Acids, Bases and Salts · Lesson 6 of 12
What Happens To An Acid Or A Base In A Water Solution?
“Water walks in and suddenly acids and bases start showing their true ionic personalities.”
• Explain why water is necessary for acidic behaviour of hydrogen chloride. • Distinguish H⁺ from H₃O⁺ in aqueous solution. • Explain dissociation of common bases in water. • Express neutralisation in ionic form. • Explain dilution and the correct safety procedure for mixing concentrated acids or bases with water.
The presence of an acid molecule is not enough by itself to produce acidic behaviour. Water plays a crucial role because it allows ions to form and move.
Hydrogen Chloride And Water
Dry hydrogen chloride gas does not turn dry blue litmus red. When moisture is present, the behaviour changes. Hydrogen chloride reacts with water and produces hydronium ions, which are responsible for acidic behaviour.
A Hydronium Ion, H₃O⁺, is formed when a hydrogen ion associates with a water molecule.
A bare hydrogen ion is extremely small and highly charged. In aqueous solution it does not remain isolated; it associates with water molecules. This is why acidic solutions are often described in terms of H⁺(aq) or H₃O⁺.
Bases In Water
Neutralisation In Ionic Form
The essential change during neutralisation is the combination of hydrogen ions from the acid with hydroxide ions from the base. They form water.
Dilution
Dilution is the process of decreasing the concentration of a solution by adding more solvent, usually water.
Dissolving concentrated acids or bases in water releases a large amount of heat. The process is highly exothermic. If water is poured directly onto concentrated acid, a small region can heat extremely rapidly. The water may boil locally and cause acid to splash.
Always add concentrated acid slowly to water with continuous stirring. Do not pour water directly into concentrated acid. The same care is required when dissolving strongly basic solids because considerable heat may be released.
When water is added, the total number of acid or base particles may remain present, but they are spread through a larger volume. Therefore the number of H₃O⁺ or OH⁻ ions per unit volume decreases.
Dilution changes how much solute is present per unit volume. Strength describes how extensively an acid or base produces ions in water. A strong acid can be dilute, and a weak acid can be concentrated.
Quiz
What is the correct safety procedure for diluting concentrated acid?
Why does dry hydrogen chloride gas not turn dry blue litmus red?
Which ion forms when H⁺ combines with a water molecule?
What is the essential ionic change during neutralisation?
Practice Problems
- Explain why dry hydrogen chloride gas does not affect dry blue litmus paper.
- Write the equation showing formation of hydronium ions from hydrogen chloride and water.
- Write equations showing the formation of hydroxide ions from sodium hydroxide and potassium hydroxide in water.
- Write and explain the ionic equation for neutralisation.
- Explain why concentrated acid must be added slowly to water rather than adding water directly to concentrated acid.
Key Takeaways
• Water is necessary for acids such as hydrogen chloride to produce hydronium ions. • Hydrogen ions in aqueous solution are represented as H⁺(aq) or H₃O⁺. • Bases produce hydroxide ions in water. • Neutralisation can be represented as H⁺(aq) + OH⁻(aq) → H₂O(l). • Dilution of concentrated acids and bases releases heat. • Concentrated acid should always be added slowly to water with stirring.