What this chapter is about
This chapter introduces the chemistry of three important categories of compounds: acids, bases and salts. You will learn how to identify acids and bases using indicators, understand their chemical properties, and see how they react with each other and with other substances like metals and carbonates.
The chapter also explains the pH scale, which measures how acidic or basic a solution is. This concept connects chemistry to everyday life—from digestion in our stomach to the soil in fields and the water we drink. You will understand why antacids relieve acidity and why farmers add certain substances to soil.
By the end, you should be able to write balanced equations for reactions involving acids and bases, predict products of neutralisation reactions, explain the importance of pH in living systems, and describe how common salts are prepared and used.
Key ideas
- Acids are substances that produce hydrogen ions (H⁺) when dissolved in water; bases produce hydroxide ions (OH⁻) in water.
- Indicators like litmus, phenolphthalein and methyl orange change colour in acidic and basic solutions, helping us identify them.
- Acids react with metals to produce hydrogen gas and a salt; they react with metal carbonates and bicarbonates to produce carbon dioxide gas, water and a salt.
- Neutralisation is the reaction between an acid and a base to form salt and water: Acid + Base → Salt + Water.
- The pH scale runs from 0 to 14; pH below 7 is acidic, pH 7 is neutral, pH above 7 is basic.
- Strong acids and bases ionise completely in water; weak acids and bases ionise only partially.
- Common salts like sodium chloride, washing soda, baking soda and bleaching powder have important uses in daily life and industry.
Formulas and facts to remember
- General reaction of acid with metal: Metal + Acid → Salt + Hydrogen gas. Example: Zn + 2HCl → ZnCl₂ + H₂↑
- Acid with metal carbonate: Na₂CO₃ + 2HCl → 2NaCl + H₂O + CO₂↑
- Acid with metal bicarbonate: NaHCO₃ + HCl → NaCl + H₂O + CO₂↑
- Neutralisation: NaOH + HCl → NaCl + H₂O
- pH scale: 0–14; pH 7 is neutral; lower pH means more acidic, higher pH means more basic.
- Baking soda (sodium bicarbonate): NaHCO₃; used in cooking, fire extinguishers, antacids.
- Washing soda: Na₂CO₃·10H₂O; used in glass, soap and paper industries.
- Bleaching powder: CaOCl₂; made by passing chlorine over dry slaked lime; used for disinfecting water.
- Plaster of Paris: CaSO₄·½H₂O; sets hard when mixed with water; used for casts and moulds.
Worked examples
Example 1: Reaction of zinc with dilute sulphuric acid
Problem: A small piece of zinc is dropped into dilute sulphuric acid. Write the balanced equation and identify the gas released.
Solution: Zinc is a metal; it reacts with the acid to form a salt and hydrogen gas.
Unbalanced: Zn + H₂SO₄ → ZnSO₄ + H₂
Check atoms: Zn (1 = 1), S (1 = 1), O (4 = 4), H (2 = 2). Already balanced.
Balanced equation: Zn + H₂SO₄ → ZnSO₄ + H₂↑
The gas released is hydrogen. It burns with a pop sound when a burning matchstick is brought near.
Example 2: Finding if a solution is acidic or basic
Problem: A solution turns blue litmus red but shows no colour change with red litmus. Its pH is measured as 3. Is the solution acidic or basic? Suggest one substance that might behave this way.
Solution: Blue litmus turning red indicates an acidic solution. Red litmus remains red in acid (no change expected).
pH 3 is below 7, confirming the solution is acidic.
Substances with pH around 3 include lemon juice or vinegar (dilute acetic acid).
Example 3: Neutralisation calculation (concept-based)
Problem: 20 mL of dilute hydrochloric acid is exactly neutralised by 20 mL of sodium hydroxide solution of the same concentration. What is formed?
Solution: The reaction is: HCl + NaOH → NaCl + H₂O
Since equal volumes of solutions of equal concentration are mixed, moles of acid equal moles of base.
Complete neutralisation occurs, forming sodium chloride (common salt) and water.
The resulting solution will have pH close to 7 (neutral).
Common mistakes
- Thinking all acids are dangerous liquids → Many acids like citric acid (in oranges) and acetic acid (in vinegar) are safe in dilute form and are part of food.
- Confusing strong acids with concentrated acids → Strong refers to complete ionisation; concentrated refers to amount of acid per unit volume. Dilute hydrochloric acid is still a strong acid.
- Writing H₂ gas release when acid reacts with carbonate → Carbonates release CO₂, not H₂; hydrogen gas comes from acid-metal reactions.
- Believing pH 0 is neutral because zero sounds like nothing → pH 7 is neutral; pH 0 is highly acidic.
- Forgetting water of crystallisation in salt formulas → Washing soda is Na₂CO₃·10H₂O, not just Na₂CO₃; the 10H₂O matters for its properties.
Quick revision
- Acids give H⁺ ions in water; bases give OH⁻ ions.
- Acid + Base → Salt + Water (neutralisation).
- Acid + Metal → Salt + H₂ gas; Acid + Carbonate → Salt + H₂O + CO₂ gas.
- pH scale: 0–14; below 7 acidic, 7 neutral, above 7 basic.
- Baking soda (NaHCO₃) neutralises excess stomach acid; washing soda (Na₂CO₃·10H₂O) softens hard water.
- Plaster of Paris sets hard by absorbing water to form gypsum.