Acids, Bases and Indicators: Unit Review

Introduction

This unit began with the sour taste of lemons and the slippery feel of soap and ended with lakes, soils and teeth. Along the way you met litmus, natural plant indicators, synthetic indicators, universal indicator and the pH scale, and you learned how acids and bases cancel each other out in neutralisation. This review draws those threads together so that you can see how the ideas connect, check your understanding and prepare for tests.

Core explanation

Acids, bases and neutral substances. Acids, such as citric acid and hydrochloric acid, have a pH below 7 and form hydrogen ions (H⁺) in water. Bases neutralise acids; soluble bases, such as sodium hydroxide, are called alkalis and form hydroxide ions (OH⁻) in water, giving a pH above 7. Neutral substances, such as pure water and sugar solution, have a pH of 7. We never taste or touch substances to test them, because many are corrosive or toxic.

Indicators. An indicator changes colour depending on whether a solution is acidic, neutral or alkaline.

Indicator In acid In neutral In alkali --- --- --- --- Blue litmus Red Blue Blue Red litmus Red Red Blue Red cabbage Red/pink Purple Green to yellow Turmeric Yellow Yellow Red-brown Phenolphthalein Colourless Colourless Pink Methyl orange Red Yellow Yellow Universal indicator Red to orange/yellow Green Blue to purple

Litmus and single indicators tell you only whether a solution is acidic or alkaline. Universal indicator , a mixture of several indicators, shows a range of colours that match numbers on the pH scale . Plant pigments such as anthocyanins change colour because their molecules gain or lose hydrogen ions, which alters their structure. Olfactory indicators, such as onion or vanilla, change smell rather than colour.

The pH scale. pH 0 to 3 is strongly acidic, 4 to 6 weakly acidic, 7 neutral, 8 to 10 weakly alkaline and 11 to 14 strongly alkaline. A lower pH means more acidic.

Neutralisation. An acid reacts with a base to form a salt and water:

acid + base → salt + water

For example, hydrochloric acid + sodium hydroxide → sodium chloride + water. With a carbonate, carbon dioxide is also formed. The reaction is exothermic, so the temperature rises. The particle-level change is H⁺ + OH⁻ → H₂O.

Applications. Antacids neutralise excess stomach acid; baking soda soothes acidic ant and nettle stings; lime neutralises acidic soils and lakes; toothpaste neutralises mouth acids that attack enamel; and factories neutralise acidic or alkaline effluent before release.

Safety and investigation. Wear eye protection, use dilute solutions, never taste or touch, and clean spills promptly. Fair indicator investigations change one variable, keep others constant and record colours precisely.

Step-by-step reasoning

To answer a typical review question about an unknown solution:

1. Test it with litmus to decide if it is acidic, neutral or alkaline. 2. Use universal indicator to estimate its pH. 3. Decide whether it is strongly or weakly acidic or alkaline. 4. Choose a suitable substance to neutralise it. 5. Write the word equation for the neutralisation.

Visual explanation

Picture a concept map. At the centre is "Acids and bases". Branches lead to "Indicators" (litmus, natural, synthetic, universal), "pH scale" (0 to 14, colour chart) and "Neutralisation" (salt and water, heat), with smaller branches from neutralisation to stomach, stings, soil, teeth and industry.

Real-world analogy

Acids and bases are like two sides of a seesaw. Add acid and the balance tips towards the acidic side; add base and it tips the other way. Indicators are the pointer that shows which side is down, and neutralisation brings the seesaw back to level.

Real-world example

A gardener tests soil with a kit containing universal indicator and finds pH 5. Knowing the crop prefers pH 6.5, the gardener adds powdered limestone. The calcium carbonate neutralises the acid, releasing carbon dioxide and raising the pH. This one example uses indicators, pH and neutralisation together.

Why?

Why do we need both litmus and universal indicator? Litmus gives a quick yes-or-no answer about acidity, which is often enough. Universal indicator gives an approximate pH, which is needed when the degree of acidity matters, such as for soil, pools or medicines.

Common misconception

"Neutralisation always produces a neutral solution." It produces a neutral solution only when acid and base are in exactly the right amounts. With excess acid or excess base, the final mixture is still acidic or alkaline.

Worked example

Question: A liquid turns universal indicator orange. Give its approximate pH, describe it, and write a word equation for neutralising it with sodium hydroxide if it is hydrochloric acid.

Reasoning: Orange corresponds to about pH 4 to 5, which is weakly acidic. Acid + alkali gives a salt and water; hydrochloric acid gives a chloride.

Answer: About pH 4 to 5, weakly acidic. hydrochloric acid + sodium hydroxide → sodium chloride + water.

Quick check

1. What are the two products when an acid reacts with an alkali? Answer: A salt and water.

Exam focus

Learn the indicator colour table, the pH ranges and the general word equation for neutralisation. Practise naming salts, choosing suitable neutralising substances for everyday problems, and explaining why strong alkalis are unsuitable for use on the body.

Advanced insight

The ideas in this unit lead directly to later chemistry. Chemists define acids as proton (H⁺) donors and bases as proton acceptors, measure pH precisely with meters, and use titrations with indicators to find the exact concentration of an acid or alkali. The simple colour changes you have studied are the foundation of those quantitative methods.

Summary

Acids have pH below 7, alkalis above 7, and neutral substances pH 7. Indicators such as litmus, plant extracts, phenolphthalein and methyl orange show acidity by colour, while universal indicator gives an approximate pH. In neutralisation an acid and a base form a salt and water, releasing heat. This reaction is used in medicine, gardening, dental care and industry, always with safe practice.

Practice questions

1. What colour is phenolphthalein in an alkali? Answer: Pink. 2. Give the pH range of a strongly alkaline solution. Answer: About 11 to 14. 3. Write the word equation for the reaction of nitric acid with potassium hydroxide. Answer: nitric acid + potassium hydroxide → potassium nitrate + water. 4. Suggest a suitable substance to treat acidic soil and explain your choice. Answer: Powdered limestone (calcium carbonate); it is a cheap, safe base that neutralises the acid in the soil. 5. Why is universal indicator more useful than litmus for testing a swimming pool? Answer: It shows an approximate pH, so you can tell how acidic or alkaline the water is, not just which side of neutral it is.