What Is a Solution?
A solution is a type of mixture formed when one substance (the solute) dissolves completely in another substance (the solvent). The result is a clear, uniform (homogeneous) mixture.
Key vocabulary:
- Solute — the substance that dissolves (e.g. table salt)
- Solvent — the substance that does the dissolving (e.g. water)
- Solution — the solute + solvent mixture
- Aqueous solution — a solution where water is the solvent
How Does Dissolving Work?
When a solute dissolves, its particles are surrounded and pulled apart by the solvent particles. The solute particles spread out evenly to fill the whole volume of the solvent.
For example, when salt (NaCl) dissolves in water:
- Water molecules (which are slightly polar) are attracted to the sodium (Na⁺) and chloride (Cl⁻) ions.
- The water molecules pull the ions away from the crystal surface.
- The ions spread throughout the water, forming a uniform solution.
Dissolving is a physical change, not a chemical one. The salt is still NaCl when dissolved — you can recover it by evaporating the water. However, dissolving does not always look physical — some substances (like acids) react with water as they dissolve, releasing heat or changing chemically.
Describing Solutions
Dilute solution: Contains a small amount of solute relative to the amount of solvent. Concentrated solution: Contains a large amount of solute relative to the amount of solvent. Saturated solution: No more solute can dissolve at that temperature — the solution has reached its maximum concentration.
Water as a Universal Solvent
Water is often called the "universal solvent" because it dissolves more substances than any other common liquid. This is because water molecules are polar (one end is slightly positive, the other slightly negative), which allows them to attract and surround ionic and polar molecules.
However, water does not dissolve everything:
- Non-polar substances like oil, wax, and most plastics do not dissolve in water.
- To dissolve non-polar substances, a non-polar solvent (like hexane or acetone) is needed.
"Like dissolves like" is a helpful rule in chemistry. Polar solvents (like water) dissolve polar and ionic substances. Non-polar solvents (like cooking oil) dissolve non-polar substances. This is why water and oil do not mix — they are very different types of molecules.
Solvents Other Than Water
| Solvent | Common use |
|---|---|
| Water | Drinks, cooking, biological systems |
| Ethanol (alcohol) | Perfumes, medicines, hand sanitiser |
| Acetone (propanone) | Nail polish remover |
| Petrol/hexane | Degreasing engines, dry cleaning |
Real world: Sugar dissolving in hot tea
When you stir sugar into hot tea, you are making an aqueous solution — sugar is the solute, hot water is the solvent. Hot water dissolves sugar faster and in greater quantities than cold water, because the faster-moving water molecules pull sugar particles apart more efficiently. When tea cools, you might notice undissolved sugar grains at the bottom if you added too much — the solution became saturated at room temperature, but had dissolved more sugar when hot.
Real world: Hand sanitiser uses ethanol, not water
Hand sanitisers contain 60–70% ethanol (alcohol) rather than water. Ethanol is the solvent that dissolves the oils and fats in bacterial cell membranes, killing the bacteria. Water alone cannot do this — water is polar and cannot dissolve non-polar fats effectively. This is "like dissolves like" in action: ethanol is less polar than water and can dissolve fatty compounds. The same principle explains why acetone (nail varnish remover) dissolves nail polish, but water cannot.
Real world: Seawater — Earth's largest aqueous solution
Earth's oceans contain about 3.5% dissolved salts — mainly sodium chloride, magnesium sulfate, and potassium chloride. This makes seawater a complex aqueous solution. The dissolved ions affect everything from the boiling point of seawater to how marine animals regulate their body fluids. Desalination plants in water-scarce countries (Saudi Arabia, UAE, Australia) use distillation or reverse osmosis to remove the dissolved salts and produce drinking water from seawater.
Where do you see this in real life? Sugar dissolving in hot drinks (solute in solvent), hand sanitiser using ethanol to dissolve fats (non-polar solvent), oil and vinegar dressing separating in a bottle (non-polar oil won't dissolve in polar water), contact lens solution keeping proteins dissolved — solutions are the most common form of matter in living systems and everyday chemistry.
Practice Problems
50 g of sugar is dissolved in 200 cm³ of water. Name the solute and the solvent, and describe the resulting solution.
Show answer
Why does oil not dissolve in water even if you stir the mixture vigorously?