NDA Chemistry · Formula sheet
Matter and Its States formulas
1 formula, 12 reference tables and 19 common traps for NDA Chemistry Matter and Its States, grouped by subtopic.
States of Matter, Phase Changes and Diffusion
Learn this subtopic in the notesThe three states of matter
| State | Shape | Volume | Compressibility |
|---|---|---|---|
| Solid | Fixed | Fixed | Almost none |
| Liquid | Takes container's shape | Fixed | Almost none |
| Gas | Takes container's shape | Fills container | High |
| Liquid elements at 25 °C | — | — | — Only mercury (Hg) and bromine (Br₂) are liquid at room temperature and normal pressure. Gallium and caesium are solids at 25 °C (they melt only just above it). |
Solid = fixed shape + fixed volume; Liquid = fixed volume only; Gas = neither.
The six phase changes
| Phase change | Direction | Heat | Example |
|---|---|---|---|
| Melting | Solid → liquid | Absorbed | Ice → water |
| Freezing | Liquid → solid | Released | Water → ice |
| Vaporisation | Liquid → gas | Absorbed | Water → steam |
| Condensation | Gas → liquid | Released | Steam → water droplets |
| Sublimation | Solid → gas | Absorbed | Dry ice → CO₂ gas; camphor Sublimation skips the liquid state entirely — solid goes straight to gas. |
| Deposition | Gas → solid | Released | Sulphur vapour → solid crust; frost Deposition is the reverse of sublimation: gas straight to solid, no liquid. |
Sublimation and deposition are the direct solid↔gas pair, skipping liquid.
Dry ice — solid carbon dioxide
| Question asked | Answer |
|---|---|
| What is dry ice? | Solid carbon dioxide (CO₂) |
| Is dry ice frozen water? | No — it is solid CO₂ |
| Phase change when dry ice 'disappears' | Sublimation (solid → gas) Dry ice never melts to a liquid at normal pressure — it sublimes straight to gas. |
| Why mist forms on a stage | Cold CO₂ gas condenses atmospheric water vapour |
Diffusion
| State | Diffusion rate | Everyday example |
|---|---|---|
| Gas | Fastest | Perfume smell spreading across a room |
| Liquid | Moderate | Ink drop colouring water |
| Solid | Slowest (negligible) | Two metals welded together over years Solids diffuse extremely slowly because their particles are locked in place. |
Diffusion is fastest in gases, slowest in solids, and faster when hotter.
Common traps
Gallium is a solid at room temperature
Gallium feels like it should be liquid (it melts in your hand at 30 °C), but at room temperature (25 °C) it is a solid. The only elements liquid at 25 °C are mercury and bromine.
Boiling is bulk, evaporation is surface
The correct statement is: boiling is a bulk phenomenon (throughout the liquid, only at the boiling point) but evaporation is a surface phenomenon (only at the surface, at any temperature). Any statement that swaps these two, or calls both 'surface' or both 'bulk', is wrong.
Deposition ≠ condensation
A gas turning into a solid directly is deposition, not condensation. Condensation is gas → liquid. The sulphur-crust and frost examples are deposition.
Dry ice is CO₂, not ice
Dry ice is solid carbon dioxide, not frozen water. It sublimes (solid → gas) with no liquid stage — that is why it is called 'dry'.
Physical vs Chemical Changes
Learn this subtopic in the notesThe test: is a new substance formed?
| Feature | Physical change | Chemical change |
|---|---|---|
| New substance? | No | Yes |
| Molecular composition | Unchanged | Changed |
| Reversible? | Usually yes | Usually no |
| Examples | Melting, boiling, dissolving | Rusting, burning, souring Burning a candle is BOTH: wax melting is physical, vapour burning is chemical. |
The whole distinction is one question: is a new substance formed?
Examples: which side is each change on?
| Event | Change type | Why |
|---|---|---|
| Rusting of iron | Chemical | Iron → iron oxide (new substance) |
| Burning of coal | Chemical | Carbon → CO₂ + ash |
| Souring of milk | Chemical | Bacteria turn lactose into lactic acid Souring of milk LOOKS physical but is chemical — a new acid is formed. |
| Greying of hair (natural) | Chemical | Pigment chemically lost — irreversible Natural greying of hair is a chemical change, not a physical one. |
| Melting of ice | Physical | Still H₂O, just a state change |
| Reaction of acid with base | Chemical | Forms salt + water |
Common traps
Physical change keeps the composition
A common false statement is 'during a physical change, the chemical composition of molecules changes' — that is NOT correct. In a physical change the molecules are unchanged; only the form/state differs.
Souring and greying are chemical
Souring of milk and natural greying of hair both look harmless and physical, but each forms a new substance — they are chemical changes. Melting, boiling and dissolving are the physical ones.
Compounds, Mixtures and Solutions
Learn this subtopic in the notesMass percentage of a solution
Mass percentage of solute
- mass of solutemass of the dissolved substance (e.g. salt)
- mass of solventmass of the dissolving medium (e.g. water)
Elements, compounds and mixtures
| 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 solutions
| 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 |
Common traps
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.
