(Years ago, a student asked me why lemon juice stings a cut but soap doesn't. That one question led us straight into this chapter — and by the end of the lesson, she was explaining pH to her younger brother at the dinner table.) Right now, somewhere in your home, there's a bottle of vinegar, a box of baking soda, a lemon, and a bar of soap. You probably think of them as just kitchen and bathroom items. But to a chemist, they're members of two great families: acids and bases. Acids taste sour — think of lemons, vinegar, and fizzy drinks. Bases feel slippery — think of soap, toothpaste, and baking soda. When they meet, they react. When they're balanced, they're neutral. Understanding acids and bases is not just exam material — it's how we make medicines, keep our water safe, cook food, and even keep our own bodies alive. Your stomach is full of acid right now. Your blood is very slightly basic. You are, quite literally, a walking chemistry experiment. Let's understand how it all works.
Let's start with something you already know. Taste a lemon. It's sour. That sourness comes from an acid — in this case, citric acid. Taste vinegar — sour again. That's ethanoic acid (also called acetic acid).
But chemistry isn't about taste — it's about what's happening at the particle level. An acid is a substance that produces hydrogen ions (H⁺) when dissolved in water. Those hydrogen ions are what make acids behave the way they do.
(Picture a hydrogen atom — just one proton and one electron. Now imagine it loses its electron. You're left with a bare proton, which is what H⁺ really is. These tiny, positively charged particles are the "active ingredient" in every acid. Remember that, and everything else in this chapter falls into place.)
Common acids you'll meet:
| Acid | Found In | Type |
|---|---|---|
| Hydrochloric acid (HCl) | Stomach acid | Strong |
| Sulphuric acid (H₂SO₄) | Car batteries | Strong |
| Nitric acid (HNO₃) | Fertiliser production | Strong |
| Ethanoic acid (CH₃COOH) | Vinegar | Weak |
| Citric acid | Lemons, oranges | Weak |
| Carbonic acid (H₂CO₃) | Fizzy drinks | Weak |
Now think of soap. It feels slippery. That slipperiness is a clue that you're dealing with a base. A base is a substance that accepts hydrogen ions (H⁺) — or, put another way, produces hydroxide ions (OH⁻) when dissolved in water.
An alkali is simply a base that dissolves in water. So all alkalis are bases, but not all bases are alkalis. For example, copper(II) oxide is a base but it doesn't dissolve in water, so it's not an alkali. Sodium hydroxide dissolves readily, so it is an alkali.
| Base / Alkali | Found In | Type |
|---|---|---|
| Sodium hydroxide (NaOH) | Soap-making, drain cleaners | Strong alkali |
| Potassium hydroxide (KOH) | Soft soaps | Strong alkali |
| Calcium hydroxide (Ca(OH)₂) | Limewater, cement | Weak alkali |
| Ammonia (NH₃) | Cleaning products | Weak alkali |
| Magnesium hydroxide (Mg(OH)₂) | Antacids, laxatives | Weak alkali |
| Sodium hydrogencarbonate (NaHCO₃) | Baking soda | Weak alkali |
Here's a side-by-side comparison you should know by heart:
| Property | Acids | Bases / Alkalis |
|---|---|---|
| Taste | Sour | Bitter (never taste in the lab!) |
| Feel | Not slippery | Slippery / soapy |
| Effect on litmus | Turn blue litmus red | Turn red litmus blue |
| pH | Below 7 | Above 7 |
| Ions produced in water | Hydrogen ions (H⁺) | Hydroxide ions (OH⁻) |
| Reaction with metals | Produce hydrogen gas | Generally no reaction |
| Reaction with carbonates | Produce carbon dioxide gas | Generally no reaction |
| Examples | Lemon juice, vinegar, stomach acid | Soap, baking soda, toothpaste |
Not all acids are equally fierce. The difference between "strong" and "weak" is about how completely the acid or base breaks apart (dissociates) in water.
