Lemon juice, soap, baking soda and stomach acid are all described by one idea: how many hydrogen ions are present. Learn the reactions, tests and calculations.
Sour lemons, bitter soap, fizzing antacids and the colour change of litmus paper all trace back to acids and bases. These are among the most useful ideas in chemistry, because they explain digestion, cleaning, farming and the manufacture of countless materials. In this article you will learn how to recognise acids and bases, predict their reactions, make salts and calculate concentrations by titration.
An acid is a substance that produces hydrogen ions, H⁺, when dissolved in water. Common examples are hydrochloric acid (HCl), sulfuric acid (H₂SO₄), nitric acid (HNO₃), ethanoic acid (CH₃COOH, in vinegar) and citric acid (in citrus fruits). Acids taste sour and turn blue litmus red, and they have a pH below 7.
A base is a substance that reacts with an acid to form a salt and water. Most bases are metal oxides or metal hydroxides, such as copper(II) oxide (CuO) and magnesium hydroxide (Mg(OH)₂). A base that dissolves in water is called an alkali, and it produces hydroxide ions, OH⁻. Common alkalis are sodium hydroxide (NaOH), potassium hydroxide (KOH), calcium hydroxide (limewater) and ammonia solution. All alkalis are bases, but not all bases are alkalis, because many bases do not dissolve in water.
Alkalis feel slippery, turn red litmus blue and have a pH above 7. A solution that is neither acidic nor alkaline is neutral and has a pH of 7; pure water is neutral.
Never taste or touch laboratory acids and alkalis. Concentrated acids and alkalis are corrosive and can burn the skin and eyes. Wear eye protection, and if a chemical gets on your skin or in your eyes, rinse with plenty of running water and tell a teacher at once. When diluting an acid, always add the acid slowly to the water, never the other way round.
An indicator is a substance that changes colour depending on whether it is in acid or alkali. The pH scale runs from 0 to 14 and measures how acidic or alkaline a solution is. Lower pH values mean a higher concentration of H⁺ ions: pH 1 is much more acidic than pH 6. Each step on the scale is a tenfold change in hydrogen-ion concentration.
| Indicator | In acid | In neutral | In alkali |
|---|---|---|---|
| Litmus | Red | Purple | Blue |
| Methyl orange | Red | Orange | Yellow |
| Phenolphthalein | Colourless | Colourless | Pink |
| Universal indicator | Red / orange / yellow (pH 1–6) | Green (pH 7) | Blue / purple (pH 8–14) |
Universal indicator is a mixture that gives a range of colours, so it can show an approximate pH. A pH meter gives a more precise reading. Remember that the exact colours at each pH can vary slightly between brands.
“Strong” and “weak” describe how much an acid or alkali ionises (splits into ions) in water. This is different from “concentrated” and “dilute”, which describe how much substance is dissolved.
| Type | Examples |
|---|---|
| Strong acids (fully ionised) | Hydrochloric, sulfuric, nitric |
| Weak acids (partly ionised) | Ethanoic, citric, carbonic |
| Strong alkalis (fully ionised) | Sodium hydroxide, potassium hydroxide |
| Weak alkali (partly ionised) | Ammonia solution |
Equal concentrations of a strong and a weak acid have different pH values: the strong acid has the lower pH because it produces more H⁺ ions. A very dilute strong acid can still have a higher pH than a concentrated weak acid, which shows why strength and concentration are separate ideas.
Acids have four reactions you must know. In each, a salt is formed. A salt is a compound formed when the hydrogen of an acid is replaced by a metal (or ammonium) ion. The first part of the salt’s name comes from the metal or base, and the second part from the acid: hydrochloric acid gives chlorides, sulfuric acid gives sulfates, nitric acid gives nitrates, and ethanoic acid gives ethanoates.
| Acid reacts with… | Products | Example equation | Observation / test |
|---|---|---|---|
| A reactive metal | Salt + hydrogen | Mg + 2HCl → MgCl₂ + H₂ | Fizzing; hydrogen gives a squeaky pop with a lighted splint |
| A base (oxide or hydroxide) | Salt + water | CuO + H₂SO₄ → CuSO₄ + H₂O | Black solid disappears; blue solution forms |
| An alkali | Salt + water | NaOH + HCl → NaCl + H₂O | Temperature rises; indicator changes colour |
| A carbonate | Salt + water + carbon dioxide | CaCO₃ + 2HCl → CaCl₂ + H₂O + CO₂ | Fizzing; CO₂ turns limewater milky |
Metals low in the reactivity series, such as copper and silver, do not react with dilute hydrochloric or sulfuric acid. Very reactive metals such as sodium and potassium react far too violently to be used in this way.
