Why care about acids, bases and salts?
Ever wondered why lemons make your tongue tingle or why soap feels slippery? Those everyday sensations are all about acids, bases and the salts they form. Mastering them not only earns marks, it explains the chemistry behind kitchen, cleaning and even your body.
💡 In Simple Words: Acids are substances that give you a sour taste and can donate a special particle called a hydrogen ion. Bases feel bitter, feel slippery and can accept that hydrogen ion. When they meet, they cancel each other out and make a neutral salt plus water.
What exactly is an acid?
Acid means a chemical that releases hydrogen ions (H⁺) when it dissolves in water. Think of hydrogen ions as tiny messengers that rush out like water flowing out of a leaky faucet. The more H⁺ it releases, the stronger the acid.
Common examples
- Hydrochloric acid (HCl) – the stomach’s digestive juice.
- Sulphuric acid (H₂SO₄) – found in car batteries.
- Acetic acid (CH₃COOH) – gives vinegar its sour bite.
What is a base?
Base is a substance that accepts hydrogen ions or releases hydroxide ions (OH⁻) in water. Hydroxide ions are like the opposite of hydrogen ions – they’re the “negative” side of the same coin. When you dissolve a base, it feels slippery, much like how soap makes your hands smooth.
Everyday bases
- Sodium hydroxide (NaOH) – the strong cleaning agent in drain openers.
- Ammonia solution (NH₃) – the smell you notice when cleaning windows.
- Magnesium hydroxide (Mg(OH)₂) – the active ingredient in some antacids.
What do we call the product of an acid‑base reaction?
The neutral product is a salt. A salt is any ionic compound that forms when the hydrogen ion from an acid swaps places with a metal or ammonium ion from a base. For instance, when hydrochloric acid meets sodium hydroxide, the result is sodium chloride (common table salt) and water.
Neutralisation – the classic acid‑base dance
Neutralisation is simply the process where an acid and a base combine to cancel each other's effects, leaving a salt and water. It’s like two friends with opposite opinions meeting and agreeing on a compromise.
Strength matters: Strong vs Weak
Not all acids or bases behave the same. A strong acid or base dissociates (splits apart) completely in water, giving lots of ions. A weak one only partially dissociates, leaving many molecules untouched.
| Property | Strong Acid | Weak Acid |
|---|---|---|
| Ionisation in water | Almost 100 % (e.g., HCl) | Only a few % (e.g., CH₃COOH) |
| pH of 0.1 M solution | ≈1 | ≈3–4 |
Same idea applies to bases. Remember: the larger the concentration of H⁺, the lower the pH; the larger the concentration of OH⁻, the higher the pH.
Indicators – the colour‑changing detectives
Indicators are substances that change colour depending on the acidity of a solution. Litmus paper turns red in acid and blue in base. Phenolphthalein stays colourless in acid but turns pink in a basic medium. They’re handy tools in the lab and in exam questions.
Key reactions you must know
1. Acid + Metal → Salt + Hydrogen gas
Example: Zn + 2HCl → ZnCl₂ + H₂↑. Zinc metal (Zn) reacts with hydrochloric acid, giving zinc chloride (a salt) and bubbling hydrogen gas.
2. Base + Acid → Salt + Water (Neutralisation)
Example: NaOH + H₂SO₄ → Na₂SO₄ + 2H₂O. Sodium hydroxide (a base) neutralises sulphuric acid, forming sodium sulphate (salt) and water.
3. Salt + Water → Acid + Base (Hydrolysis)
Some salts react with water to give a weak acid or base, explaining why certain solutions feel slightly acidic or basic. Example: NH₄Cl + H₂O → NH₄⁺ + Cl⁻ → NH₃ (weak base) + H₃O⁺ (acidic).
Quick revision checklist
- Define acid, base, salt, pH, strong, weak.
- Remember the ion‑exchange idea: H⁺ from acid swaps with metal/ NH₄⁺ from base.
- Be able to write balanced equations for the three main reaction types.
- Identify the correct indicator colour change for a given solution.
- Compare strong vs weak acids/bases in a table format.
📝 Likely Exam Questions
1. Write the balanced chemical equation for the reaction between sodium hydroxide and hydrochloric acid. State the type of reaction.
Answer: NaOH + HCl → NaCl + H₂O. It is a neutralisation reaction.
2. Explain why sulphuric acid is called a strong acid, using the concept of ionisation.
Answer: In water, H₂SO₄ dissociates almost completely into 2H⁺ and SO₄²⁻ ions, giving a very high concentration of hydrogen ions, which characterises a strong acid.
3. A student adds phenolphthalein to a solution and observes a pink colour. What does this indicate about the solution’s pH?
Answer: The solution is basic (pH > 8.2) because phenolphthalein turns pink only in alkaline conditions.
4. Compare the properties of a strong acid and a weak acid in a short table.
Answer: See the table in the article – strong acids fully ionise, have low pH, conduct electricity well; weak acids partially ionise, have higher pH, conduct electricity poorly.
5. Describe what happens when copper(II) sulphate solution is treated with sodium hydroxide.
Answer: CuSO₄ + 2NaOH → Cu(OH)₂ (blue precipitate) + Na₂SO₄. This is a double‑displacement reaction forming a sparingly soluble base (copper hydroxide) and a soluble salt (sodium sulphate).