| Term | Meaning | Examples |
|---|---|---|
| Strong acid | Completely dissociates into ions in water — every molecule releases H⁺. | Hydrochloric acid, sulphuric acid, nitric acid |
| Weak acid | Only partially dissociates — most molecules stay intact, only a few release H⁺. | Ethanoic acid, citric acid, carbonic acid |
| Strong alkali | Completely dissociates into ions in water — every unit releases OH⁻. | Sodium hydroxide, potassium hydroxide |
| Weak alkali | Only partially dissociates — few OH⁻ ions released. | Ammonia, calcium hydroxide |
Important distinction: "Strong" does not mean "concentrated." A strong acid can be dilute, and a weak acid can be concentrated. "Strong" refers to how completely it dissociates; "concentrated" refers to how much of it is dissolved in a given volume of water.
(Here's an analogy I love: imagine a crowd of people entering a building. A strong acid is like a crowd where everyone walks straight in through the open door — 100% dissociation. A weak acid is like a crowd where only a few people go in and the rest stay outside chatting — partial dissociation. The concentration is about how big the crowd is; the strength is about how many actually go in. Two different things.)
How do we know if something is an acid or a base? We don't taste it (never in the lab!). We use an indicator — a substance that changes colour depending on whether it's in an acidic, neutral, or basic solution.
| Indicator | Colour in Acid | Colour in Neutral | Colour in Base |
|---|---|---|---|
| Litmus | Red | Purple | Blue |
| Methyl orange | Red | Orange | Yellow |
| Phenolphthalein | Colourless | Colourless | Pink |
| Bromothymol blue | Yellow | Green | Blue |
| Universal indicator | Red / orange | Green | Blue / purple |
Notice that universal indicator gives a whole spectrum of colours, not just three. That's because it's a mixture of several indicators, and each one changes colour at a different pH. This lets universal indicator tell you not just whether something is acidic or basic, but how acidic or basic it is.
(Here's a trick I teach: "Red before blue" — acids turn litmus red, bases turn it blue. Say it out loud a few times. It'll stick.)
The pH scale is a numerical way of expressing how acidic or basic a solution is. It runs from 0 to 14.
| pH Value | Nature | Examples |
|---|---|---|
| 0–3 | Strongly acidic | Battery acid, stomach acid |
| 4–6 | Weakly acidic | Lemon juice, vinegar, black coffee |
| 7 | Neutral | Pure water, salt solution |
| 8–10 | Weakly basic | Baking soda, soap, toothpaste |
| 11–14 | Strongly basic | Ammonia, bleach, drain cleaner |
Key insight: The pH scale is logarithmic. This means each step represents a tenfold change in acidity. So a solution of pH 3 is ten times more acidic than one of pH 4, and a hundred times more acidic than one of pH 5. That's why small changes in pH matter so much — in your blood, in a lake, in a swimming pool.
Acids are reactive substances. They react with four main groups of substances. Learn these four reactions — they appear again and again.
When an acid reacts with a reactive metal, it produces a salt and hydrogen gas. You can test for the hydrogen with a lit splint — it pops.
Example: Magnesium + hydrochloric acid → magnesium chloride + hydrogen
Mg + 2HCl → MgCl₂ + H₂
When an acid reacts with a base, they neutralise each other, producing a salt and water. This is called neutralisation.
Example: Sodium hydroxide + hydrochloric acid → sodium chloride + water
NaOH + HCl → NaCl + H₂O
When an acid reacts with a carbonate, it produces a salt, water, and carbon dioxide gas. You can test for the CO₂ by bubbling it through limewater — the limewater turns milky.
Example: Calcium carbonate + hydrochloric acid → calcium chloride + water + carbon dioxide
CaCO₃ + 2HCl → CaCl₂ + H₂O + CO₂
Very similar to carbonates. Hydrogencarbonates (like baking soda) also produce carbon dioxide when they react with acid.
Example: Sodium hydrogencarbonate + hydrochloric acid → sodium chloride + water + carbon dioxide
NaHCO₃ + HCl → NaCl + H₂O + CO₂
(Notice the pattern: in every reaction, the acid gives up its hydrogen, and the metal from the base or carbonate takes its place, forming a salt. Once you see that pattern, you can predict the products of almost any acid reaction.)
