Vinegar and lemon juice share similar chemical properties despite being completely different substances. This shared behaviour comes down to the release of a single type of particle when they dissolve in water.
For example, strong acids like hydrochloric acid and nitric acid dissociate in water:
For example, solid sodium hydroxide dissolves in water to release its constituent ions:
A student tests an acidic solution and finds it has a pH of 5. They modify the solution so that the ion concentration increases by a factor of 100. What is the new pH of the solution?
Step 1: Determine the number of 10-fold changes.
Step 2: Relate the factor change to pH units.
Step 3: Calculate the final pH.
Students often think hydrogen ions remain part of the acid molecule in solution, but OCR mark schemes require you to state clearly that the molecules separate (dissociate) into free ions.
Do not confuse bases and alkalis — remember that all alkalis are bases, but only bases that physically dissolve in water are classified as alkalis.
In equations involving dissociation or neutralisation, always include the (aq) state symbol for the H+ and OH- ions to secure full marks in the exam.
When defining a strong acid, do not just say it has a low pH; you must explicitly state that it undergoes 'complete dissociation' in water.
Higher Tier students must remember to use the reversible arrow symbol (⇌) when writing equations for the dissociation of weak acids to show that the reaction does not go to completion.
Acid
A substance that releases hydrogen ions (H+) when dissolved in an aqueous solution.
Hydrogen ions (H+)
Positively charged particles released by acids in water that are responsible for acidic chemical properties.
Aqueous solution
A solution in which the solvent is water, indicated in chemical equations by the state symbol (aq).
Dissociation
The process by which a molecule splits into smaller particles, such as ions, when placed in a solvent like water.
Ionisation
Another term for dissociation, describing the formation of ions from molecules in a solvent.
Alkali
A soluble base that releases hydroxide ions (OH-) when dissolved in an aqueous solution.
Hydroxide ions (OH-)
Negatively charged polyatomic ions released by alkalis in water that are responsible for alkaline chemical properties.
Neutralisation ionic equation
The fundamental equation showing hydrogen ions reacting with hydroxide ions to form water: H+(aq) + OH-(aq) → H2O(l).
Strong acid
An acid that fully dissociates in an aqueous solution, releasing all of its hydrogen ions.
Weak acid
An acid that only partially dissociates in an aqueous solution, establishing a reversible equilibrium.
Concentration
A measure of the number of moles of acid or alkali particles dissolved in a given volume (usually 1 dm³).
Put your knowledge into practice — try past paper questions for Chemistry A
Acid
A substance that releases hydrogen ions (H+) when dissolved in an aqueous solution.
Hydrogen ions (H+)
Positively charged particles released by acids in water that are responsible for acidic chemical properties.
Aqueous solution
A solution in which the solvent is water, indicated in chemical equations by the state symbol (aq).
Dissociation
The process by which a molecule splits into smaller particles, such as ions, when placed in a solvent like water.
Ionisation
Another term for dissociation, describing the formation of ions from molecules in a solvent.
Alkali
A soluble base that releases hydroxide ions (OH-) when dissolved in an aqueous solution.
Hydroxide ions (OH-)
Negatively charged polyatomic ions released by alkalis in water that are responsible for alkaline chemical properties.
Neutralisation ionic equation
The fundamental equation showing hydrogen ions reacting with hydroxide ions to form water: H+(aq) + OH-(aq) → H2O(l).
Strong acid
An acid that fully dissociates in an aqueous solution, releasing all of its hydrogen ions.
Weak acid
An acid that only partially dissociates in an aqueous solution, establishing a reversible equilibrium.
Concentration
A measure of the number of moles of acid or alkali particles dissolved in a given volume (usually 1 dm³).