Acids & Bases (CIE A Level Chemistry)

• An acid is a substance that neutralises a base forming a salt and water:

acid	+	base		salt	+	water
2HCl (aq)	+	CaO (s)		CaCl2 (aq)	+	H2O (l)

• Acids are also substances that release hydrogen ions when they dissolve in water:
  • HCl(g) → H⁺(aq) + Cl^- (aq)

Acid dissociation

Acids dissociate in water to release a hydrogen ion

Common acids table

Name	Formula	Ions formed in water
Hydrochloric acid	HCl	H+ + Cl–
Nitric acid	HNO3	H+ + NO3–
Sulfuric acid	H2SO4	H+ + SO42–
Ethanoic acid	CH3COOH	H+ + CH3COO–

• Monoprotic inorganic acids, such as hydrochloric acid, fully dissociate into their ions
• Organic acids, such as carboxylic acids, do not fully dissociate into their ions
  • Only some of the hydrogen atoms can form ions 

• A base is a compound that neutralises an acid forming a salt and water

acid	+	base		salt	+	water
2HCl (aq)	+	CaO (s)		CaCl2 (aq)	+	H2O (l)

• A base is a substance that accepts hydrogen ions or a compound that contains oxide or hydroxide ions
  • For example, when the base ammonia is added to water, the ammonium ion and hydroxide ions are formed:

NH₃ (g) + H₂O (l) → NH₄⁺ (aq) + OH^– (aq)

• • For example, when sodium hydroxide is dissolved in solution, sodium ions and hydroxide ions are formed:

NaOH (s) + aq → Na⁺ (aq) + OH^– (aq)

• A base that is soluble in water is called an alkali

Common alkalis table

Name	Formula	Ions formed in water
Sodium hydroxide	NaOH	Na+ + OH–
Potassium hydroxide	KOH	K+ + OH–
Aqueous ammonia	NH3	NH4+ + OH–