, called the solute, can mix with another substance, called the solvent, to make a solution. If a substance can’t dissolve, we call this insolubility.

The amount of a substance that can dissolve is measured when the solution is saturated. This means no more of the solute can dissolve. At this point, the solute and solvent are balanced, called the solubility equilibrium. Sometimes, two substances can mix in any amount and are said to be "miscible in all proportions."

Solubility depends on many things, such as the types of substances, their pH, temperature, and pressure. It is important in many areas of science and daily life, including geology, biology, physics, oceanography, engineering, medicine, agriculture, and even activities like painting, cleaning, cooking, and brewing. Water is the most common solvent where many important chemical reactions happen.

Quantification of solubility

The solubility of a substance in a liquid can be measured in different ways, usually by how much of the substance can dissolve. One common way is to measure how many grams of the substance can dissolve in 100 millilitres of the liquid. Another way is to measure how many grams can dissolve in a kilogram of the liquid. Sometimes, scientists use the number of tiny parts called moles instead of grams.

Solubility can also be measured based on the whole mixture, not just the liquid. For example, it might be measured as moles of the substance in one litre of the mixture. In some special cases, it is measured as a part of the whole, like how many moles of the substance there are compared to all the moles in the mixture.

When measuring liquids or gases dissolving in other liquids, sometimes the amount is measured in litres instead of grams. Converting between these different ways of measuring solubility can be tricky because the final space the mixture takes might not be the same as the space the liquid and the substance took before mixing.

Qualifiers used to describe extent of solubility

The amount of a substance that can dissolve in another can vary a lot. Some things mix completely, like ethanol in water. Others hardly mix at all, like titanium dioxide in water. Different words are used to describe how much something can dissolve, depending on what you need it for.

For instance, the U.S. Pharmacopoeia uses certain terms based on how much of a substance dissolves in a given amount of liquid. One source says something is "insoluble" if less than 0.1 g of it dissolves in 100 mL of liquid.

Molecular view

Solubility happens when a substance mixes with another substance. It is a balance between dissolving and coming back together, like when a solid forms again. This balance means the amount of dissolved and undissolved material stays the same. If you take away the liquid, the dissolved part comes back.

Sometimes, the word "solubility" is used when the substance changes when it mixes. For example, some metals mix with special liquids but change forever. When a substance dissolves, it can change into different forms in the liquid.

Factors affecting solubility

Solubility is how well a substance mixes with another, called a solvent. For example, different forms of calcium carbonate may mix with water differently.

Several things can change how much a substance will mix with a solvent. Temperature and pressure can make a big difference. For example, some substances mix better when it’s hot, while others mix better when it’s cold. Also, if there are already other substances in the water, this can change how much more will mix in.

Temperature

How much a substance mixes with water can change with temperature. For most solids and liquids, they mix better when it’s warmer. But for gases, they usually mix less as it gets warmer. Some special salts mix less when it’s warmer.

Pressure

For solids and liquids, pressure doesn’t change solubility much. But for gases dissolved in water, pressure can matter a lot.

Solubility of gases

Henry's law helps us learn how gases mix with liquids. It says that more gas will dissolve if the pressure of the gas above the liquid is higher.

The way gases dissolve can also change with temperature and the chemicals in the liquid. For example, the amount of carbon dioxide that mixes into seawater depends on the water's temperature and its chemistry. This process is important for Earth's climate because changes in ocean temperature can affect how much carbon dioxide is in the air.

Polarity

A useful idea for guessing if something will dissolve is "like dissolves like". This means a substance will dissolve best in another substance that is similar to it. The ability of a liquid to dissolve other things mainly depends on how polar it is.

For example, a very polar substance like urea mixes well with very polar water, but not as well with fairly polar methanol, and almost not at all with non-polar benzene. On the other hand, a non-polar substance like naphthalene does not mix with water, mixes fairly well with methanol, and mixes very well with non-polar benzene.

A simple ionic compound like sodium chloride, also known as common salt, dissolves easily in a highly polar liquid like water, which is why the sea is salty—it has collected dissolved salts over many years.

