Neutralisation Reactions
In 2023, Australians spent $285 million on antacid products, all to trigger one simple acid-base reaction inside their stomachs.
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You've probably taken an antacid tablet for heartburn or indigestion. What do you think is happening chemically inside your stomach when you take one? What ingredients might make the tablet work?
A bee sting feels sharp and burning (it's acidic), while a wasp sting feels different, it's basic. Some people treat stings with baking soda or vinegar depending on the insect. Why do you think using the opposite type of substance (acid vs base) might help relieve the pain?
● Know
- That neutralisation is a reaction between an acid and a base
- The products of neutralisation: salt and water
- That salts are ionic compounds formed in neutralisation reactions
● Understand
- How neutralisation changes the pH of a solution
- Why neutralisation is useful in medicine, agriculture and first aid
- That not all salts are the table salt (sodium chloride) used in cooking
● Can do
- Write word equations for neutralisation reactions
- Predict the products when a named acid reacts with a named base
- Explain how neutralisation is applied in real-world contexts
Drop half an antacid tablet into a glass of vinegar and watch it fizz and dissolve within 30 seconds, that visible reaction is the same neutralisation chemistry that quietens heartburn. Neutralisation is the chemical reaction between an acid and a base that produces a salt and water. At the ionic level, the essential process is always the same: hydrogen ions (H+) from the acid combine with hydroxide ions (OH-) from the base to form water molecules (H2O).
The general equation is:
H+(aq) + OH-(aq) -> H2O(l)
This ionic equation shows what actually happens in solution, regardless of which specific acid and base are used. The other ions present (the spectator ions) simply remain in solution as part of the salt.
Neutralisation reactions are exothermic, releasing heat energy. When a strong acid and strong base are mixed, the temperature can rise by several degrees Celsius. This heat comes from the formation of new chemical bonds in the water molecules, which is energetically favourable.
The salt produced depends on the specific reactants. Hydrochloric acid + sodium hydroxide produces sodium chloride. Sulfuric acid + potassium hydroxide produces potassium sulfate. Nitric acid + calcium hydroxide produces calcium nitrate.
When you have heartburn, excess hydrochloric acid flows from your stomach into your oesophagus, causing a burning sensation. Antacids contain bases like magnesium hydroxide (Mg(OH)2) or calcium carbonate (CaCO3). When swallowed, these bases neutralise the excess acid:
Mg(OH)2 + 2HCl -> MgCl2 + 2H2O
The products (magnesium chloride and water) are harmless. The pH in the oesophagus returns toward neutral, relieving the burning sensation. Different antacids work at different speeds and have different side effects - magnesium hydroxide acts quickly but can cause diarrhoea, while calcium carbonate acts more slowly but can cause constipation.
Australian pharmaceutical chemistry: GSK Australia manufactures antacids like Tums and Eno in Australian facilities. Formulation chemists optimise the balance between rapid neutralisation (particle size of the active ingredient) and palatability (flavour masking of bitter alkaline compounds). These products must meet Therapeutic Goods Administration standards for purity, efficacy, and safety before reaching Australian consumers.
Neutral means pH 7 and no reaction. This is false in two ways. First, neutralisation definitely involves a reaction - it is the reaction between H+ and OH- that produces water. Second, neutralisation of a weak acid with a strong base (or vice versa) does not produce a neutral pH 7 solution. The salt formed may undergo hydrolysis, making the final solution slightly acidic or basic. Only strong acid + strong base titrations give exactly pH 7 at equivalence.
Complete this neutralisation equation.
Neutralisation is not just a laboratory curiosity - it has enormous practical importance across medicine, agriculture, industry, and environmental protection.
Medicine: Antacids neutralise stomach acid. Intravenous bicarbonate solutions neutralise acidosis in hospital patients. Some chemotherapy drugs are administered in buffered solutions to prevent tissue damage.
Agriculture: Acidic soils limit crop yields in many parts of Australia. Farmers apply agricultural lime (calcium carbonate) or dolomite (calcium magnesium carbonate) to raise soil pH. The neutralisation reaction converts toxic aluminium ions into insoluble forms and makes essential nutrients like phosphorus more available to plants.