Question: Write a balanced equation for zinc reacting with dilute sulfuric acid, and name the salt.
Answer: Zn + H₂SO₄ → ZnSO₄ + H₂; the salt is zinc sulfate.
Neutralisation is the reaction between an acid and a base (or alkali) to form a salt and water. In terms of ions, every neutralisation in water is the same reaction between hydrogen ions and hydroxide ions:
Neutralisation is exothermic, so the temperature rises. It is used in many places: antacid tablets neutralise excess stomach acid; farmers add lime to neutralise acidic soil; factories neutralise acidic waste before releasing it; and a bee sting (acidic) or wasp sting (alkaline) can be soothed with a mild base or acid, though medical advice is better for severe stings.
The method depends on whether the salt dissolves in water. First, know the solubility rules for common salts:
This makes copper(II) sulfate from copper(II) oxide: CuO + H₂SO₄ → CuSO₄ + H₂O. Using an excess of the solid makes sure no acid is left in the product.
Sodium, potassium and ammonium salts cannot be made by Method A because the alkali dissolves, so you cannot see when it has all reacted or filter out an excess. Instead, a measured volume of alkali is neutralised exactly with acid using an indicator. The experiment is then repeated without the indicator, using the same volumes, and the solution is crystallised.
Mix solutions of two soluble salts that contain the ions of the insoluble salt. For example, barium sulfate forms when barium chloride and sodium sulfate solutions are mixed: Ba²⁺(aq) + SO₄²⁻(aq) → BaSO₄(s). The solid precipitate is filtered off, washed with distilled water and dried.
A titration finds the concentration of an unknown acid or alkali. A burette delivers acid into a measured volume of alkali in a conical flask, with an indicator, until the colour changes at the end-point. Phenolphthalein or methyl orange are the usual indicators. The steps for the calculation are always the same:
Question: 25.0 cm³ of sodium hydroxide solution is exactly neutralised by 20.0 cm³ of 0.100 mol/dm³ hydrochloric acid. Find the concentration of the sodium hydroxide.
Answer: 0.0800 mol/dm³.
Question: What volume of 0.0500 mol/dm³ sulfuric acid is needed to neutralise 25.0 cm³ of 0.100 mol/dm³ sodium hydroxide?
Answer: 25.0 cm³. Always check the mole ratio from the balanced equation.
| Where | Chemistry |
|---|---|
| Stomach and antacids | Stomach acid is hydrochloric acid; antacids such as magnesium hydroxide or calcium carbonate neutralise excess acid. |
| Farming | Lime (calcium oxide or hydroxide) raises the pH of acidic soil; ammonium nitrate and ammonium sulfate are fertilisers made by neutralising ammonia with acids. |
| Food | Vinegar (ethanoic acid) and citric acid are used for flavour and preservation; baking soda reacts with acid to release carbon dioxide. |
| Industry | Sulfuric acid is one of the most widely produced industrial chemicals, used for fertilisers, detergents, batteries and many other products. |
| Environment | Acid rain from sulfur dioxide and nitrogen oxides damages buildings, soil and lakes; limestone and lime are used to reduce acidity. |
| Dental care | Bacteria turn sugar into acids that dissolve tooth enamel; toothpaste is mildly alkaline. |
This article is general education and not medical advice. Do not treat burns or poisoning at home; seek medical help.
Try each question first, then tap it to reveal the answer.
Answer: Acids produce H⁺; alkalis produce OH⁻.
Answer: Magnesium chloride and hydrogen.
Answer: H⁺(aq) + OH⁻(aq) → H₂O(l).
Answer: Pink.
Answer: A reaction of an acid with a carbonate gives calcium nitrate, water and carbon dioxide.
Answer: Precipitation, because barium sulfate is insoluble: mix a soluble barium salt with a soluble sulfate and filter.
Answer: Moles HCl = 0.250 × 0.0100 = 0.00250 mol; KOH = 0.00250 ÷ 0.0250 = 0.100 mol/dm³.
Suggested use: red-cabbage indicator is a safe way to test household liquids before moving to universal indicator. Titration practice needs burettes, eye protection and careful reading of the meniscus; repeat until two results agree within 0.10 cm³. Parents can connect the topic to antacids, vinegar and soil. Please follow your own syllabus for the required indicator colours and solubility rules.
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Copyright note: This article is original EDUSAMBAM educational writing. The diagrams are original inline SVG schematics created for this page and are not copied from any source. Links open external websites; the sources were used to check facts and are suggested for further study.
Last updated: 21 October 2026. Written for students up to age 18 and checked against standard school-level chemistry references. This is general education, not a substitute for your teacher, textbook or syllabus.
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