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
Intuition
First split everything into pure substances (fixed composition, one kind of particle throughout) and mixtures (two or more substances just stirred together). Pure substances split again into elements (one type of atom) and compounds (atoms chemically bonded in a fixed ratio).
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. |
Pure = element or compound (fixed composition). Mixture = physically mixed, any proportion.
Practice this conceptself-check · 4 quick reps
Try it yourself
Which statement about a compound is NOT correct? (a) Two or more elements chemically combined in a fixed proportion (b) Properties differ from its constituent elements (c) A compound is an impure substance (d) Constituents separable only by chemical reactions.
Practice — Level 1 (4 reps)
Quick reps to lock in the method. Try each, then check.
- 1.Of tin, brass, air and sandy water, which is NOT a mixture?
- 2.Is ice a pure substance or a mixture?
- 3.Is a compound a pure or impure substance?
- 4.How can the constituents of a compound be separated?
From the bank · past-year question
Which one of the following statements about a compound is NOT correct?
[Q80 · Apr · 2025]
A compound is pure, not impure
A compound has a fixed composition, so it is a pure substance. The statement 'a compound is an impure substance' is the wrong one. (Mixtures are the variable-composition ones.)
Ice and tin are not mixtures
Ice is frozen water (a pure compound) and tin is a pure element — neither is a mixture. The mixtures in a list are things like air, brass, or sandy water.
Concept 2 of 3
Homogeneous vs heterogeneous mixtures, and solutions
Intuition
Mixtures split by how uniform they look. A homogeneous mixture is the same throughout with no visible boundaries (a solution — salt water, air, brass). A heterogeneous mixture has visible different parts (sandy water, oil and water, milk).
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
Try it yourself
Which is NOT a heterogeneous mixture? (a) Sulphur in carbon disulphide (b) Sugar and salt crystals (c) Sandy water (d) Sulphur in water.
Practice — Level 1 (4 reps)
Quick reps to lock in the method. Try each, then check.
- 1.Is a solution homogeneous or heterogeneous?
- 2.Is milk a homogeneous or heterogeneous mixture?
- 3.Of alloy, milk, air and salt water, which is NOT a solution?
- 4.Why is sulphur in carbon disulphide homogeneous?
From the bank · past-year question
Which one of the following is NOT a heterogeneous mixture ?
[Q102 · Sep · 2025]
Milk is heterogeneous, not a solution
Milk looks uniform but is a heterogeneous mixture (a colloid of fat droplets in water). In a 'which is NOT a solution' list, milk is the answer.
Sulphur dissolves in CS₂ but not water
Sulphur is soluble in carbon disulphide (giving a homogeneous solution) but insoluble in water (giving a heterogeneous mixture). Watch which solvent the question uses.
Concept 3 of 3
Mass percentage of a solution
Intuition
The strength of a solution by mass is just the fraction of the total mass that is the solute, expressed as a percentage. Remember the denominator is the mass of the WHOLE solution (solute + solvent), not just the solvent.
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
20 g of common salt is dissolved in 180 g of water. What is the mass percentage of salt in the solution?
- Mass of solution = mass of salt + mass of water = 20 + 180 = 200 g.
- Mass % = (20 ÷ 200) × 100.
- = 0.1 × 100 = 10%.
Answer:10%.
Practice this conceptself-check · 3 quick reps
Try it yourself
25 g of sugar is dissolved in 75 g of water. What is the mass percentage of sugar?
Practice — Level 1 (3 reps)
Quick reps to lock in the method. Try each, then check.
- 1.10 g of salt in 90 g of water — mass %?
- 2.5 g of solute in 45 g of solvent — mass %?
- 3.In a mass-% calculation, the denominator is the mass of the solvent or the whole solution?
From the bank · past-year question
20 g of common salt is dissolved in 180 g of water. What is the mass percentage of the salt in the solution ?
[Q92 · Apr · 2017]
Divide by the whole solution, not the solvent
For 20 g salt in 180 g water, the answer is 20 ÷ 200 × 100 = 10%, NOT 20 ÷ 180 × 100 ≈ 11%. The denominator is the total solution mass.
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. |
Pure = element or compound (fixed composition). Mixture = physically mixed, any proportion.
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
Mastery check — 4 interleaved questions
Try each one before clicking. Questions are interleaved across the concepts above, not grouped — interleaving sharpens transfer.
Which one of the following substances is a mixture?
[Q75 · Sep · 2019]
Which one of the following is a solution?
[Q57 · Apr · 2022]
Which one of the following substances is NOT a mixture?
[Q103 · Apr · 2019]
Which one of the following is a heterogeneous mixture?
[Q64 · Sep · 2018]
Drill every past-year question on this subtopic
7 questions from the bank — paginated, with cart and Word-export support.