Acids and Bases: A Student Guide

By Thread Academy · 11 September 2026 · Chemistry

Acids and bases are among the most useful chemicals in the world. The tang of a lemon, the sting of a bee, the cleaner under your kitchen sink, the antacid tablet that settles an upset stomach — acids and bases are behind all of them. This guide explains what they are, how we measure them, how they react, and how to handle them safely.

What is an acid?

An acid is a substance that releases hydrogen ions (H+ ions) when dissolved in water. At IGCSE, the important thing is to recognise acids and know how they behave.

Acids share a set of characteristic properties:

They have a sour taste. (Citric acid gives lemons and oranges their sharpness. Never taste chemicals in the laboratory, though — this is an observation from food, not a test method.)
They turn blue litmus paper red.
They have a pH less than 7.
They react with metals such as magnesium and zinc, producing hydrogen gas.
They react with carbonates, producing carbon dioxide gas.
They react with bases, producing a salt and water. This is called neutralisation.

Common acids include hydrochloric acid (HCl), sulfuric acid (H2SO4) and nitric acid (HNO3) in the laboratory, and citric acid, ethanoic acid (vinegar) and carbonic acid in food and drink.

What is a base? What is an alkali?

A base is a substance that neutralises an acid, producing a salt and water. Bases include metal oxides (such as copper oxide), metal hydroxides (such as sodium hydroxide) and ammonia.

An alkali is a base that dissolves in water to form a solution with a pH greater than 7. So every alkali is a base, but not every base is an alkali: copper oxide is a base but it does not dissolve in water, so it is not an alkali. Sodium hydroxide and potassium hydroxide are classic alkalis.

Alkalis have their own characteristic properties:

They feel soapy or slippery to the touch.
They turn red litmus paper blue.
They have a pH greater than 7.
They react with acids in neutralisation reactions.
Many are corrosive, especially concentrated ones — sodium hydroxide can cause serious burns.

Common bases and alkalis include sodium hydroxide (NaOH), potassium hydroxide (KOH), calcium hydroxide (Ca(OH)2) and ammonia solution.

The pH scale explained

The pH scale measures how acidic or alkaline a solution is. It runs from 0 to 14:

pH less than 7 — acidic. The lower the number, the stronger the acid. Battery acid sits near pH 1; lemon juice is around pH 2.
pH equal to 7 — neutral. Pure water is pH 7.
pH greater than 7 — alkaline. The higher the number, the stronger the alkali. Oven cleaner is around pH 13.

A crucial detail: the pH scale is not a simple straight-line scale. Each whole-number step represents a tenfold change in acidity. A solution of pH 3 is ten times more acidic than a solution of pH 4, and a hundred times more acidic than pH 5. That is why even small pH changes matter — in a lake, in soil, or in your blood.

Note the difference between strong and concentrated, which students often mix up. "Strong" describes how completely an acid releases its hydrogen ions; "concentrated" describes how much acid is dissolved in the water. You can have a dilute strong acid or a concentrated weak acid — they are independent ideas.

Indicators: litmus and universal indicator

We cannot see pH directly, so we use indicators — substances that change colour depending on whether a solution is acidic or alkaline.

Litmus is the simplest indicator, used as paper strips or solution:

Blue litmus paper turns red in acid, and stays blue in alkali.
Red litmus paper turns blue in alkali, and stays red in acid.
Neither changes colour in a neutral solution.

A handy memory aid: acids turn litmus red, and "alkali" and "blue" both contain the letter sequence you can link — or simply remember that the colours swap to the opposite.

Universal indicator is more informative: it is a mixture of dyes that shows a range of colours across the whole pH scale, from red through orange, yellow and green to blue and purple.

Red/orange — strongly acidic (pH 1 to 3).
Yellow — weakly acidic (pH 4 to 6).
Green — neutral (pH 7).
Blue — weakly alkaline (pH 8 to 10).
Purple — strongly alkaline (pH 11 to 14).

