Stir sugar into tea and nothing chemically new is created — the sugar is still sugar, hiding among the water molecules, and a spoon, a filter, or a little patience is all it takes to separate them again.
Salt dissolved in seawater, sugar stirred into tea, the mix of gases you breathe as air — none of these involve a single chemical bond forming or breaking. Every substance keeps its own identity, just distributed among the others, which is exactly what makes mixtures fundamentally different from the chemical reactions covered earlier in this series.
A pure substance is either a single element or a single compound with a fixed, unchanging composition, such as pure gold or pure water. A mixture, by contrast, is a physical combination of two or more substances that are not chemically bonded together, meaning each substance keeps its own individual properties and can, at least in principle, be separated back out again.
| Mixture Type | Description | Example |
|---|---|---|
| Homogeneous | Uniform composition throughout; components not visibly distinguishable | Salt water, air |
| Heterogeneous | Non-uniform composition; components remain visibly distinct | Sand and iron filings, oil and water |
A solution is a specific type of homogeneous mixture in which one substance is completely and evenly dissolved within another. Every solution has two parts: the solute, the substance being dissolved and typically present in the smaller amount, and the solvent, the substance doing the dissolving and typically present in the larger amount. In salt water, salt is the solute and water is the solvent.
Air itself is a solution, though we rarely think of it that way: it's a homogeneous mixture of gases, roughly 78% nitrogen (the solvent, being the largest component) and 21% oxygen (a solute), along with small amounts of argon, carbon dioxide, and other gases.
Solubility is the maximum amount of a solute that can dissolve in a given amount of solvent at a specific temperature. Depending on how much solute has been added relative to this maximum, a solution falls into one of three categories.
| Type | Description |
|---|---|
| Unsaturated | Less solute than the maximum is dissolved; more could still dissolve |
| Saturated | The maximum possible amount of solute is dissolved at that temperature |
| Supersaturated | Contains more dissolved solute than should be possible; unstable, usually made by cooling a hot saturated solution carefully |
For most solids dissolved in a liquid, solubility increases as temperature rises, which is why sugar dissolves more easily in hot tea than in iced tea. Gases behave in the opposite way, dissolving less easily as temperature increases — which is exactly why a warm fizzy drink goes flat faster than a cold one.
Because the substances in a mixture are only physically combined, not chemically bonded, they can always be separated again using techniques that exploit their different physical properties.
| Technique | Property Used | Example |
|---|---|---|
| Filtration | Particle size | Separating tea leaves from tea |
| Evaporation | Boiling point of the solvent | Recovering salt from salt water |
| Distillation | Different boiling points of liquids | Separating alcohol from water |
| Chromatography | Different solubility or affinity for a surface | Separating pigments in ink |
Paper chromatography separates the different coloured pigments within a single ink sample, based on how far each travels up the paper.
Not every mixture fits neatly into "solution" or "visibly separate." A colloid contains particles larger than a true solution's dissolved particles but too small to settle out or be seen individually, giving milk and fog their cloudy, uniform appearance without ever fully dissolving. A suspension contains even larger particles that will eventually settle out under gravity if left undisturbed, such as muddy water clearing over time.
Desalination plants, which convert seawater into drinking water, rely on the same separation principles covered in this article at an industrial scale. Distillation-based plants boil seawater and condense the resulting water vapour, leaving the dissolved salt behind, while other modern plants use specialised filtration membranes fine enough to let water molecules through while blocking the larger, dissolved salt ions — supplying drinking water to millions of people in places where fresh water is scarce.
Every mixture, from the air filling your lungs to a glass of salt water, is a reminder that not every combination of substances needs a chemical reaction to be interesting. Sugar in tea, ink on paper, seawater turned to drinking water — in each case, the substances involved never stopped being themselves, waiting patiently to be separated back out by nothing more than a filter, some heat, or a sheet of paper. The next article in this series moves from mixtures to a far more precise kind of chemical counting: the mole and chemical calculations.
14 questions. Select an answer for each, then submit to see your score instantly.