Neutralisation is not just a lab technique. It happens all around you, and sometimes inside you.
| Situation | What Happens |
|---|---|
| Indigestion / heartburn | Stomach produces too much acid. An antacid (a base like magnesium hydroxide) neutralises the excess acid. |
| Tooth decay | Bacteria in the mouth produce acid, which attacks tooth enamel. Toothpaste (a base) neutralises the acid. |
| Soil treatment | If soil is too acidic, farmers add lime (calcium oxide or hydroxide) to neutralise it. If too alkaline, they add compost or acidic fertiliser. |
| Insect stings | Bee stings are acidic — a mild base like baking soda can soothe them. Wasp stings are alkaline — a mild acid like vinegar can soothe them. |
| Industrial waste | Factories neutralise acidic or alkaline waste before releasing it into rivers, to protect aquatic life. |
| Swimming pools | The pH of pool water must be carefully controlled, usually between 7.2 and 7.6, to keep it safe and comfortable. |
Question: A clear liquid turns red litmus paper blue. Is it an acid, a base, or neutral? What would phenolphthalein show in it?
Solution: Red litmus turning blue is the classic test for a base (or alkali). Phenolphthalein is colourless in acid and neutral, but turns pink in a base. So in this liquid, phenolphthalein would turn pink.
Question: Zinc reacts with hydrochloric acid. Write the word equation and the chemical equation.
Solution:
• Word equation: Zinc + hydrochloric acid → zinc chloride + hydrogen gas
• Chemical equation: Zn + 2HCl → ZnCl₂ + H₂
• Reasoning: Zinc is a metal, so it reacts with acid to produce a salt and hydrogen. The salt is zinc chloride (zinc + chloride from HCl).
Question: 25 cm³ of sodium hydroxide solution is neutralised by 20 cm³ of 0.1 mol/dm³ hydrochloric acid. Write the balanced equation and state what the products are.
Solution:
• Balanced equation: NaOH + HCl → NaCl + H₂O
• Products: sodium chloride (common table salt) and water.
• Reasoning: This is a classic acid + base neutralisation. The Na⁺ from the sodium hydroxide pairs with the Cl⁻ from the hydrochloric acid to form NaCl; the H⁺ and OH⁻ combine to form H₂O.
1. Define an acid and a base in terms of the ions they produce in water.
An acid produces hydrogen ions (H⁺) when dissolved in water. A base accepts hydrogen ions, or produces hydroxide ions (OH⁻) when dissolved in water.
2. What colour does each of these indicators turn in an acidic solution? (a) litmus, (b) methyl orange, (c) phenolphthalein.
(a) Litmus — red. (b) Methyl orange — red. (c) Phenolphthalein — colourless.
3. What is the pH of a neutral solution? Give an example.
pH 7. Pure water is a neutral solution at room temperature.
4. Write the general word equation for the reaction between an acid and a carbonate.
Acid + carbonate → salt + water + carbon dioxide.
5. Why is a solution of pH 3 more acidic than one of pH 5?
Because the pH scale is logarithmic, each step represents a tenfold change. A pH 3 solution has 100 times more H⁺ concentration than a pH 5 solution.
6. Explain why antacids are used to treat indigestion.
Indigestion is often caused by excess stomach acid. Antacids are bases (like magnesium hydroxide) that neutralise the excess acid, relieving the discomfort.
7. What is the difference between a base and an alkali?
A base is any substance that accepts H⁺ ions. An alkali is a base that dissolves in water. All alkalis are bases, but not all bases are alkalis.
Question 1 [8 marks]
(a) State what is meant by the term "acid". [2]
(b) Describe two differences between acids and bases. [4]
(c) Explain why the pH scale is described as logarithmic. [2]
(a) An acid is a substance that produces hydrogen ions (H⁺) when dissolved in water. (2 marks: 1 for "produces H⁺", 1 for "in water".)
(b) Any two of: Acids taste sour, bases taste bitter. Acids turn blue litmus red, bases turn red litmus blue. Acids have pH below 7, bases have pH above 7. Acids react with metals to produce hydrogen, bases generally do not. (4 marks: 2 marks per difference, 1 for statement and 1 for clarity.)
