State that aqueous solutions of acids contain $mathrm{H}^{+}$ ions and aqueous solutions of alkalis contain $mathrm{OH}^{-}$ ions

Acids, Bases and Salts – Characteristic Properties

Quick fact: In water, acids give off \$H^+\$ ions, while alkalis (bases) give off \$OH^-\$ ions. Think of \$H^+\$ as tiny “acidic” helpers that love to join with water molecules, and \$OH^-\$ as “basic” helpers that prefer to stay on the other side of the water bridge. 🍋🧼

Acids – The Sour Side of Chemistry

  • 🔹 Common examples: \$HCl\$ (table salt), \$H2SO4\$ (sulphuric acid), \$HNO_3\$ (nitric acid).

  • 🔹 Key property: Release \$H^+\$ ions when dissolved in water.

  • 🔹 Analogy: Imagine a crowded dance floor where \$H^+\$ ions are the energetic dancers that keep the room lively.

  • 🔹 Behaviour:

    1. They taste sour (like lemon). 🍋

    2. They turn blue litmus paper red.

    3. They react with metals to produce hydrogen gas.

Bases – The Calm, Alkaline Zone

  • 🔹 Common examples: \$NaOH\$ (sodium hydroxide), \$KOH\$ (potassium hydroxide).

  • 🔹 Key property: Release \$OH^-\$ ions into water.

  • 🔹 Analogy: Think of \$OH^-\$ ions as calm, cool swimmers that keep the water less acidic.

  • 🔹 Behaviour:

    1. They taste bitter or soapy.

    2. They turn red litmus paper blue.

    3. They feel slippery (like soap).

Salts – The Neutral Middle Ground

Salts are formed when an acid reacts with a base. They usually do not give off \$H^+\$ or \$OH^-\$ ions in water, so they are neutral. Example: \$NaCl\$ (table salt) comes from \$HCl\$ + \$NaOH\$.

Quick Reference Table

PropertyAcidBaseSalt
Ion released in water\$H^+\$\$OH^-\$None (neutral)
TasteSourBitter/soapNeutral
Litmus changeBlue → RedRed → BlueNo change
Reaction with metalProduces H₂ gasNo reactionNo reaction

Exam Tip: When you see a reaction like \$HCl + NaOH → NaCl + H_2O\$, remember that the acid provides \$H^+\$ and the base provides \$OH^-\$. They combine to form water, leaving behind a neutral salt. This is a classic neutralisation reaction. 🔍

Quick Question: If you add \$H2SO4\$ to a solution of \$NaOH\$, what ions will be released into the water?


Answer: \$H^+\$ from the acid and \$OH^-\$ from the base. They combine to form water, leaving behind \$Na^+\$ and \$SO_4^{2-}\$ ions in the solution. ??