Acids & Bases
What Are Acids?
Acids are substances that release hydrogen ions (H+) when dissolved in water. They have a pH below 7 and taste sour (never taste lab chemicals). Common acids include:
- Hydrochloric acid (HCl) — found in your stomach to aid digestion
- Sulfuric acid (H2SO4) — used in car batteries
- Citric acid — found in lemons and oranges
- Ethanoic acid — found in vinegar
Strong acids (like hydrochloric and sulfuric acid) fully dissociate in water, releasing many H+ ions. Weak acids (like citric acid) only partially dissociate, releasing fewer H+ ions.
What Are Bases and Alkalis?
A base is any substance that can neutralise an acid. An alkali is a base that dissolves in water, releasing hydroxide ions (OH-). Alkalis have a pH above 7.
Common alkalis:
- Sodium hydroxide (NaOH) — used in drain cleaner
- Ammonia solution (NH3) — used in cleaning products
- Calcium hydroxide — limewater, used to treat acidic soil
Properties of alkalis: they feel soapy, turn red litmus blue, and have a pH above 7.
The pH Scale
The pH scale runs from 0 to 14 and measures how acidic or alkaline a solution is:
| pH Range | Classification | Example |
|---|---|---|
| 0 – 3 | Strongly acidic | Stomach acid (pH 1), lemon juice (pH 2) |
| 4 – 6 | Weakly acidic | Vinegar (pH 3), black coffee (pH 5) |
| 7 | Neutral | Pure water |
| 8 – 10 | Weakly alkaline | Baking soda (pH 9), seawater (pH 8) |
| 11 – 14 | Strongly alkaline | Bleach (pH 13), oven cleaner (pH 14) |
Each step on the pH scale represents a 10-fold change in acidity. So pH 3 is 10 times more acidic than pH 4, and 100 times more acidic than pH 5.
Indicators
Indicators are substances that change colour depending on whether a solution is acidic or alkaline:
| Indicator | In acid | In neutral | In alkali |
|---|---|---|---|
| Litmus | Red | Purple | Blue |
| Universal indicator | Red/orange | Green | Blue/purple |
| Phenolphthalein | Colourless | Colourless | Pink |
Universal indicator gives a range of colours allowing you to estimate the exact pH. Litmus only tells you acid or alkali, not the strength.
Neutralisation
When an acid reacts with an alkali, they cancel each other out in a reaction called neutralisation:
acid + alkali → salt + water
Example: hydrochloric acid + sodium hydroxide → sodium chloride + water
The pH of the solution moves towards 7 as neutralisation occurs. At the equivalence point, all the acid and alkali have reacted and the solution is neutral (pH 7).
Neutralisation is used in everyday life — antacid tablets neutralise excess stomach acid, and farmers add lime (calcium hydroxide) to acidic soil.
Reactions of Acids with Metals
Many metals react with acids to produce a salt and hydrogen gas:
acid + metal → salt + hydrogen
Example: sulfuric acid + zinc → zinc sulfate + hydrogen
Test for hydrogen gas: hold a lit splint near the gas — it burns with a squeaky pop.
| Acid | Metal | Salt produced |
|---|---|---|
| Hydrochloric acid | Zinc | Zinc chloride |
| Sulfuric acid | Magnesium | Magnesium sulfate |
| Hydrochloric acid | Iron | Iron(II) chloride |
Not all metals react with acids. Very unreactive metals like gold and platinum do not react.
Reactions of Acids with Carbonates
Acids react with carbonates to produce a salt, water, and carbon dioxide:
acid + carbonate → salt + water + carbon dioxide
Example: hydrochloric acid + calcium carbonate → calcium chloride + water + carbon dioxide
Test for carbon dioxide: bubble the gas through limewater — it turns milky/cloudy.
This reaction explains why acid rain damages limestone buildings and marble statues.
Worked Example
Q: Sulfuric acid reacts with potassium hydroxide. Write the word equation and name the salt formed.
Step 1: Identify the reaction type — acid + alkali = neutralisation.
Step 2: Write the word equation: sulfuric acid + potassium hydroxide → potassium sulfate + water
Step 3: Name the salt — take the metal from the alkali (potassium) and combine with the acid name (sulfuric → sulfate) to give potassium sulfate.
Key Terms
| Term | Definition |
|---|---|
| Acid | Substance with pH below 7; releases H+ ions in water |
| Alkali | Base that dissolves in water; releases OH- ions |
| Neutralisation | Reaction between an acid and an alkali producing salt and water |
| Salt | Ionic compound formed when an acid reacts with a base |
| Indicator | Substance that changes colour to show whether a solution is acidic or alkaline |
| pH scale | Scale from 0 to 14 measuring acidity or alkalinity |
| Dissociation | Breaking apart of an acid into ions in water |
Common Mistakes
- Confusing base and alkali — all alkalis are bases, but not all bases dissolve in water (e.g., copper oxide is a base but not an alkali).
- Thinking neutral means "no acid present" — neutral means equal concentrations of H+ and OH- ions (pH 7).
- Confusing strong and concentrated — a strong acid fully dissociates; concentration refers to how much acid is dissolved per litre.
- Forgetting that pH < 7 is acidic and pH > 7 is alkaline — do not mix these up.
Tips and Tricks
- Salt naming rule: first part from the metal/alkali (e.g., sodium), second from the acid (hydrochloric → chloride, sulfuric → sulfate, nitric → nitrate).
- The squeaky pop test (hydrogen) and cloudy limewater test (CO2) are classic exam questions — memorise both.
- Universal indicator colour order: red, orange, yellow, green, blue, purple — think of the rainbow going from acid to alkali.
- Antacids work by neutralisation — a real-world context that often appears in exam questions.