Solutions

A solution is a mixture in which one substance is completely dissolved in another. The substance that does the dissolving is called the solvent; the substance that gets dissolved is called the solute. Salt water is a solution (salt dissolved in water), as is the sugar in your tea, the air in your lungs (oxygen dissolved in nitrogen), and the bronze in an old coin (tin dissolved in copper). Solutions are everywhere in life and chemistry; understanding them is one of the most useful things you can learn.

• SolventDoes the dissolvingUsually the larger amount
• SoluteGets dissolvedUsually the smaller amount
• Most common solventWater"The universal solvent"
• ConcentrationHow much solutePer amount of solvent
• Saturated solutionCannot hold more soluteExtra solute settles out
• PhasesSolid, liquid or gasSolutions can be any phase

Solvent and solute

Every solution has two parts.

• The solvent is what does the dissolving. It is usually a liquid (most commonly water), but can also be a gas (nitrogen in air) or even a solid (the copper in bronze).
• The solute is what gets dissolved. It can be a solid (salt or sugar), a liquid (alcohol in beer), or a gas (oxygen in water).

The solute breaks up into tiny particles (often individual molecules or ions) and spreads evenly through the solvent. The mixture looks uniform throughout; you cannot see the solute as separate from the solvent.

Water: the universal solvent

Water dissolves more substances than any other common liquid, which is why it is often called "the universal solvent". Water molecules have a slightly positive end and a slightly negative end (they are polar), so they can break apart and surround many different kinds of molecules, especially salts and other charged substances.

Water is so good at dissolving things that almost everything you drink is a water-based solution: tea, coffee, fruit juice, fizzy drinks, even pure tap water has dissolved minerals in it. The same is true inside your body: blood, lymph, saliva, urine and the fluid in your cells are all water-based solutions full of dissolved substances.

Concentration

The amount of solute dissolved in a given amount of solvent is called the concentration.

• A dilute solution has very little solute.
• A concentrated solution has a lot of solute.
• A saturated solution contains as much solute as it can possibly hold. Any extra solute will not dissolve and stays as solid.

Concentration can be increased by adding more solute, or by removing solvent (for example, by evaporation). This is how sea salt is harvested: pump sea water into shallow ponds, let the water evaporate, and the salt is left behind.

Fact The Dead Sea in the Middle East is one of the saltiest bodies of water on Earth. It contains approximately 34% dissolved salts, almost 10 times saltier than ordinary sea water. The water is so dense that humans float effortlessly on its surface. No fish or plants can live in it (which is why it is called the Dead Sea), but it has been famous for thousands of years for its supposed healing properties.

What makes things dissolve?

Whether a substance dissolves in a particular solvent depends on the chemistry of both. The simple rule is "like dissolves like".

• Polar solvents (like water) dissolve polar solutes (salt, sugar) but not non-polar solutes (oil).
• Non-polar solvents (like cooking oil or petrol) dissolve non-polar solutes but not polar ones.

This is why oil and water do not mix. They are too different chemically. To clean greasy dishes, you need a detergent: a special molecule that is partly polar (water-loving) and partly non-polar (grease-loving), so it can hold both together.

How temperature affects solubility

• Most solids dissolve faster and more completely in hot water than cold. Sugar dissolves quickly in tea but slowly in iced water.
• Most gases are the opposite: they dissolve less in hot water. This is why warm fizzy drinks go flat faster than cold ones, and why fish struggle in warm rivers (less oxygen dissolved).
• Stirring speeds up dissolving by bringing fresh solvent into contact with the solute.
• Grinding into smaller pieces speeds up dissolving by giving the solvent more surface area to attack.

Did you know? The carbonated water in fizzy drinks is a solution of carbon dioxide gas dissolved in water, kept under pressure. When you open the bottle, the pressure drops and the dissolved CO2 immediately starts coming out of solution as bubbles. This is why opening a warm bottle of fizzy drink is so much more dramatic than a cold one: warm water holds less dissolved gas, so it all rushes out at once.

Deeper dive: how plants and animals depend on dissolved gases

One of the most important things solutions do is carry dissolved gases through living things. Fish breathe by absorbing oxygen dissolved in water through their gills. The amount of oxygen water can hold is small (typically only 5 to 10 mg per litre) and decreases sharply when the water is warm or polluted. This is why warming rivers and lakes is so dangerous for fish: even a small temperature rise can cause oxygen levels to drop below what fish need.

Plants are equally dependent. Carbon dioxide dissolved in the water inside leaves is what powers photosynthesis. Minerals dissolved in soil water are taken up by plant roots and used to build leaves, fruit and seeds.

Even your own breathing depends on solutions. The oxygen you breathe in passes from your lungs into your blood by dissolving in the watery lining of your alveoli. The CO2 your cells produce dissolves in the watery blood and is carried back to your lungs to breathe out. Without water as a solvent, none of this would work.

This is why scientists looking for life on other planets focus so much on finding liquid water. Other liquids could in theory work (some scientists have speculated about life in liquid methane lakes on Saturn's moon Titan), but water is so good at dissolving the chemistry of life that it remains our best bet for finding extraterrestrial life elsewhere in the solar system.

For more, see solubility and separating mixtures.