CDS Chemistry · Teaching notes
Matter and Its States — CDS Chemistry
Matter and Its States has 19 past-year questions in CDS General Knowledge, from 2017 (II) to 2026 (I), and only one of them is HARD. It is the Class 9 chapter in CDS: what a compound is, which method separates which mixture, what speeds up evaporation. Most questions are a definition read carefully, so the traps matter more than new facts. A compound has a fixed composition; evaporation ignores the mass of liquid; an emulsion is two liquids.
Every subtopic, worked example, formula and trap in one printable document — answers shown, ready to share.
Subtopic notes
States of Matter and Evaporation
6 PYQsWhat the particle picture of matter says, the five states including plasma and the Bose-Einstein condensate, the two liquid elements, and what does and does not change the rate of evaporation.
Elements, Compounds and Mixtures
6 PYQsWhat 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.
Separation Techniques
4 PYQsWhich method separates which mixture, from sublimation and centrifugation to chromatography, and why the parts of a compound need a chemical method instead.
Colloids and Suspensions
3 PYQsSolutions, colloids and suspensions by particle size, the kinds of colloid named by phase and medium, and the Tyndall effect.
Formula & revision sheet
1 formulas · 6 reference tables · 12 gotchas across all subtopics — the exam-eve cheat-sheet
Formula & revision sheet
1 formulas · 6 reference tables · 12 gotchas across all subtopics — the exam-eve cheat-sheet
Reference tables (2)
Particles of matter and the five states5 rows
| State | How particles behave | Example |
|---|---|---|
| Solid | Fixed places, strong attraction | Ice, iron |
| Liquid | Move past each other, moderate attraction | Water; bromine and mercury among elements |
| Gas | Move freely, weak attraction | Hydrogen, nitrogen, oxygen, carbon dioxide |
| Plasma (4th state) | Ionised: free electrons and ions | Lightning, the Sun |
| Bose-Einstein condensate (5th state) | Atoms near absolute zero share one quantum state | Made in laboratoriesQ |
Watch out for (4)
- Fluorine is a gas, rubidium a solid→ Particles of matter and the five states
- Plasma is the fourth state, BEC the fifth→ Particles of matter and the five states
- More liquid does not mean faster evaporation→ What changes the rate of evaporation
- Humidity slows evaporation→ What changes the rate of evaporation
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
Reference tables (1)
Choosing a separation method5 rows
| Mixture | Method | Difference used |
|---|---|---|
| Salt and ammonium chloride | Sublimation | NH₄Cl sublimes, salt does notQ |
| Very fine insoluble particles in a liquid | Centrifugation | DensityQ |
| Colours in a dye; drugs in blood | Chromatography | How strongly each part is held by the stationary phaseQ CDS 2019 (II): chromatography cannot separate radio-isotopes. |
| Acetone and water | Fractional distillation | Boiling point |
| The elements in a compound (water) | Electrolysis | Breaks the chemical bondsQ |
Watch out for (2)
- Chromatography cannot tell isotopes apart→ Choosing a separation method
- A compound needs a chemical method→ Choosing a separation method
Reference tables (2)
Kinds of colloid5 rows
| Kind | Dispersed phase | Dispersion medium | Example |
|---|---|---|---|
| Emulsion | Liquid | Liquid | MilkQ |
| Foam | Gas | Liquid | Shaving cream, soap latherQ CDS 2024 (II) asked which has a liquid medium: foam and shaving cream both do, so the question has two right options. |
| Gel | Liquid | Solid | Jelly, cheese |
| Aerosol | Liquid or solid | Gas | Mist, fog, smoke |
| Sol | Solid | Liquid | Paint |
The Tyndall effect3 rows
| System | Tyndall effect? |
|---|---|
| Soap solution (micelles) | Yes — micelles scatter light, so it looks cloudyQ |
| Milk | Yes |
| Salt solution | No — a true solution |
Watch out for (3)
- Mist is liquid in gas, not gas in liquid→ Kinds of colloid
- An emulsion is two liquids→ Kinds of colloid
- Cloudiness is scattering, not refraction→ The Tyndall effect
PYQ weightage by concept
7 concepts · 19 PYQs — where the marks actually sit, so you know what to drill first
PYQ weightage by concept
7 concepts · 19 PYQs — where the marks actually sit, so you know what to drill first
| Concept | PYQs | Share |
|---|---|---|
| Particles of matter and the five states | 4 | 21% |
| What changes the rate of evaporation | 2 | 11% |
| Concept | PYQs | Share |
|---|---|---|
| Elements, compounds and mixtures | 5 | 26% |
| Concentration as mass by mass percentage | 1 | 5% |
Test yourself on a real paper
Sit a past CDS paper, timed and marked the way the exam marks it. You see your score and every answer the moment you finish. Free to start.