Solvents, solutes and solutions for iGCSE Chemistry
A solution is a mixture made when a solute dissolves into a solvent. The solute is the substance being dissolved (often a solid like salt or sugar), and the solvent is the liquid doing the dissolving (most commonly water). When the two combine and you can no longer see the solute particles, you have a solution.
This guide covers the Cambridge iGCSE Chemistry definitions you need to memorise, how to describe a solution in mark-scheme language, the difference between dilute, concentrated and saturated solutions, and the practical work examiners love to test.
Three terms to know
Solute (what dissolves), solvent (what does the dissolving), solution (the mixture you end up with).
Mark-scheme wording
Examiners want precise language. A solution is a homogeneous mixture of a solute dissolved in a solvent.
Cambridge iGCSE focus
Section 12.1 (Experimental design) covers the learning outcome on solvents, solutes, solutions and solubility. Expect short-answer definition questions and one practical.
Defining solute, solvent and solution
A solute is the substance that dissolves into a liquid. A solvent is the liquid that does the dissolving. A solution is the homogeneous mixture formed when a solute is fully dissolved in a solvent. Homogeneous means evenly mixed all the way through, so a single drop from anywhere in the beaker looks the same.
The most common example is salt water. Salt is the solute, water is the solvent, and salt water is the solution. Other examples include sugar in tea, copper sulfate in water, and iodine in ethanol.
Aqueous solutions When the solvent is water, the solution is called an aqueous solution. You will see this written as (aq) in chemical equations. So NaCl (aq) means sodium chloride dissolved in water.
Key vocabulary you need for the exam
Cambridge iGCSE mark schemes reward precise vocabulary. Mixing up dilute and concentrated, or soluble and miscible, is a common way to drop easy marks. Learn these terms and use them deliberately in your answers.
| Term | What it means | Example |
|---|---|---|
| Soluble | A substance that can dissolve in a particular solvent | Salt is soluble in water |
| Insoluble | A substance that does not dissolve in a particular solvent | Sand is insoluble in water |
| Dilute | A solution with a small amount of solute relative to solvent | A weak salt solution |
| Concentrated | A solution with a large amount of solute relative to solvent | A strong sugar syrup |
| Saturated | A solution that has dissolved as much solute as it can at that temperature | Sea water near full salinity |
| Miscible | Two liquids that mix completely in any proportion | Ethanol and water |
| Immiscible | Two liquids that do not mix and form separate layers | Oil and water |
Saturated solutions and solubility
A saturated solution is one that cannot dissolve any more solute at a given temperature. If you keep adding salt to water, eventually no more will dissolve and the extra sits at the bottom of the beaker. The solution is saturated.
Solubility is the maximum mass of solute that dissolves in 100 g of solvent at a given temperature. It is usually written in g per 100 g of water. Solubility increases with temperature for most solids, which is why you can dissolve more sugar in hot tea than cold.
A common iGCSE exam pattern You are given a solubility curve and asked to read off how much solute dissolves at a particular temperature. Always check the axis units (g per 100 g of water is the standard), and quote temperature to the nearest degree. Then calculate the mass that crystallises out when the solution cools.
Solvents are not always water
Water is the most common solvent, but it is not the only one. Many substances that are insoluble in water dissolve readily in organic solvents like ethanol, propanone or hexane. This matters in industry (dry cleaning uses organic solvents) and in the lab (chromatography often uses ethanol or hexane).
A good rule of thumb is like dissolves like: Polar solvents like water dissolve polar solutes like ionic salts, and non-polar solvents like hexane dissolve non-polar solutes like waxes and oils. You will not be tested on the full polarity theory at iGCSE, but you should know that different solvents dissolve different things.
Required practical: Making and testing a solution
A standard Cambridge iGCSE practical is to investigate the solubility of a salt at different temperatures. The method below gives you the mark-scheme steps.
Solubility practical: Method steps
Follow this sequence and you will pick up the practical marks.
- Measure a fixed volume of water (usually 25 cm³) into a boiling tube
- Heat the water to the target temperature using a water bath
- Add the salt in small known masses (e.g. 1 g at a time) and stir until dissolved
- Keep adding until no more dissolves and undissolved solid remains visible
- Record the total mass of salt added and the temperature
- Repeat at different temperatures to plot a solubility curve
- Express solubility as g of solute per 100 g of water
Where students lose marks on solutions questions
Examiner reports from Cambridge International flag the same handful of issues every year. Most of them are about loose vocabulary rather than a lack of chemistry knowledge.
Common mistakes that cost marks Writing that a solution is "a liquid with stuff in it" instead of using the words solute, solvent and dissolved. Confusing saturated with concentrated (they are not the same). Forgetting to state the temperature when quoting a solubility. Saying oil dissolves in water (it does not, it is immiscible). Writing soluable instead of soluble.
Worked example: Calculating solubility
A student dissolves 18 g of potassium nitrate in 50 g of water at 30 °C to form a saturated solution. Calculate the solubility of potassium nitrate in g per 100 g of water at 30 °C.
Step 1: Identify the ratio. 18 g of solute dissolves in 50 g of water.
Step 2: Scale up to 100 g of water. Multiply both sides by 2. 18 × 2 = 36 g of potassium nitrate dissolves in 100 g of water.
The solubility of potassium nitrate at 30 °C is 36 g per 100 g of water. Always quote the temperature, because solubility changes with it.
Key facts to memorise for the exam
- Solute: The substance that dissolves
- Solvent: The liquid that does the dissolving
- Solution: A homogeneous mixture of solute in solvent
- Aqueous solution: A solution where the solvent is water, written as (aq)
- Saturated: No more solute will dissolve at that temperature
- Solubility: The mass of solute that dissolves in 100 g of solvent at a stated temperature
- Miscible vs immiscible: Used for liquids only, not solids in liquids
- Solubility of most solids increases with temperature