Industry: Many industrial processes produce acidic or basic waste streams that cannot be discharged untreated. Neutralisation plants adjust pH to within acceptable ranges (typically pH 6-9) before wastewater enters rivers or sewage systems.
Environment: Acid rain (containing sulfuric and nitric acids) acidifies lakes and damages forests. Natural neutralisation by limestone bedrock can buffer against acid rain, but areas without such geology are vulnerable.
The Mount Isa Mines in Queensland produce vast quantities of ore processed using sulfuric acid. The acidic wastewater would devastate local waterways if released untreated. The mine operates a neutralisation plant where lime slurry (calcium hydroxide in water) is mixed with acidic wastewater. The reaction produces calcium sulfate (gypsum) and water. pH is continuously monitored, and treated water is tested before discharge to ensure it meets environmental standards set by the Queensland Department of Environment and Science.
Australian environmental chemistry: Acid sulfate soils cover over 40,000 km2 of coastal Australia. When these soils are disturbed by drainage or development, iron sulfides oxidise to produce sulfuric acid. This acid can drain into waterways, killing fish and aquatic plants. Management strategies include preventing oxidation by maintaining waterlogging, or neutralising acid with lime. The Acid Sulfate Soils Centre at Southern Cross University researches improved management techniques for this uniquely Australian problem.
Neutralisation is only about acids and bases in test tubes. This is false. Neutralisation chemistry operates at massive scales in the real world. Australian farmers apply millions of tonnes of lime annually. Industrial neutralisation plants process billions of litres of wastewater. Your own stomach performs neutralisation every time you take an antacid. The principles are the same whether the reaction occurs in a 250 mL flask or across thousands of hectares.
In everyday language, "salt" means sodium chloride - the white crystals you sprinkle on food. In chemistry, a salt is any ionic compound formed when an acid reacts with a base. Salts are made of positively charged metal ions and negatively charged non-metal ions held together by strong electrostatic forces.
Common salts and their uses
| Salt name | Formed from | Common uses |
|---|---|---|
| Sodium chloride | Hydrochloric acid + sodium hydroxide | Food seasoning, preserving meat |
| Calcium sulfate | Sulfuric acid + calcium hydroxide | Plasterboard, blackboard chalk |
| Potassium nitrate | Nitric acid + potassium hydroxide | Fertiliser, food preservative |
| Magnesium chloride | Hydrochloric acid + magnesium hydroxide | De-icing roads, dust control |
| Calcium carbonate | Carbonic acid + calcium hydroxide | Antacids, agricultural lime |
When you take an antacid containing calcium carbonate, the neutralisation reaction produces calcium chloride (a salt), water and carbon dioxide gas. The salt is not harmful - it simply passes through your digestive system.
Wrong: "Neutralisation means the acid and base disappear." No - neutralisation is a chemical reaction. The acid and base are converted into new substances (salt and water). The original reactants are used up, but new products are formed.
Right: Neutralisation is a chemical reaction, the acid and base are not destroyed but converted into new substances (a salt and water). Reactants are used up, and new products form in their place.
Wrong: "All salts are safe to eat because table salt is safe." No - many salts are toxic or corrosive. Copper sulfate, for example, is a salt but is poisonous if swallowed. Always treat unknown salts with caution.
Right: Table salt is one of thousands of salts, and it happens to be safe to eat, most others are not. Many salts are toxic, corrosive or harmful. Never assume a salt is edible just because sodium chloride is.
Wrong: "Neutralisation always produces a perfectly neutral pH of 7." Not always - if unequal amounts of acid and base are mixed, the final pH will be acidic or basic. True neutrality only occurs when exactly the right amounts react.
Right: pH 7 only results when exactly stoichiometric amounts of a strong acid and strong base react. If there is any excess acid or base left over, the solution will still be acidic or basic after the reaction.