Universal indicator can be used as a solution added to a test tube, or as pH paper strips dipped into the solution and matched against a colour chart. It tells you not just whether a solution is acidic or alkaline, but roughly how strongly.

Neutralisation reactions

When an acid reacts with a base, they cancel each other out. This is neutralisation, and it always produces a salt plus water:

acid + base -> salt + water

The salt's name comes from the acid and the base: the first part of the name comes from the metal in the base, and the second part comes from the acid (hydrochloric acid makes chlorides, sulfuric acid makes sulfates, nitric acid makes nitrates).

Worked example 1: Hydrochloric acid and sodium hydroxide

HCl + NaOH -> NaCl + H2O

The hydrogen from the acid and the hydroxide from the alkali join to make water, leaving sodium chloride — common table salt — dissolved in solution. Notice the equation is already balanced: one of every atom on each side.

Worked example 2: Sulfuric acid and potassium hydroxide

Step 1 — write the word equation: sulfuric acid + potassium hydroxide -> potassium sulfate + water.

Step 2 — write the formulae: H2SO4 + KOH -> K2SO4 + H2O.

Step 3 — balance it. The sulfate group needs two potassium ions, so we need two KOH; that gives two waters:

H2SO4 + 2KOH -> K2SO4 + 2H2O

Check: left side has 2 K, 1 S, 6 O and 4 H; right side has 2 K, 1 S, 6 O and 4 H. Balanced.

Neutralisation is not just a laboratory curiosity. Farmers add lime (calcium hydroxide, an alkali) to acidic soil to neutralise it. Antacid tablets contain weak bases like magnesium hydroxide that neutralise excess stomach acid. And bee stings, which are acidic, can be soothed with a weak alkali such as baking soda solution.

Everyday examples of acids and bases

Vinegar (ethanoic acid) — used in cooking and cleaning.
Citrus fruits — citric acid gives the sharp taste.
Fizzy drinks — carbonic acid and phosphoric acid.
Car batteries — sulfuric acid, dangerously concentrated.
Soap and detergents — mildly alkaline, which is why they feel slippery.
Baking soda (sodium hydrogencarbonate) — a weak base used in cooking and as a gentle cleaner.
Bleach and oven cleaner — strongly alkaline and corrosive.
Stomach acid — hydrochloric acid, which helps digest food.

Safe handling basics

Acids and alkalis demand respect, especially concentrated ones.

Always wear eye protection when handling acids or alkalis — splashes can damage eyesight permanently.
Never taste laboratory chemicals to identify them, even if you know food acids taste sour.
If you spill acid or alkali on your skin, rinse immediately with plenty of cold water and tell your teacher.
Add acid to water, not water to acid, when diluting — diluting concentrated acid releases a lot of heat, and adding water to acid can cause dangerous spitting. (Your teacher will demonstrate; do not try this yourself unsupervised.)
Store acids and alkalis in labelled containers, away from each other, and wipe up spills at once.
Treat "weak" and "dilute" as relative terms: even vinegar stings in a cut, and oven cleaner can burn skin badly.

Key takeaways

Acids release H+ ions in water; they taste sour, turn blue litmus red, have pH less than 7, and react with metals, carbonates and bases.
A base neutralises an acid; an alkali is a base that dissolves in water to give pH greater than 7. Every alkali is a base, but not every base is an alkali.
The pH scale runs 0 to 14: below 7 is acidic, 7 is neutral, above 7 is alkaline. Each step is a tenfold change in acidity.
Litmus tells you acid versus alkali (blue to red in acid, red to blue in alkali); universal indicator gives the approximate pH by colour.
Neutralisation: acid + base -> salt + water. Name the salt from the base's metal and the acid (chlorides, sulfates, nitrates).
Neutralisation matters in real life: treating acidic soil with lime, antacids for indigestion, soothing stings.
Handle acids and alkalis with eye protection, rinse spills with plenty of water, never taste lab chemicals, and always add acid to water when diluting.