Acids and Bases
Acids and bases are common in Chemistry and everyday life. They react with each other in neutralisation reactions to form salt and water.
What is an acid?
- An acid is a substance that releases hydrogen ions (H⁺) when dissolved in water
- Acids have a pH less than 7
Examples:
- Hydrochloric acid (HCl)
- Sulfuric acid (H₂SO₄)
- Nitric acid (HNO₃)
What is a base?
- A base is a substance that can neutralise an acid
- A soluble base is called an alkali
- Alkalis release hydroxide ions (OH⁻) in water
- Alkalis have a pH greater than 7
Examples:
- Sodium hydroxide (NaOH)
- Potassium hydroxide (KOH)
- Ammonia (NH₃)
pH scale
The pH scale runs from 0 to 14 and measures how acidic or alkaline a solution is.
pH | Type of solution |
0–3 | Acidic (low pH) |
4–6 | Mildly acidic |
7 | Neutral |
8–10 | Mildly alkaline |
11–14 | Alkaline (high pH) |
- Measured using universal indicator or a pH probe
- A lower pH = more H⁺ ions
- A higher pH = more OH⁻ ions
Concentrated vs Dilute
- A concentrated acid has a large amount of acid particles in a given volume
- A dilute acid has a small amount of acid particles in the same volume
- The concentration affects pH – more concentrated = lower pH
💡 Concentration refers to how much acid is present, not how it ionises
Neutralisation
Acid + Base → Salt + Water
This is called a neutralisation reaction – the H⁺ from the acid reacts with the OH⁻ from the base to make H₂O.
Examples:
- HCl + NaOH → NaCl + H₂O
- H₂SO₄ + 2KOH → K₂SO₄ + 2H₂O
Uses of acids and alkalis
- Acids – car batteries (sulfuric acid), cleaning products
- Alkalis – oven cleaner, indigestion tablets (neutralise stomach acid)
- Neutralisation – used in agriculture, treating insect stings, and in lab reactions
Questions
- What ion do acids release in water?
- What is the pH of a neutral solution?
- What is the name of a soluble base?
- What are the products of acid + alkali?
- What’s the difference between concentrated and dilute?
Summary
- Acids release H⁺ ions and have a pH less than 7
- Bases (alkalis) release OH⁻ ions and have a pH above 7
- The pH scale shows how acidic or alkaline a solution is
- Concentration affects pH – more concentrated = more extreme pH
- Neutralisation produces salt and water when acids and bases react