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.
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.
Colloids and Suspensions
Learn this subtopic in the notesTrue solution, colloid and suspension
| Property | True solution | Colloid | Suspension |
|---|---|---|---|
| Particle size | < 1 nm | 1–1000 nm | > 1000 nm |
| Appearance | Homogeneous | Heterogeneous | Heterogeneous |
| Tyndall effect | No | Yes | Yes (if not settled) |
| Settle on standing? | No | No (needs centrifuge) | Yes Colloid particles do NOT settle on their own — but centrifugation can separate them. |
| Visible to naked eye? | No | No | Yes Colloidal particles cannot be seen by the naked eye — only suspension particles can. |
| Example | Salt water, CuSO₄ solution | Milk, fog, soap solution | Muddy water, chalk in water |
Sorted by particle size: solution < colloid < suspension. Only the true solution is homogeneous.
Soaps, micelles and cleansing action
| Question asked | Answer |
|---|---|
| What kind of salt is a soap? | Sodium/potassium salt of a long-chain carboxylic acid A soap is a carboxylate (Na/K salt), NOT an ammonium salt — that is the bank's trap statement. |
| Where does dirt collect? | In the centre of the micelle |
| Principle of cleansing | Lowering surface tension (and emulsifying oil) |
| Does soap solution scatter light? | Yes — it is colloidal (Tyndall effect) |
| What forms in hard water? | Insoluble precipitate (scum) with Ca²⁺ and Mg²⁺ |
| Soap with water forms… | A lyotropic liquid crystal Lyotropic = order set by concentration/solvent. Thermotropic = order set by temperature. Soap micelles are lyotropic. |
Common traps
A colloid is NOT homogeneous
The statement 'a colloidal solution is homogeneous in nature' is false. Colloids are heterogeneous — they only appear uniform. Only a true solution is homogeneous.
Colloid particles are invisible to the eye
Colloidal particles cannot be seen with the naked eye (they are 1–1000 nm). Only suspension particles (> 1000 nm) are visible. Don't accept 'particles of colloids and suspensions can be seen by the naked eye'.
Soap is a carboxylate, not an ammonium salt
Soaps are sodium or potassium salts of long-chain carboxylic acids. The statement that 'soaps are ammonium salts of long-chain carboxylic acids' is the NOT true one.
Lyotropic, not thermotropic
Soap in water forms a lyotropic liquid crystal — its order depends on concentration (the solvent). A thermotropic liquid crystal's order depends on temperature; that is the wrong choice for soap.
Separation Techniques
Learn this subtopic in the notesDistillation, fractional distillation and the separating funnel
| Method | Use it when… | Example |
|---|---|---|
| Distillation | Two miscible liquids, boiling points far apart | Acetone and water; two miscible liquids |
| Fractional distillation | Miscible liquids, boiling points close | Petrol and kerosene; refining petroleum Close boiling points → you need the fractionating column → fractional distillation. |
| Separating funnel | Two immiscible liquids (don't mix) | Water and kerosene oil; oil and water Immiscible = they form separate layers → separating funnel, not distillation. |
Miscible → (fractional) distillation by boiling point; immiscible → separating funnel by layers.
Centrifugation, chromatography, sublimation, evaporation and crystallization
| Method | Separates | Example |
|---|---|---|
| Centrifugation | Suspended solid from liquid (by density, by spinning) | Cream from milk; blood tests Blood tests in diagnostic labs use centrifugation to spin cells from plasma. |
| Chromatography | Components by rate of movement / adsorption | Pigments from plant extract; ink dyes |
| Sublimation | A sublimable solid from a non-sublimable one | Anthracene from salt; camphor from sand Use sublimation only when ONE component sublimes (turns solid → gas). |
| Evaporation | Non-volatile solute from solvent (dry off solvent) | Salt from salt water |
| Crystallization | Pure solute crystals from a hot solution | Pure crystals from a non-volatile solute A non-volatile solid solute can be recovered by EITHER evaporation OR crystallization. |
Common traps
Miscible vs immiscible decides the method
Two liquids that mix (miscible) are separated by distillation; two that don't mix (immiscible, like oil and water) are separated by a separating funnel. Picking distillation for oil-and-water is the classic error.
Close boiling points need the column
When miscible liquids have close boiling points (petrol/kerosene, crude-oil fractions), simple distillation fails — use fractional distillation (with the fractionating column).
Non-volatile solute → evaporation OR crystallization
A non-volatile solid solute can be recovered from its solution by both evaporation and crystallization — so in a 'which process(es) work' question, the answer is often 'both'. Crystallization gives purer crystals.
Sublimation needs a sublimable component
Reach for sublimation only when one component turns solid → gas directly (camphor, naphthalene, iodine, ammonium chloride, anthracene). To separate anthracene from salt, the anthracene sublimes and the salt stays.