CDS Chemistry · Matter and Its States

Elements, Compounds and Mixtures

What separates a compound from a mixture, how to sort everyday substances into each, and how to state a solution's concentration as a mass percentage.

All Matter and Its States notes

Why this matters

Six CDS questions. Five ask the same thing in different words: a compound has a fixed composition and cannot be separated by physical means, a mixture can. One is a short concentration calculation.

Concept 1 of 2: Elements, compounds and mixtures

An element is one kind of atom. A compound is two or more elements chemically joined in a fixed ratio, so it has its own properties and can only be split by a chemical change. A mixture is substances simply put together, in any ratio, and can be separated by physical means.

Definition

The differences:

Element: cannot be broken into simpler substances by chemical means (silicon, tin).
Compound: elements joined in a fixed ratio by mass; one kind of particle; constituents cannot be separated by physical methods (sugar, calcium carbonate, water).
Mixture: variable composition; separable by physical methods such as evaporation (air, milk, tea, sugar solution).
Homogeneous mixture (solution): uniform throughout, such as sulphur dissolved in carbon disulphide, or salt in water.

Property	Compound	Mixture
Composition	Fixed	VariableQ CDS 2018 (I) and 2026 (I): 'a compound has variable composition' is the wrong statement.
Particles	One kind	Two or more kinds
Separation	Only by chemical means	By physical means (evaporation, filtration)Q
Examples	Sugar, calcium carbonate, water	Air, milk, tea, sugar solutionQ Asked in CDS 2023 (II) (sugar) and 2025 (I) (calcium carbonate).

Practice this conceptself-check · 4 quick reps

The same idea in a real exam question:

CDS · 2026 · CDS (I) 2026 — General Knowledge · Q22Moderate

Example 1 · Matter and Its States · Compounds, Mixtures and Solutions

Consider the following statements regarding matter : 1. Elements cannot be broken down into simpler substances. 2. A compound has variable composition and can be broken down into elements. 3. The solution of sulphur in carbon disulphide is an example of a homogeneous mixture. Which of the statements given above is/are not correct ?

A compound's composition is fixed

A compound always has its elements in the same ratio by mass (law of constant proportions). 'Variable composition' describes a mixture.

Sugar solution is a mixture

Sugar is a compound, but sugar dissolved in water is a mixture: you can change how much sugar it holds and recover the sugar by evaporation.

Drill 4 more on elements, compounds and mixtures

Concept 2 of 2: Concentration as mass by mass percentage

Concentration says how much solute is in the whole solution. The trap is the denominator: divide by the mass of the solution (solute plus solvent), not by the solvent alone.

Definition

The formula:

Mass of solution = mass of solute + mass of solvent.
Mass by mass % = (mass of solute ÷ mass of solution) × 100.

Mass by mass percentage

\text{mass \%} = \frac{\text{mass of solute}}{\text{mass of solute} + \text{mass of solvent}} \times 100

Worked example

25 g of salt is dissolved in 100 g of water. What is the concentration as a mass by mass percentage?

Practice this conceptself-check · 3 quick reps

The same idea in a real exam question:

CDS · 2021 · CDS (I) 2021 — General Knowledge · Q100Moderate

Example 2 · Matter and Its States · Compounds, Mixtures and Solutions

A solution contains 20 g of solute in 180 g of solvent. If the solvent is water, what is the concentration of the solution in terms of mass by mass percentage ?

Divide by the solution, not the solvent

Dividing the solute by the solvent alone gives a number that is too large. Always add the solute to the solvent first.

Summary — formulas & gotchas at a glance

A revision cheat-sheet for the formulas and gotchas above. Click any concept name to jump back to its full explanation.

Formulas (1)

Concentration as mass by mass percentage
Mass by mass percentage
$\text{mass \%} = \frac{\text{mass of solute}}{\text{mass of solute} + \text{mass of solvent}} \times 100$

Reference tables (1)

Elements, compounds and mixtures4 rows

Property	Compound	Mixture
Composition	Fixed	VariableQ CDS 2018 (I) and 2026 (I): 'a compound has variable composition' is the wrong statement.
Particles	One kind	Two or more kinds
Separation	Only by chemical means	By physical means (evaporation, filtration)Q
Examples	Sugar, calcium carbonate, water	Air, milk, tea, sugar solutionQ Asked in CDS 2023 (II) (sugar) and 2025 (I) (calcium carbonate).

Watch out for (3)

A compound's composition is fixed
→ Elements, compounds and mixtures
Sugar solution is a mixture
→ Elements, compounds and mixtures
Divide by the solution, not the solvent
→ Concentration as mass by mass percentage

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