Solutions: Solute, Solvent and Solution

Introduction

Stir a spoonful of sugar into a cup of tea and it seems to vanish. The tea still tastes sweet, so the sugar has not disappeared — it has dissolved . Chemists use three precise words to describe what has happened: the solute , the solvent and the solution . These words appear in almost every topic in chemistry, from separating mixtures to measuring concentrations, so it pays to learn them carefully now and use them correctly every time.

Core explanation

The solute is the substance that dissolves. In sweet tea the sugar is the solute; in seawater the salts are the solutes.

The solvent is the substance that does the dissolving. It is usually the substance present in the larger amount. Water is by far the most common solvent, which is why it is sometimes called the "universal solvent" — although it does not dissolve everything.

The solution is the mixture that forms when the solute dissolves in the solvent. The simple word equation is:

solute + solvent → solution

Solutions are homogeneous mixtures. Once dissolved, the solute is spread evenly through the solvent as individual particles far too small to see. Every part of the solution has the same composition, so a sip from the top of a cup of sweet tea is as sweet as a sip from the bottom. A true solution is usually transparent (see-through), although it may be coloured: copper(II) sulfate solution is clear but blue.

Solutions are mixtures, not compounds. The solute and solvent are not chemically bonded together. You can make a weak or a strong sugar solution simply by changing how much sugar you add, so the composition is not fixed. You can also get the solute back unchanged — evaporating the water from salt solution leaves solid salt behind.

Aqueous solutions. A solution in water is called an aqueous solution and is given the state symbol (aq) . Sodium chloride solution is written NaCl(aq). Other solvents are also used: nail-varnish remover contains propanone (acetone), and some paints use white spirit. These are non-aqueous solvents.

Mass is conserved. When 5 g of salt dissolves in 100 g of water, the solution has a mass of 105 g. No particles are lost; they are simply spread out among the water particles.

Not only solids dissolve. Gases and liquids can be solutes too. Fizzy drinks contain dissolved carbon dioxide gas, and vinegar is a solution of ethanoic acid (a liquid) in water.

Step-by-step reasoning

To name the parts of any solution:

1. Ask which substance has "disappeared" into the other — that is the solute. 2. Ask which substance there is more of and which keeps its state — that is usually the solvent. 3. Name the whole uniform mixture as the solution. 4. If the solvent is water, add the state symbol (aq).

Visual explanation

Imagine a beaker drawn in three stages. First, a pile of blue crystals sits at the bottom of clear water. Next, blue streaks spread upwards from the crystals. Finally, the crystals are gone and the whole beaker is an even, see-through blue. The first picture shows solute and solvent separately; the last shows one uniform solution.

Real-world analogy

A solution is like a drop of food colouring in a swimming pool that has been stirred for an hour. The colouring (solute) has spread through all the water (solvent), and every bucketful you scoop out contains the same tiny share of it.

Real-world example

Oral rehydration solution, used to treat dehydration, is made by dissolving measured amounts of glucose and salts in clean water. Because a solution is homogeneous, every mouthful delivers the same balance of sugar and salts, which is exactly what the body needs to absorb water effectively.

Why?

Why does a dissolved solute not settle to the bottom, even after days? Its particles are individual molecules or ions, mixed among the solvent particles and constantly moving. They are far too small to sink out, so the solution stays uniform for as long as nothing evaporates.

Common misconception

"When sugar dissolves, it disappears." The sugar is still there, broken up into particles too small to see. The solution tastes sweet, has a greater mass than the water alone, and the sugar can be recovered by evaporating the water.

Worked example

Question: 12 g of copper(II) sulfate is dissolved in 150 g of water to give a clear blue liquid. Name the solute, the solvent and the solution, and give the mass of the solution.

Reasoning: Copper(II) sulfate dissolves, so it is the solute. Water does the dissolving, so it is the solvent. Mass is conserved: 12 + 150 = 162 g.

Answer: Solute: copper(II) sulfate. Solvent: water. Solution: copper(II) sulfate solution, CuSO₄(aq), with a mass of 162 g.

Quick check

1. In salty seawater, which substance is the solvent? Answer: Water is the solvent; the salts are the solutes.

Exam focus

Use the three terms precisely: the solute dissolves, the solvent does the dissolving, and the solution is the mixture. Do not say that a solid "melts" in water — melting needs heat and involves only one substance. Remember that mass is conserved on dissolving and that (aq) means dissolved in water.

Advanced insight

A solution does not have to be a liquid. Air is a gaseous solution of oxygen, argon and other gases in nitrogen, and many alloys are solid solutions of one metal in another. What makes something a solution is that it is a uniform mixture at the level of individual particles, whatever its state.

Summary

A solute dissolves in a solvent to form a solution. A solution is a homogeneous mixture: the solute particles are spread evenly and are too small to see, so it is usually transparent. The composition can vary, the parts are not chemically bonded, and the total mass equals the mass of solute plus solvent. Solutions in water are aqueous, written (aq).

Practice questions

1. Define the word solvent. Answer: The substance in which a solute dissolves, usually present in the larger amount. 2. Vinegar is ethanoic acid dissolved in water. Identify the solute and the solvent. Answer: The solute is ethanoic acid and the solvent is water. 3. 20 g of sugar is dissolved in 250 g of water. What is the mass of the solution? Answer: 270 g, because mass is conserved when a substance dissolves. 4. Give two reasons why salt solution is described as a mixture rather than a compound. Answer: Its composition can vary (more or less salt can be added), and the salt can be recovered unchanged by evaporating the water, so the substances are not chemically bonded.