NDA Chemistry · Matter and Its States
Compounds, Mixtures and Solutions
Matter is either a pure substance (element or compound, fixed composition) or a mixture (two or more substances physically mixed); mixtures are homogeneous (uniform, like a solution) or heterogeneous (non-uniform).
Why this matters
Seven PYQs, mostly EASY classification: 'which is NOT a mixture / solution / heterogeneous mixture?', plus a 'compound statement that is NOT correct' and one mass-percentage sum. The whole subtopic is one classification tree — element vs compound vs mixture, then homogeneous vs heterogeneous — plus the one calculation. Learn where the trick examples sit (ice and tin are pure; milk is heterogeneous; sugar is a pure compound, not a solution).
Concept 1 of 3: Elements, compounds and mixtures
Definition
The classification tree:
- Element — a pure substance made of one type of atom (iron, tin, oxygen, gold). Cannot be broken down chemically.
- Compound — a pure substance of two or more elements chemically combined in a fixed proportion (water H₂O, salt NaCl, sugar C₁₂H₂₂O₁₁). Its properties differ from its constituent elements, and it is a pure substance (NOT impure). Its constituents are separable only by chemical or electrochemical means.
- Mixture — two or more substances physically mixed, in any proportion, separable by physical methods (air, brass, sand-and-salt, milk).
- Trap examples: ice is pure (frozen water — a compound), tin is pure (an element), sugar is pure (a compound) — none of these is a mixture.
| Substance | Category | Why |
|---|---|---|
| Tin | Element (pure) | One type of atom — not a mixture |
| Ice | Compound (pure) | Frozen H₂O — not a mixture |
| Sugar | Compound (pure) | C₁₂H₂₂O₁₁ — a pure substance, not a solution |
| Air | Mixture | Mainly N₂ + O₂, variable proportion |
| Brass | Mixture (alloy) | Copper + zinc, variable proportion |
| A compound | Pure substance | Fixed ratio; NOT an impure substance A compound is a PURE substance with fixed composition — the claim 'a compound is an impure substance' is false. |
Practice this conceptself-check · 4 quick reps
The same idea in a real exam question:
Example 1 · Matter and Its States · Compounds, Mixtures and Solutions
A compound is pure, not impure
Ice and tin are not mixtures
Concept 2 of 3: Homogeneous vs heterogeneous mixtures, and solutions
Definition
The second split, and what a solution is:
- Homogeneous mixture — uniform throughout, single phase, no visible boundaries. A solution is a homogeneous mixture (salt water, air, alloys like brass).
- Heterogeneous mixture — non-uniform, you can see the separate components or phases (sandy water, sulphur in water, a mixture of sugar and salt crystals, oil and water).
- A solution = solute (dissolved) + solvent (dissolving medium), homogeneous. Air (gas in gas), brass (solid in solid) and salt water (solid in liquid) are all solutions.
- Trap examples: milk is a heterogeneous mixture (a colloid — fat droplets in water); sulphur dissolved in carbon disulphide is homogeneous (sulphur IS soluble in CS₂, so it forms a true solution); pure sugar is a compound, not a solution.
| Mixture | Homogeneous or heterogeneous? | Note |
|---|---|---|
| Salt dissolved in water | Homogeneous (a solution) | Uniform, single phase |
| Air | Homogeneous (a solution) | Gases evenly mixed |
| Brass / alloy | Homogeneous (a solution) | Solid solution of metals |
| Sulphur in carbon disulphide | Homogeneous | Sulphur dissolves in CS₂ → true solution Sulphur IS soluble in carbon disulphide, so this is the NON-heterogeneous (homogeneous) one. Sulphur in WATER would be heterogeneous. |
| Milk | Heterogeneous (a colloid) | Fat droplets dispersed in water Milk is heterogeneous, not a true solution — it is a colloid. |
| Sugar and salt crystals | Heterogeneous | Visible separate crystals |
Practice this conceptself-check · 4 quick reps
The same idea in a real exam question:
Example 2 · Matter and Its States · Compounds, Mixtures and Solutions
Milk is heterogeneous, not a solution
Sulphur dissolves in CS₂ but not water
Concept 3 of 3: Mass percentage of a solution
Definition
The mass percentage of a solute in a solution:
- Mass % = (mass of solute ÷ mass of solution) × 100, where mass of solution = mass of solute + mass of solvent.
- The classic error is dividing by the mass of the solvent alone — always use the total solution mass.
Mass percentage of solute
- mass of solutemass of the dissolved substance (e.g. salt)
- mass of solventmass of the dissolving medium (e.g. water)
Worked example
Practice this conceptself-check · 3 quick reps
The same idea in a real exam question:
Example 3 · Matter and Its States · Compounds, Mixtures and Solutions
Divide by the whole solution, not the solvent
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)
- Mass percentage of a solution
Mass percentage of solute
Reference tables (2)
Elements, compounds and mixtures6 rows
| Substance | Category | Why |
|---|---|---|
| Tin | Element (pure) | One type of atom — not a mixture |
| Ice | Compound (pure) | Frozen H₂O — not a mixture |
| Sugar | Compound (pure) | C₁₂H₂₂O₁₁ — a pure substance, not a solution |
| Air | Mixture | Mainly N₂ + O₂, variable proportion |
| Brass | Mixture (alloy) | Copper + zinc, variable proportion |
| A compound | Pure substance | Fixed ratio; NOT an impure substance A compound is a PURE substance with fixed composition — the claim 'a compound is an impure substance' is false. |
Homogeneous vs heterogeneous mixtures, and solutions6 rows
| Mixture | Homogeneous or heterogeneous? | Note |
|---|---|---|
| Salt dissolved in water | Homogeneous (a solution) | Uniform, single phase |
| Air | Homogeneous (a solution) | Gases evenly mixed |
| Brass / alloy | Homogeneous (a solution) | Solid solution of metals |
| Sulphur in carbon disulphide | Homogeneous | Sulphur dissolves in CS₂ → true solution Sulphur IS soluble in carbon disulphide, so this is the NON-heterogeneous (homogeneous) one. Sulphur in WATER would be heterogeneous. |
| Milk | Heterogeneous (a colloid) | Fat droplets dispersed in water Milk is heterogeneous, not a true solution — it is a colloid. |
| Sugar and salt crystals | Heterogeneous | Visible separate crystals |
Watch out for (5)
- A compound is pure, not impure→ Elements, compounds and mixtures
- Ice and tin are not mixtures→ Elements, compounds and mixtures
- Milk is heterogeneous, not a solution→ Homogeneous vs heterogeneous mixtures, and solutions
- Sulphur dissolves in CS₂ but not water→ Homogeneous vs heterogeneous mixtures, and solutions
- Divide by the whole solution, not the solvent→ Mass percentage of a solution
Test yourself on Matter and Its States
15 past NDA questions from this chapter, timed at 15 minutes and marked the way the exam marks it. You see your score and every answer the moment you finish. Free to start.