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Acids and Hydrogen Ions

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.

An acid is defined as a substance that releases hydrogen ions ( $H^+$ ) when it dissolves in water to form an aqueous solution.
When acid molecules enter water, they undergo a process called dissociation (also known as ionisation).
During dissociation, the covalent bond holding the hydrogen atom breaks, releasing a positively charged $H^+$ ion and a negative ion.
The chemical properties of all acids are entirely due to the presence of these ions.

Alkalis and Hydroxide Ions

An alkali is a soluble base that dissolves in water. While all alkalis act as bases, it is important to note that not all bases are alkalis—only those that are soluble in water qualify.
When alkalis dissolve in an aqueous solution, they release negatively charged hydroxide ions ( $OH^-$ ).
The specific presence of these $OH^-$ ions is what makes a solution alkaline and gives alkalis their characteristic chemical properties.

For example, solid sodium hydroxide dissolves in water to release its constituent ions:

\text{NaOH(s)} \xrightarrow{\text{H}_2\text{O}} \text{Na}^+\text{(aq)} + \text{OH}^-\text{(aq)}

The Neutralisation Equation

When an acid and an alkali react, their key ions interact directly to cancel each other out.
The fundamental neutralisation ionic equation shows aqueous hydrogen ions reacting with aqueous hydroxide ions to form liquid water:

\text{H}^+\text{(aq)} + \text{OH}^-\text{(aq)} \rightarrow \text{H}_2\text{O(l)}

Strong vs. Weak Acids

Acids are categorised by their degree of ionisation. A strong acid (such as $\text{HCl}$ or $\text{H}_2\text{SO}_4$ ) dissociates completely in water, meaning every single acid molecule splits to release $H^+$ ions.

\text{CH}_3\text{COOH(aq)} \rightleftharpoons \text{CH}_3\text{COO}^-\text{(aq)} + \text{H}^+\text{(aq)}

You must distinguish between the strength of an acid and its concentration. Strength refers to the degree of dissociation, whereas concentration refers to the total number of moles of acid particles dissolved in a given volume.

pH and Ion Concentration

The pH scale is a direct measure of the concentration of $H^+$ ions in a solution.
Solutions with a high concentration of $OH^-$ ions have a pH greater than 7 and are classified as alkaline.
The pH scale is logarithmic. This means that a change of exactly 1 pH unit represents a 10-fold (factor of 10) change in the concentration of $H^+$ ions.

Worked Example

A student tests an acidic solution and finds it has a pH of 5. They modify the solution so that the $H^+$ 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.

An increase by a factor of 100 is $10 \times 10$ , which represents two factors of 10.

Step 2: Relate the factor change to pH units.

Two factors of 10 mean the pH will shift by exactly 2 units.

Step 3: Calculate the final pH.

Because the $H^+$ concentration increased, the solution has become more acidic, meaning the pH must decrease.
$\text{New pH} = 5 - 2 = 3$
Final Answer: pH 3

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.

A substance that releases hydrogen ions (H+) when dissolved in an aqueous solution.

Positively charged particles released by acids in water that are responsible for acidic chemical properties.

A solution in which the solvent is water, indicated in chemical equations by the state symbol (aq).

The process by which a molecule splits into smaller particles, such as ions, when placed in a solvent like water.

Another term for dissociation, describing the formation of ions from molecules in a solvent.

A soluble base that releases hydroxide ions (OH-) when dissolved in an aqueous solution.

Negatively charged polyatomic ions released by alkalis in water that are responsible for alkaline chemical properties.

The fundamental equation showing hydrogen ions reacting with hydroxide ions to form water: H+(aq) + OH-(aq) → H2O(l).

An acid that fully dissociates in an aqueous solution, releasing all of its hydrogen ions.

An acid that only partially dissociates in an aqueous solution, establishing a reversible equilibrium.

A measure of the number of moles of acid or alkali particles dissolved in a given volume (usually 1 dm³).