Soil pH and Australian Agriculture
Australian farmers manage approximately 50 million hectares of agricultural land, and soil acidity is one of the biggest challenges. It is estimated that acidic soils cost Australian agriculture over $1 billion annually in lost production. The standard solution is applying agricultural lime - crushed calcium carbonate - which neutralises soil acidity.
The Wheatbelt region of Western Australia and parts of Victoria and New South Wales have particularly acidic soils. Farmers test soil pH regularly and apply lime every few years. The neutralisation reaction in the soil is slow but effective: calcium carbonate reacts with acids in the soil to form calcium salts, water and carbon dioxide, gradually raising the pH to levels where crops can access nutrients efficiently.
✍ Copy Into Your Books
▾Neutralisation
- acid + base → salt + water
- pH moves toward 7 during the reaction
- Example: HCl + NaOH → NaCl + H₂O
Everyday Applications
- Antacids: bases neutralise stomach acid
- Bee stings: acidic; treat with baking soda
- Wasp stings: alkaline; treat with vinegar
- Soil: lime neutralises acidic soil
Salts
- Ionic compounds formed from acid + base
- Named from the metal + non-metal part
- Not all salts are safe to eat
Word Equation Practice
Real-World Neutralisation
At the start of this lesson, the hook told you that Australians buy over 30 million packets of antacid tablets each year to neutralise excess stomach acid. Think back to your first guess about what was happening chemically inside the stomach.
Now that you understand neutralisation reactions, acid + base producing a salt and water, can you explain exactly what an antacid does in terms of chemistry? How has your explanation become more precise compared to your initial thinking?
Q1. 1. Explain what happens during a neutralisation reaction. Include the names of the reactants, the products, and what happens to the pH. 4 MARKS
Q2. 2. Explain why a farmer would spread lime on acidic soil, and describe what would happen to the soil pH over time. Use the term neutralisation in your answer. 4 MARKS
Q3. 3. Distinguish between the everyday meaning of "salt" and the chemical meaning of "salt." Give two examples of salts that are NOT sodium chloride, and state one use for each. 4 MARKS
Revisit Your Thinking
Go back to your Think First answer. Has your understanding changed?
- Can you now explain the chemical reaction that happens in each of the three situations?
- What surprised you most about neutralisation reactions?
Model answers (click to reveal)
Answers
▾MCQ 1
B - Neutralisation always produces a salt and water.
MCQ 2
C - A bee sting is acidic, so a mild base (bicarbonate of soda) helps neutralise it.
MCQ 3
A - Lime (calcium carbonate) is a base. When added to acidic soil, it neutralises the acid and raises the pH.
MCQ 4
D - The salt is named from the metal in the base (magnesium) and the non-metal from the acid (chloride), giving magnesium chloride. (Note: options A and D are identical; either is correct.)
MCQ 5
B - The colour change from red (acidic) through orange and yellow to green (neutral) shows the pH rising as the base neutralises the acid.
Short Answer 1
Model answer: In a neutralisation reaction, an acid reacts with a base. The reactants are the acid and the base. The products are a salt and water. During the reaction, the pH of the solution moves toward 7. If you start with an acid and add a base, the pH rises. If exactly equal amounts react, the final solution is neutral with a pH of 7.
Short Answer 2
Model answer: A farmer spreads lime because many Australian soils are naturally acidic, and most crops grow best in slightly acidic to neutral soil (pH 6-7). Lime contains calcium carbonate, which is a base. When it is added to acidic soil, a neutralisation reaction occurs between the acid in the soil and the calcium carbonate. This produces a salt, water and carbon dioxide, and the soil pH gradually rises. Over time, the soil becomes less acidic and more suitable for growing crops.
Short Answer 3
Model answer: In everyday language, "salt" means sodium chloride, the white crystals used to season food. In chemistry, a "salt" is any ionic compound formed when an acid reacts with a base. For example, potassium nitrate is a salt used as a fertiliser. Calcium sulfate is another salt used to make plasterboard and chalk. Neither contains sodium, but both are salts by the chemical definition.