(c) The pH scale is logarithmic because each step represents a tenfold change in the concentration of H⁺ ions. (2 marks: 1 for "tenfold change", 1 for "H⁺ concentration".)
Question 2 [10 marks]
(a) Name four common laboratory indicators and state their colours in acidic and basic solutions. [4]
(b) Describe the reaction between an acid and a metal carbonate, including a test for one of the products. [4]
(c) Explain why universal indicator is more useful than litmus paper for determining pH. [2]
(a) Any four of: Litmus — red in acid, blue in base. Methyl orange — red in acid, yellow in base. Phenolphthalein — colourless in acid, pink in base. Bromothymol blue — yellow in acid, blue in base. (4 marks: 1 per correct indicator with both colours.)
(b) Acid + carbonate → salt + water + carbon dioxide. The CO₂ can be tested by bubbling it through limewater — the limewater turns milky/cloudy. (4 marks: 1 for word equation, 1 for CO₂, 1 for limewater test, 1 for "turns milky".)
(c) Universal indicator gives a range of colours corresponding to different pH values, so it tells you how acidic or basic a solution is. Litmus only tells you whether a solution is acidic or basic, not how strong it is. (2 marks: 1 for range of colours, 1 for comparison with litmus.)
Question 3 [10 marks]
(a) Write balanced chemical equations for the reaction of hydrochloric acid with: (i) magnesium, (ii) sodium hydroxide, (iii) calcium carbonate. [6]
(b) Explain, using the idea of neutralisation, why toothpaste helps prevent tooth decay. [4]
(a)
(i) Mg + 2HCl → MgCl₂ + H₂
(ii) NaOH + HCl → NaCl + H₂O
(iii) CaCO₃ + 2HCl → CaCl₂ + H₂O + CO₂
(6 marks: 2 per correct balanced equation.)
(b) Bacteria in the mouth produce acid, which attacks tooth enamel. Toothpaste contains mild bases that neutralise this acid, raising the pH in the mouth and reducing the damage to enamel. (4 marks: 1 for acid production, 1 for basic toothpaste, 1 for neutralisation, 1 for protection of enamel.)
Challenge 1: A student is given two unlabelled bottles, both containing clear, colourless liquids. One is a strong acid, the other is a weak acid. Describe how the student could distinguish between them using only universal indicator and a pH meter.
Use universal indicator first. Both will turn red or orange, but the strong acid will show a deeper red (lower pH) than the weak acid, which may show orange or yellow-orange. Then use a pH meter: the strong acid will have a pH closer to 0–1, while the weak acid will typically show a pH of 3–5. The pH meter gives a precise numerical reading, which is more reliable than colour comparison alone.
Challenge 2: A farmer finds that a field has a pH of 5.2. Explain why this may be a problem, and describe two ways the farmer could raise the pH to a more suitable level for crops.
A pH of 5.2 is acidic, which can be too acidic for many crops — essential nutrients become less available, and some toxic metals become more soluble. To raise the pH, the farmer could add lime (calcium oxide or calcium hydroxide), which is a base and neutralises the acid in the soil. Alternatively, the farmer could add crushed limestone (calcium carbonate), which also neutralises the acid but acts more slowly. Both approaches raise the pH towards neutral.
Challenge 3: Explain why a strong acid can be dilute, and a weak acid can be concentrated. Use the ideas of dissociation and concentration in your answer.
"Strong" refers to how completely an acid dissociates into ions in water. A strong acid dissociates completely — every molecule releases H⁺. "Concentrated" refers to how much acid is dissolved in a given volume of water. A strong acid can be dilute if only a small amount of it is dissolved in a large volume of water — even though each molecule dissociates fully, there aren't many molecules present. Similarly, a weak acid can be concentrated if a large amount of it is dissolved in a small volume of water — even though only a small fraction dissociates, the sheer number of molecules means a significant number of H⁺ ions are still produced. Strength and concentration are independent properties.
"Acids and bases — the sour and the slippery — teach us that balance is everything. Now go find your balance."
