Solubility

Solubility is a measure of how much of a particular substance (the solute) will dissolve in a particular solvent at a particular temperature. Sugar is highly soluble in water (about 2 kg per litre at room temperature). Salt is moderately soluble (about 360 g per litre). Sand is not soluble in water at all. Solubility depends on the chemistry of the two substances and is also strongly affected by temperature, pressure and stirring. Knowing what dissolves in what is one of the most useful pieces of practical chemistry.

• Solubility of sugar in waterApproximately 2 kg/LAt room temperature
• Solubility of salt in waterApproximately 360 g/LAt room temperature
• Solubility of CO2 in waterApprox. 1.4 g/LHigher in cold water
• Rule of thumbLike dissolves likePolar with polar, non-polar with non-polar
• Speed depends onTemperature, stirring, surface area
• Saturated solutionCannot dissolve more soluteExtra solute settles out as solid

What is solubility?

Solubility tells you the maximum amount of a solute that will dissolve in a given amount of solvent at a given temperature. You cannot dissolve more than that, no matter how long you stir. Above the limit, the extra solute simply settles to the bottom as a solid (or floats to the top, or escapes as a gas, depending on what it is).

The "like dissolves like" rule

Whether two substances are soluble in each other depends on whether they have similar chemistry. The simple rule of thumb is "like dissolves like".

• Polar solvents (those with a slightly positive and slightly negative end, like water) dissolve polar solutes (salt, sugar, alcohol).
• Non-polar solvents (like cooking oil, petrol) dissolve non-polar solutes (other oils, fats, wax).
• Polar and non-polar substances do not dissolve each other well. This is why oil and water do not mix.

How temperature affects solubility

Temperature has a big effect, but it works differently for solids and gases.

• Solids dissolve better in hot water. Sugar disappears in hot tea but lingers in iced water. This is why fudge and toffee are made by dissolving lots of sugar in hot water, then cooling it slowly.
• Gases dissolve better in cold water. This is why warm fizzy drinks go flat faster: the CO2 cannot stay dissolved at higher temperatures.
• This also explains why fish struggle in warm rivers: less oxygen dissolved per litre means less to breathe.

How pressure affects solubility

Pressure mainly affects how much gas can dissolve in a liquid. Higher pressure means more gas can dissolve. This is the entire principle behind fizzy drinks: CO2 is forced to dissolve into the water under high pressure in the factory, then sealed in. When you open the bottle, the pressure drops and the CO2 immediately starts coming out of solution as bubbles. The result is the satisfying fizz of a freshly opened drink.

The same effect causes decompression sickness (also called "the bends") in divers. When a diver goes deep, the high pressure dissolves extra nitrogen into their blood. If they surface too quickly, the pressure drops fast and the nitrogen bubbles out of solution inside their body, causing painful and dangerous bubbles in their tissues. Slow ascent allows the gas to escape gradually through the lungs without forming bubbles.

Fact The world record for the fastest sugar dissolving is held by microwaved water. Hot water can dissolve over 5 kg of sugar per litre (more than two and a half times the room-temperature limit). When you cool such a saturated solution slowly, the excess sugar comes out as beautiful sugar crystals: the basis of rock candy and many other sweets.

Why oil and water do not mix

The classic example of insoluble substances is oil and water. The reason they refuse to mix is in their chemistry.

• Water molecules are polar: each has a slightly positive end (the hydrogens) and a slightly negative end (the oxygen). Water molecules attract each other strongly.
• Oil molecules are non-polar: long chains of carbon and hydrogen with no charge separation. Oil molecules do not interact with water molecules.
• When you mix them, the water molecules cling to each other and exclude the oil. The oil floats to the surface, where its molecules cling to each other.

To get oil and water to mix, you need a detergent or surfactant: a molecule with one polar end and one non-polar end. The polar end loves water and the non-polar end loves oil, so the detergent links the two together. This is how soap cleans grease off plates and how washing-up liquid breaks up oily spills.

Did you know? A supersaturated solution is a remarkable thing: a solution containing more solute than it should be able to. They are made by dissolving lots of solute in hot water then cooling it very slowly without disturbing it. The solution is in an unstable state and waiting for the slightest trigger (a dust particle, a sudden tap on the container) to crystallise dramatically. Hand-warmers full of sodium acetate use this principle: clicking a metal disk inside triggers crystallisation, which releases the heat that had originally been used to dissolve the salt.

Deeper dive: how seawater stays so salty

The oceans contain approximately 3.5% dissolved salts, mostly sodium chloride (table salt) but also magnesium chloride, calcium sulphate, potassium chloride and many trace minerals. Total dissolved salt in the world's oceans adds up to roughly 50 million billion tonnes: enough to cover all the world's land in a layer 150 metres thick.

Where did all that salt come from, and why does the sea keep getting saltier? The salts come from weathering of rocks on land. Rain dissolves tiny amounts of mineral from rocks; rivers carry the dissolved minerals to the sea. Once there, the water can evaporate (going back to the air, leaving the salt behind) but the dissolved salts cannot.

Over billions of years, the slow accumulation of dissolved minerals has built the oceans up to their current saltiness. The sea is slowly becoming saltier still, but very slowly: a few parts per million per million years. The Dead Sea is much saltier than the open ocean because it has no outlet, so all the water that flows in evaporates and leaves all its dissolved salts behind. The Great Salt Lake in Utah works the same way.

Some salt is removed from the oceans by being incorporated into sediments and rocks on the seabed, then eventually subducted into Earth's mantle. This is why the sea is at a relatively stable level of saltiness rather than getting saltier without limit. The cycle of dissolved minerals between rocks, rivers and seas is one of the biggest chemical cycles on the planet.

For more, see solutions and suspensions and colloids.