Water and Solutions

What you'll learn

This revision guide covers everything you need to know about water and solutions for WJEC GCSE Chemistry. You'll master how substances dissolve, how to calculate concentrations, and the key techniques used to separate and purify solutions. These concepts form the foundation for many practical chemistry skills tested in both written papers and practicals.

Key terms and definitions

Solvent — the liquid that dissolves a solute; water is the most common solvent in chemistry and is called the universal solvent

Solute — the substance that dissolves in a solvent to form a solution

Solution — a mixture formed when a solute dissolves in a solvent; solutions are clear (not necessarily colourless) and cannot be separated by filtration

Solubility — the maximum mass of solute that will dissolve in 100 g of solvent at a given temperature; measured in g per 100 g of solvent

Saturated solution — a solution containing the maximum amount of dissolved solute at a particular temperature; no more solute can dissolve

Concentration — the amount of solute dissolved in a given volume of solution; commonly measured in g/dm³ or mol/dm³

Aqueous solution — a solution where water is the solvent, denoted by the state symbol (aq)

Miscible — describes liquids that mix completely to form a solution (e.g. ethanol and water)

Core concepts

Water as a solvent

Water is often called the universal solvent because it dissolves more substances than any other liquid. This property makes water essential for life and for many industrial processes.

Why water is such a good solvent:

• Water molecules are polar, with a slight positive charge on the hydrogen atoms and a slight negative charge on the oxygen atom
• Polar water molecules attract charged particles (ions) and other polar molecules
• The attractions between water molecules and solute particles overcome the forces holding the solute together

Substances that dissolve in water:

• Ionic compounds (e.g. sodium chloride, copper sulfate)
• Polar covalent compounds (e.g. sugar, ethanol)
• Some acids (e.g. hydrochloric acid, sulfuric acid)

Substances that don't dissolve in water:

• Non-polar covalent compounds (e.g. oil, wax, iodine)
• Most molecular substances with large non-polar regions

When ionic compounds dissolve in water, the ions separate and become surrounded by water molecules. This process is called dissociation. For example, when sodium chloride dissolves:

NaCl(s) → Na⁺(aq) + Cl⁻(aq)

Solubility and temperature

The solubility of most solid solutes increases with temperature. This means more solute can dissolve in hot water than in cold water.

Key points about solubility:

• Solubility is measured as mass of solute per 100 g of solvent at a specific temperature
• Different substances have different solubilities
• Solubility curves show how solubility changes with temperature
• When a hot saturated solution cools, the excess solute crystallises out

Solubility of gases:

• The solubility of gases in water decreases as temperature increases
• This is why fizzy drinks go flat more quickly when warm
• This is also why fish struggle in warm water (less dissolved oxygen)

Practical applications:

• Recrystallisation uses differences in solubility at different temperatures to purify substances
• Sugar dissolves better in hot tea than cold tea
• Industrial processes often use hot water to dissolve more reactants

Concentration calculations

Concentration tells you how much solute is dissolved in a given volume of solution. You must be able to calculate concentration and rearrange the formula.

Standard formula:

Concentration (g/dm³) = mass of solute (g) / volume of solution (dm³)

Key conversion:

• 1 dm³ = 1000 cm³
• To convert cm³ to dm³, divide by 1000
• To convert dm³ to cm³, multiply by 1000

Important points:

• Concentration refers to the whole solution volume, not just the solvent
• Adding more solute increases concentration
• Adding more solvent decreases concentration (dilution)
• Concentrations can also be expressed in mol/dm³ for mole calculations

Triangle method: Draw a triangle with concentration at the top, mass on the bottom left, and volume on the bottom right. Cover the value you want to find.

• Mass = concentration × volume
• Volume = mass ÷ concentration
• Concentration = mass ÷ volume

Separation techniques for solutions

Different techniques separate mixtures based on physical properties. You need to know which technique to use and when.

Simple distillation:

• Separates a liquid solvent from a solution
• Used to obtain pure water from salt water
• The solution is heated until the solvent boils
• Vapour rises, passes through a condenser, and is collected as pure liquid
• The solute (e.g. salt) remains in the flask
• Only suitable when the components have very different boiling points

Fractional distillation:

• Separates miscible liquids with different boiling points
• Uses a fractionating column packed with glass beads
• The liquid with the lower boiling point is collected first
• Used to separate ethanol and water, or to separate crude oil
• More efficient than simple distillation for liquids with closer boiling points

Evaporation:

• Used to obtain a solid solute from a solution
• The solution is heated in an evaporating basin
• The solvent evaporates, leaving solid crystals behind
• Quick method but can decompose heat-sensitive solutes
• Used for substances that don't decompose on heating

Crystallisation:

• Gentler method to obtain pure solid crystals
• The solution is heated to evaporate some solvent
• The solution is then left to cool slowly
• Crystals form as the solution becomes saturated
• Crystals are filtered off and dried
• Produces larger, purer crystals than evaporation

Filtration:

• Separates insoluble solids from liquids
• The mixture is poured through filter paper in a funnel
• The liquid (filtrate) passes through
• The solid (residue) remains on the filter paper
• Cannot separate solutions because dissolved particles are too small

Water purity and testing

Pure water contains only H₂O molecules and nothing else. Water from taps, rivers or seas contains dissolved substances.

