Solubility

In chemistry, solubility is the ability of a substance, called the solute, to mix and form a solution with another substance, called the solvent. When a substance can’t dissolve, we call this insolubility.

The amount of a substance that can dissolve in a solvent is measured when the solution is saturated, meaning no more of the solute can dissolve. At this point, the solute and solvent are in balance, 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 factors, such as the types of substances involved, their pH, temperature, and pressure. It is important in many areas of science and everyday 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 take place.

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. These terms can change depending on the use, too. 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 in 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. For instance, a liquid with cobalt can make different kinds of cobalt mixes that change into each other.

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, even though they have the same basic recipe of atoms.

Several things can change how much a substance will mix with a solvent. The pull between molecules and changes in energy play a big role. Temperature and pressure can also 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 because breaking apart needs energy. But for gases, they usually mix less as it gets warmer. Some special salts mix less when it’s warmer, which is called “inverse” solubility.

Pressure

For solids and liquids, pressure doesn’t change solubility much. But for gases dissolved in water, pressure can matter a lot. For example, less pressure can cause some substances to come out of solution in oil fields.

Solubility of gases

Henry's law helps us understand how gases dissolve in liquids. It tells us that the amount of gas that can dissolve in a liquid is directly related to the pressure of that gas above the liquid. This means if you increase the pressure of the gas, more of it will dissolve.

The solubility of gases can also be affected by other factors like temperature and the presence of certain chemicals in the liquid. For example, the amount of carbon dioxide that can dissolve in seawater changes with temperature and the water's chemistry. This process plays a role in Earth's climate, as changes in ocean temperature can affect the amount of carbon dioxide in the atmosphere.

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, matching what the equations predict.

Theories of solubility

Solubility constants help us understand how much of certain substances can dissolve in water. These constants show the balance between the parts of a substance that dissolve and the parts that stay solid. Temperature can change these constants.

Other ways to think about solubility include special theories for polymers and methods using physical properties. Scientists also measure how well substances dissolve in water compared to other liquids to understand their chemical behavior. The energy needed for a substance to dissolve is also an important factor.

Applications

Solubility is very important in many areas of science and everyday life. It helps us in tasks like getting useful materials from ores, using medicines, and understanding 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 slightly negative oxygen in water (H₂O). The negative part of a salt, like Cl⁻, is pulled toward the slightly positive hydrogen atoms in water.

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

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, making it not very soluble. In comparison, table salt (NaCl) is much more soluble.

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 describes how well an alloying element mixes into a base metal without creating a new separate material. The solvus, or solubility line, shown on a phase diagram, marks the most of a component that can mix into another and remain a solid solution.

In a metal’s crystal structure, the added element can either replace part of the metal’s structure (substitutional) or fit into spaces between the metal’s atoms (interstitial).

In making tiny electronic parts, solid solubility means the most impurities that can be added to a material without changing its properties much.

For solid compounds—not single elements—the ability of one element to mix into another can change based on what other materials form during the process. Studying these mixtures can be tricky because many different combinations are possible, and finding the exact limit of how much can mix in requires testing many samples. Knowing the common types of defects in the material can help reduce the number of tests needed.

Incongruent dissolution

Some substances dissolve in a way where what dissolves does not match what was originally there. This can change the solid and sometimes create 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 understand these processes.

Solubility prediction

Solubility is important in many areas of science and industry. It helps us understand how well substances mix, especially with water, which is crucial for life and transportation. Predicting solubility accurately can save a lot of money, especially in making medicines.

Scientists use different methods to predict solubility. Some methods rely on physical theories and thermodynamic cycles, which involve calculating energy changes during processes like turning a solid into a gas or mixing substances. These methods help estimate how much of a substance can dissolve in a solvent. There are also well-known equations that help predict solubility based on a substance’s properties.