The Ions in Acids and Alkalis

Section 1 of 7Which ions make a solution acidic or alkaline

Acids and H⁺

• Acids and H⁺ An acid is a substance that releases hydrogen ions, H⁺, when it dissolves in water.
• Hydrochloric acid, HCl, is a typical example: in water it separates into H⁺(aq) and Cl^-(aq).

Alkalis and OH^-

• Alkalis and OH^- An alkali is a soluble base. It releases hydroxide ions, OH^-, in water.
• Sodium hydroxide, NaOH, gives Na⁺(aq) and OH^-(aq).

Why this matters

• Why this matters The ions, not the whole compounds, are what give a solution its acidic or alkaline properties and decide how it behaves with an indicator.

Section 2 of 7Neutralisation at the ion level

1

An acid and an alkali are mixed. The acid supplies H⁺(aq) ions and the alkali supplies OH^-(aq) ions.

2

The H⁺ ions and OH^- ions join together in a one-to-one ratio to make water molecules.

3

The ionic equation is the same for every acid-alkali neutralisation: H⁺(aq) + OH^-(aq) → H₂O(l).

4

The other ions (for example Na⁺ and Cl^-) stay in solution as spectator ions and form the salt when the water is removed.

5

When exactly enough alkali has been added to use up all the H⁺, the solution is neutral with a pH of 7.

Section 3 of 7Reading the pH scale

pH range	Description	Examples
0 to 2	Strong acid	Hydrochloric acid, sulfuric acid
3 to 6	Weak acid	Ethanoic acid, citric acid
7	Neutral	Pure water
8 to 11	Weak alkali	Ammonia solution, sodium hydrogencarbonate
12 to 14	Strong alkali	Sodium hydroxide, potassium hydroxide

Section 4 of 7pH and hydrogen ion concentration

pH and H⁺ concentration

Inverse link

• Inverse link The more hydrogen ions per unit volume, the lower the pH.
  • A solution with a high H⁺ concentration is strongly acidic.

Alkalis

• Alkalis As the concentration of OH^- ions rises, the pH rises above 7.

Tenfold steps

• Tenfold steps The scale is logarithmic: each drop of 1 pH unit means the H⁺ concentration is ten times greater.
• A solution of pH 2 therefore has ten times more H⁺ than one of pH 3 and a hundred times more than one of pH 4.

Measuring pH

• Measuring pH Universal indicator gives an approximate pH by its colour;
  • a pH meter gives a more precise digital reading.

Section 5 of 7Not every acid reaction is neutralisation

Metal plus acid

• Metal plus acid Magnesium reacts with hydrochloric acid to form a salt and hydrogen gas, but no water is made.
• Because water is not a product, this is not classed as a neutralisation.

Test for neutralisation

• Test for neutralisation A true neutralisation always involves H⁺ and OH^- (or O^2- or CO₃^2- in bases) forming water, and the pH moves towards 7.

Section 6 of 7Key terms

Term	Meaning
Acid	A substance that releases hydrogen ions, H+, in aqueous solution.
Alkali	A soluble base that releases hydroxide ions, OH-, in aqueous solution.
Neutralisation	The reaction of H+ ions with OH- ions to form water.
pH	A number that shows how acidic or alkaline a solution is; it depends inversely on the H+ concentration.
Spectator ion	An ion present in the mixture that does not take part in the reaction.
Strong acid	An acid with a very low pH (about 0 to 2) because it has a high concentration of H+ ions.

Section 7 of 7Exam technique

1

Saying that alkalis contain H⁺ ions. Alkalis are defined by OH^- ions; the H⁺ concentration is simply very low.

2

Thinking pH 3 is only slightly more acidic than pH 4. It has ten times the H⁺ concentration because the scale is logarithmic.

3

Calling every acid reaction a neutralisation. Metal plus acid makes a salt and hydrogen, not water.

4

Writing the ionic equation without state symbols or with unbalanced charges, such as H + OH → H₂O.