Properties of pure water:

• Boiling point: exactly 100°C at standard atmospheric pressure
• Freezing point: exactly 0°C at standard atmospheric pressure
• Distilled or deionised water is pure
• Pure substances have sharp, fixed melting and boiling points

Testing for water:

• Anhydrous copper sulfate turns from white to blue in the presence of water
• Cobalt chloride paper turns from blue to pink when wet
• These tests show water is present but don't prove it's pure

Testing for pure water:

• Measure the boiling point (should be exactly 100°C)
• Measure the melting point (should be exactly 0°C)
• Any dissolved impurities will alter these values

Types of water:

• Potable water — water that is safe to drink but not pure (contains dissolved minerals and salts)
• Distilled water — pure water produced by distillation
• Hard water — contains dissolved calcium or magnesium ions
• Soft water — does not contain significant dissolved calcium or magnesium ions

Calculating percentage by mass

Solutions can be described by the percentage mass of solute present. This is important in many industrial processes and product formulations.

Formula:

Percentage by mass = (mass of solute / total mass of solution) × 100

Key points:

• Total mass of solution = mass of solute + mass of solvent
• Answer is always a percentage (no units)
• Useful for comparing concentrations in different sized samples
• Used on product labels (e.g. 5% glucose solution)

Worked examples

Example 1: Calculating concentration

Question: A student dissolves 15 g of sodium chloride in water to make 500 cm³ of solution. Calculate the concentration in g/dm³. [3 marks]

Solution:

• Convert volume to dm³: 500 cm³ ÷ 1000 = 0.5 dm³ [1 mark]
• Use formula: concentration = mass ÷ volume [1 mark]
• Concentration = 15 g ÷ 0.5 dm³ = 30 g/dm³ [1 mark]

Example 2: Finding mass of solute

Question: A solution of copper sulfate has a concentration of 40 g/dm³. What mass of copper sulfate is dissolved in 250 cm³ of this solution? [3 marks]

Solution:

• Convert volume: 250 cm³ ÷ 1000 = 0.25 dm³ [1 mark]
• Rearrange formula: mass = concentration × volume [1 mark]
• Mass = 40 g/dm³ × 0.25 dm³ = 10 g [1 mark]

Example 3: Percentage by mass

Question: A solution is made by dissolving 8 g of sugar in 92 g of water. Calculate the percentage by mass of sugar in the solution. [3 marks]

Solution:

• Find total mass: 8 g + 92 g = 100 g [1 mark]
• Use formula: percentage = (mass of solute ÷ total mass) × 100 [1 mark]
• Percentage = (8 ÷ 100) × 100 = 8% [1 mark]

Example 4: Choosing separation techniques

Question: Describe how you would obtain pure water from a sample of sea water. [3 marks]

Solution:

• Use simple distillation [1 mark]
• Heat the sea water until it boils; water vapour rises [1 mark]
• The vapour condenses in the condenser and pure water is collected, leaving salt in the flask [1 mark]

Common mistakes and how to avoid them

• Forgetting to convert cm³ to dm³ — always divide cm³ by 1000 before using concentration formulas; show this working clearly for method marks

• Confusing solute, solvent and solution — remember: solute dissolves in solvent to make solution; use the phrase "solute in solvent = solution" as a memory aid

• Saying filtration separates solutions — filtration only separates insoluble solids from liquids; solutions pass straight through filter paper because dissolved particles are too small

• Thinking all solids are more soluble at higher temperatures — while most solids are, some (like calcium hydroxide) are less soluble at higher temperatures; always refer to data given in questions

• Using mass of solvent instead of total solution mass — concentration uses the volume/mass of the entire solution, not just the solvent; remember to add solute mass + solvent mass for total

• Writing that tests for water prove purity — copper sulfate and cobalt chloride only show water is present; to prove purity, you must check boiling/melting points are exactly 100°C and 0°C

Exam technique for "Water and Solutions"

• Command word awareness: "Calculate" requires numerical working and a final answer with units; "Describe" needs a sequence of steps in the correct order; "Explain" requires reasons using scientific principles

• Show all working for calculations: Even if your final answer is wrong, you can earn method marks by showing correct formula, correct substitution and proper unit conversion

• Use correct terminology: Write "solute dissolves in solvent" not "mixes"; write "filtrate" and "residue" not "liquid bit" and "solid bit"; precision in language earns marks

• Include practical details when describing separations: State what happens to each component, mention specific apparatus (condenser, evaporating basin, filter funnel), and give safety considerations if asked for 4+ marks

Quick revision summary

Water is the universal solvent that dissolves ionic and polar substances through dissociation. Solubility increases with temperature for most solids. Calculate concentration using mass ÷ volume, remembering to convert cm³ to dm³. Separate solutions using distillation (pure solvent) or evaporation/crystallisation (pure solute). Test for water using anhydrous copper sulfate (white to blue) but prove purity by checking boiling point equals 100°C. Filtration only works for insoluble solids, not solutions.