Chemistry Definitions: Conditions, State Symbols, Units
Definition marks are often “easy marks”… until you lose them for one missing word: aqueous, standard conditions, per mole, or (aq).
This page gives a repeatable way to write definitions that don’t get penalised in O Level (6092) and A Level (9476 H2).
- Write the core meaning (what it is).
- Add the required condition (aqueous / standard / dilute / in excess) if the meaning depends on it.
- Add the unit phrase if it’s a “per ___” quantity (per mole / per unit time / per unit volume).
- Keep it one sentence (no examples unless asked).
Quick Reference
| What students miss | What it changes | Typical mark-scheme fix |
|---|---|---|
| “in aqueous solution” | acid/alkali/electrolyte meaning changes | add (aq) or say aqueous |
| “standard conditions” | standard enthalpy / E° definitions become incomplete | add 298 K, 100 kPa, 1 mol dm⁻³ |
| “per mole” / “per unit time” | turns a definition into a vague description | add “per mole” / “per second” |
| state symbols | shows if you know what exists as ions | use (s), (l), (g), (aq) correctly |
1) Conditions you must include (when they matter)
A. Aqueous (ions in water)
Use aqueous when the definition depends on ions being present in water.
- Acid (O Level): produces H + (aq) in water.
- Alkali (O Level): produces OH-(aq) in water.
- Electrolysis of solutions: ions are in water (and water provides H⁺ / OH⁻).
- Qualitative analysis: tests are done in solution, so ions are (aq).
If you can only write the definition using ions like H⁺ or OH⁻, you almost always need (aq) or “in aqueous solution”.
B. Standard conditions (standard state + fixed conditions)
Include standard conditions when the definition uses a standard symbol: Δ H°, E°, K° (if used), etc.
For this site, use:
- Temperature: 298 K
- Pressure: 100 kPa (or 1 bar)
- Solutions: 1.0 mol dm⁻³
- All substances in standard states
If you write “standard” without stating what it means, you risk losing the definition mark.
C. Dilute / concentrated and strong / weak
These pairs are different jobs:
- Concentrated/dilute = how many moles per volume (concentration).
- Strong/weak = how much ionisation/dissociation happens in water.
If a definition is about ionisation (strong/weak), include aqueous in your wording.
2) State symbols (what (aq) really means)
State symbols are not decoration. They show you understand what exists as ions and what does not.
- (s) solid (fixed shape/volume)
- (l) liquid
- (g) gas
- (aq) dissolved in water (usually as ions)
(aq) does not mean “liquid”. For example, HCl(aq) is hydrochloric acid in water, but HCl(g) is hydrogen chloride gas.
3) Units that belong inside definitions
You don’t always need to write units in a definition, but you do need to include the unit phrase (per mole / per unit time / per unit volume) when the concept depends on it.
| Term | Definition skeleton that scores | Typical units (if asked) |
|---|---|---|
| Concentration | amount of solute (moles) per unit volume of solution | mol dm⁻³ |
| Rate of reaction | change in amount/concentration per unit time | mol dm⁻³ s⁻¹ (or cm³ s⁻¹) |
| Enthalpy change, Δ H | energy change per mole of reaction (as defined) | kJ mol⁻¹ |
| Entropy change, Δ S | entropy change per mole (system) | J K⁻¹ mol⁻¹ |
| Gibbs free energy change, Δ G | change in Gibbs free energy per mole (system) | kJ mol⁻¹ |
| Electrode potential, E | potential difference of a half-cell (relative reference) | V |
4) Definition bank (high-frequency, mark-scheme-safe)
Use these as templates. Keep them short and condition-aware.
O Level (6092) templates
- Acid: produces H + (aq) ions in aqueous solution.
- Alkali: a soluble base that produces OH-(aq) ions in aqueous solution.
- Strong acid: an acid that is fully ionised in aqueous solution.
- Concentrated acid: an acid with a high concentration (many moles per dm³).
- Oxidation: loss of electrons (increase in oxidation state).
- Reduction: gain of electrons (decrease in oxidation state).
- Oxidising agent: accepts electrons and is reduced.
- Reducing agent: donates electrons and is oxidised.
- Electrolyte: conducts electricity when molten or aqueous due to mobile ions.
- Anode: electrode where oxidation occurs.
- Cathode: electrode where reduction occurs.
Helpful hubs for these definitions:
A Level (9476 H2) templates
- Standard enthalpy change of formation, Δ H_f°: enthalpy change when 1 mol of a compound is formed from its elements in their standard states under standard conditions.
- Standard enthalpy change of combustion, Δ H_c°: enthalpy change when 1 mol of a substance is completely burned in oxygen under standard conditions.
- Standard electrode potential, E°: electrode potential of a half-cell measured under standard conditions relative to the standard hydrogen electrode.
- Lattice enthalpy (formation): enthalpy change when 1 mol of an ionic solid is formed from its gaseous ions under standard conditions.
- Rate equation: equation relating rate to reactant concentrations (with a rate constant k).
- Order of reaction: power of a reactant concentration in the rate equation (overall order = sum of powers).
- Buffer solution: solution that resists pH change when small amounts of acid or alkali are added.
Helpful hubs for these definitions:
Worked Examples
Write your answer in one sentence first. Then check: did you include the condition/unit phrase that makes it mark-scheme-complete?
Worked Example 1 (Acid vs alkali: include aqueous)
Define “acid” and “alkali” in O Level Chemistry.
Show Answer
- Acid: produces H + (aq) ions in aqueous solution.
- Alkali: a soluble base that produces OH-(aq) ions in aqueous solution.
Worked Example 2 (Strong vs concentrated: don’t mix the jobs)
State the difference between a strong acid and a concentrated acid.
Show Answer
- Strong refers to extent of ionisation in aqueous solution (strong acids fully ionise).
- Concentrated refers to concentration (moles per dm³).
Worked Example 3 (Standard definition: include “1 mol” + standard conditions)
Define standard enthalpy change of formation, Δ H_f°.
Show Answer
Δ H_f° is the enthalpy change when 1 mol of a compound is formed from its elements in their standard states under standard conditions (298 K, 100 kPa, and 1.0 mol dm⁻³ for solutions).
Worked Example 4 (Rate definition: include “per unit time”)
Define rate of reaction.
Show Answer
Rate of reaction is the change in concentration (or amount) of reactant used (or product formed) per unit time.
Worked Example 5 ((aq) is not “liquid”)
What is the meaning of (aq) in an equation?
Show Answer
(aq) means dissolved in water (aqueous solution), usually as ions. It does not mean “liquid”.
Next steps
- Exam Skills Hub: command words, data answers, graphs, and practical “suggest” wording.
- For O Level definitions: O Level Cheatsheet.