Rate of dissolution

When a solid dissolves in a liquid, it doesn't happen right away. The speed at which it dissolves can depend on things like how the solid is shaped and its surface area. Scientists use special formulas to describe this speed, such as the Noyes–Whitney equation.

The rate of dissolution can change a lot between different materials. Usually, materials that dissolve easily also dissolve quickly.

Theories of solubility

Solubility constants help us learn how much of some materials can mix into water. These numbers show the balance between the parts of a material that dissolve and the parts that stay solid. Temperature can change these numbers.

There are other ideas about solubility for special materials like polymers. Scientists also test how well things dissolve in water compared to other liquids. The energy needed for a material to dissolve is also very important.

Applications

Solubility is important in science and everyday life. It helps us get useful materials from ores, use medicines, and understand how pollution moves.

We often describe substances by how well they dissolve. For example, a blue dye called indigo doesn’t mix with water or alcohol, but it will mix with certain other liquids like sulfuric acid.

Knowing how well things dissolve helps us separate mixtures. For instance, we can mix salt (sodium chloride) with water and then filter out the undissolved sand. Chemists use solubility to separate the products they make from the materials they start with and from unwanted by-products.

One way scientists separate mixtures is by using two liquids that don’t mix. For example, when making benzoic acid from phenylmagnesium bromide and dry ice, the benzoic acid will dissolve in an organic liquid such as dichloromethane or diethyl ether, while other materials stay in the water layer. This method, called liquid–liquid extraction, is a key tool in synthetic chemistry.

Differences in how well things dissolve can also affect natural processes. For example, minerals can form in places where hot water flows deep in the Earth, creating valuable deposits. Over very long periods, low solubility can lead to the formation of caves and other landscape features.

Solubility of ionic compounds in water

Some ionic compounds, called salts, can dissolve in water. This happens because of the pull between positive and negative charges. For example, the positive part of a salt, like Ag⁺, is attracted to the oxygen in water (H₂O). The negative part of a salt, like Cl⁻, is pulled toward the hydrogen atoms in water.

But there is a limit to how much salt can dissolve in water. This limit is called solubility. It depends on the type of salt and the temperature.

Here’s a simple way to find out how much of a salt like AgCl can dissolve in one liter of water:

K_sp = [Ag⁺] × [Cl⁻] / M²

For AgCl, K_sp = 1.8 × 10⁻¹⁰. This shows that only a tiny amount of AgCl can dissolve in water, so it does not dissolve well. In comparison, table salt (NaCl) dissolves much more easily.

Solubility of organic compounds

The rule for dissolving organic materials is that similar things mix well together. For example, petroleum jelly dissolves in gasoline because both are made of non-polar hydrocarbons. However, petroleum jelly does not dissolve in ethyl alcohol or water because these liquids are too polar. Similarly, sugar does not dissolve in gasoline because sugar is too polar compared to gasoline. You can separate a mix of gasoline and sugar using filtration or extraction with water.

Solid solution

In metallurgy, a solid solution shows how well one material mixes into another without making a new material. The solvus, or solubility line, on a phase diagram, shows the most of one part that can mix into another and stay as a solid.

In a metal’s crystal shape, the added material can either take the place of some of the metal’s parts (substitutional) or fit into spaces between the metal’s tiny parts (interstitial).

When making very small electronic parts, solid solubility tells us the most impurities we can add to a material without changing how it works much.

Incongruent dissolution

Some substances dissolve in a way that what disappears does not match what was there before. This can change the solid and sometimes make a new solid. For example, when albite dissolves, it can form gibbsite. This kind of dissolving is important in geology because it helps create metamorphic rocks.

Both types of dissolving can create new solids. In Materials Science, scientists use chemical composition phase diagrams to study these processes.

Solubility prediction

Solubility is important in science and industry. It helps us understand how well substances mix, especially with water. This is important for life and transportation. Predicting solubility can save money, especially when making medicines.

Scientists use different ways to predict solubility. Some ways use physical theories and calculations. These help estimate how much of a substance can dissolve in a liquid. There are also equations that help predict solubility using a substance’s properties.