Acids and alkalis (Edexcel GCSE Chemistry): Revision Notes

Acids and alkalis

Acids

• Acids have a pH less than 7.

• In water, acids form H⁺ ions (hydrogen ions).

• The higher the concentration of hydrogen ions, the more acidic the solution, and the lower the pH. Examples of acids:

  • Hydrochloric acid ($HCl$)
  • Sulfuric acid ($H₂SO₄$)
  • Nitric acid ($HNO₃$)

Alkalis

• An alkali is a base that is soluble in water.

• Alkalis have a pH greater than 7.

• In water, alkalis form OH⁻ ions (hydroxide ions).

• The higher the concentration of hydroxide ions, the more alkaline the solution, and the higher the pH. Examples of alkalis:

  • Sodium hydroxide ($NaOH$)
  • Potassium hydroxide ($KOH$)
  • Ammonia solution ($NH₃$ in water)

Bases

Indicators

A base is a substance that reacts with an acid to produce salt and water. Bases can be solids or dissolved in water. When a base dissolves in water, it becomes an alkali.

Indicators are substances used to test whether a solution is acidic, neutral, or alkaline. The table below shows how common indicators change colour depending on the pH of the solution:

Neutralisation Reactions

A neutralisation reaction occurs when an acid reacts with a base. The products of this reaction are a salt and water, and the resulting solution is neutral (pH 7). In a neutral solution, the concentration of hydrogen ions (H⁺) is equal to the concentration of hydroxide ions ($OH⁻$).

$$\text{Acid} + \text{Base} \rightarrow \text{Salt} + \text{Water}$$

For example:

$$\text{Hydrochloric acid (HCl)} + \text{Sodium hydroxide (NaOH)} \rightarrow \text{Sodium chloride (NaCl)} + \text{Water (H₂O)}$$

In Aqueous Solutions:

• Acids in water: $HCl → H⁺ + Cl⁻$

• Alkalis in water: $NaOH → Na⁺ + OH⁻$ When an acid neutralises a base:

• H⁺ ions from the acid combine with OH⁻ ions from the base to form water (H₂O). | Indicator | Acidic (pH < 7) | Neutral (pH = 7) | Alkaline (pH > 7) | |---|---|---|---| | Litmus | Red | Purple | Blue | | Methyl Orange | Red | Yellow | Yellow | | Phenolphthalein | Colourless | Colourless | Pink |