CDS Chemistry · Formula sheet
Matter and Its States formulas
1 formula, 6 reference tables and 12 common traps for CDS Chemistry Matter and Its States, grouped by subtopic.
States of Matter and Evaporation
Learn this subtopic in the notesParticles of matter and the five states
| 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 |
Common traps
Fluorine is a gas, rubidium a solid
The liquid elements at room temperature are bromine and mercury. Fluorine is a gas, and thallium and rubidium are solids (rubidium melts at about 39 °C).
Plasma is the fourth state, BEC the fifth
The Bose-Einstein condensate is the fifth state of matter. Plasma is the fourth, and neither is a kind of solid.
More liquid does not mean faster evaporation
Evaporation is a surface process. The mass of liquid does not change its rate; surface area, temperature, wind and humidity do.
Humidity slows evaporation
Humid air already holds a lot of water vapour, so less can escape into it. An increase in humidity decreases the rate of evaporation.
Elements, Compounds and Mixtures
Learn this subtopic in the notesConcentration as mass by mass percentage
Mass by mass percentage
Elements, compounds and mixtures
| 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). |
Common traps
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.
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.
Separation Techniques
Learn this subtopic in the notesChoosing a separation method
| 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 |
Common traps
Chromatography cannot tell isotopes apart
Isotopes of an element behave the same chemically, so they stick to the stationary phase in the same way. Chromatography separates dyes, pigments and drugs, not isotopes.
A compound needs a chemical method
Distillation, filtration and sublimation only separate mixtures. The elements of a compound are held by bonds and need a chemical change, such as electrolysis.
Colloids and Suspensions
Learn this subtopic in the notesKinds of colloid
| 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 effect
| System | Tyndall effect? |
|---|---|
| Soap solution (micelles) | Yes — micelles scatter light, so it looks cloudyQ |
| Milk | Yes |
| Salt solution | No — a true solution |
Common traps
Mist is liquid in gas, not gas in liquid
In mist the liquid droplets are dispersed in air, so the medium is a gas. In foam the gas is dispersed in a liquid.
An emulsion is two liquids
An emulsion is one liquid dispersed in another (milk, cream). A solid in a liquid is a sol; a gas in a liquid is a foam.
Cloudiness is scattering, not refraction
Soap solution looks cloudy because its micelles scatter light (the Tyndall effect). Refraction, diffraction and polarisation are distractors.