Solutions, Solutes, and Solvents
A solution is a homogeneous mixture in which a substance called the solute is dissolved in another substance called the solvent. In brief: in salt water, salt is the solute and water is the solvent; the solvent is usually the component present in the larger amount, and the solute is the one that dissolves.
Solutions are everywhere: seawater, soft drinks, the air we breathe and the blood that transports nutrients are all solutions. This article defines the key terms, explains how solutions form, shows the different kinds of solutions in a table, explains how to express the amount of solute with a simple concentration calculation and corrects common errors.
What are a solute, a solvent and a solution?
The solute is the substance that is dissolved. The solvent is the substance that does the dissolving, normally the one present in the larger amount. The solution is the whole mixture. When 5 g of sugar is stirred into 100 g of water, sugar is the solute, water is the solvent and the sugar-water mixture is the solution.
When water is the solvent, the solution is called an aqueous solution, and the symbol (aq) is written after the formula of the solute in chemical equations. Water is such a common solvent that it is sometimes called the universal solvent, although it does not dissolve everything.
How does a solution form?
When a solute is added to a solvent, solvent particles surround the solute particles and pull them away from the solid. The particles of the solute then spread out evenly among the particles of the solvent. This process is called dissolving. In the dissolving of salt in water, the water molecules separate the sodium and chloride ions from the crystal.
Stirring and heating can speed up dissolving, but they do not change the identity of the substances. After dissolving, the solute can be recovered, for example by evaporating the solvent.
What types of solutions exist?
Solutions are not limited to solids dissolved in liquids. Any state of matter can act as solute or solvent.
| Solute | Solvent | Example |
|---|---|---|
| Solid | Liquid | Sugar in water |
| Liquid | Liquid | Ethanol in water (vodka) |
| Gas | Liquid | Carbon dioxide in soda |
| Gas | Gas | Oxygen in nitrogen (air) |
| Solid | Solid | Zinc in copper (brass) |
| Liquid | Solid | Mercury in silver (dental amalgam) |
What are the properties of a solution?
- Uniform: the composition is the same throughout.
- Transparent: a solution may be colored, but it is clear and does not scatter light noticeably.
- Stable: the solute does not settle out when the solution stands.
- Passes through filter paper: the dissolved particles are too small to be caught.
- Variable composition: the amount of solute can be changed, within a limit set by solubility.
How is the amount of solute described?
A solution with a small amount of solute is dilute, and one with a large amount is concentrated. For precise work, concentration is expressed in numbers. A simple measure is the mass percent:
mass percent = (mass of solute ÷ mass of solution) × 100%
Other measures, such as molarity, are introduced in later chemistry topics.
Worked example: mass percent of a solution
- Find the mass of the solution. 15 g of sodium chloride is dissolved in 135 g of water. The solution mass is 15 + 135 = 150 g.
- Apply the formula. (15 ÷ 150) × 100% = 10%.
- Find the solute needed for a target. To make 250 g of a 4.0% sugar solution, the mass of sugar is 0.040 × 250 = 10 g, and the mass of water is 250 − 10 = 240 g.
The target solution is made by dissolving 10 g of sugar in 240 g of water. Notice that the masses of solute and solvent add up to the mass of the solution, but volumes do not always add up exactly.
Where are solutions used?
- Medicine: intravenous fluids are carefully made salt solutions.
- Cooking: brine, syrup and coffee are solutions.
- Cleaning: detergents work in aqueous solutions.
- Industry: many chemical reactions take place in solution because the particles mix easily.
How did scientists explain solutions?
In the late 1800s, Svante Arrhenius proposed that salts split into charged particles, called ions, when they dissolve in water. He used this idea to explain why salt solutions conduct electricity. His theory of electrolytic dissociation, published in 1884, was at first received with doubt but was later widely accepted, and Arrhenius received the Nobel Prize in Chemistry in 1903.
Which mistakes are common?
Mistake 1: Reversing solute and solvent
The solute is the dissolved substance. In a syrup, sugar is the solute and water is the solvent.
Mistake 2: Thinking that a solution must be a liquid
Air and brass are solutions too. A solution can be a gas or a solid.
Mistake 3: Believing that a solution must be colorless
Copper sulfate in water is blue and clear. Clear means transparent, not colorless.
Mistake 4: Adding the volumes of solute and solvent
When ethanol and water are mixed, the final volume is slightly smaller than the sum. Use masses when you need exact additions.