Acids, Bases and Salts — Complete Grade 10 Chemistry Guide

Part 1: everything for your exam · Part 2: going further, with university and government references · free and printable

Which syllabus are you studying?

The chemistry of acids and bases is identical everywhere, but the words and the depth differ between boards. Check this before you begin.

1. Check yourself first

These three ideas come from the previous topic. If you cannot answer them, revise Chemical Reactions and Equations first.

1. What type of reaction is Zn + HCl?
2. Write the formula of carbon dioxide and state the valency of carbon.
3. You may have heard the word "neutralise". Guess what it means in chemistry.

2. Identifying acids and bases

Acids taste sour and produce H⁺ ions in water — lemon juice, tamarind and vinegar are everyday examples.
Bases taste bitter, feel soapy, and produce OH⁻ ions in water — lime and baking soda are examples.

Figure 1 — Blue litmus turns red in acid; red litmus turns blue in base.

3. Make an indicator at home

Litmus is extracted from lichens, so it is a natural indicator. Your kitchen has three more.

4. Reactions of acids

(a) Acid + metal → salt + hydrogen

Figure 2 — Hydrogen bubbles rise from zinc in dilute acid. A burning splint gives a squeaky pop — the standard test for hydrogen.

(b) Acid + carbonate or hydrogencarbonate → salt + water + CO₂

So that was the antacid fizz. Many antacids contain sodium hydrogencarbonate. It meets the excess stomach acid, the bubbles are CO₂, and the surplus acid is used up — so the burning stops.

(c) Acid + metal oxide → salt + water

The black copper oxide dissolves and the solution turns blue-green. Because metal oxides react with acids to give salt and water, they are called basic oxides.

5. Reactions of bases

(a) Base + metal → salt + hydrogen

Only some metals do this. Zinc does:

(b) Base + non-metal oxide → salt + water

This mirrors acid + metal oxide — which is why non-metal oxides are called acidic oxides.

(c) Neutralisation

Figure 3 — Neutralisation: an acid and a base react to form a salt and water.

6. The role of water

Acids and bases share one feature: they only release their ions when dissolved in water — and only then do they conduct electricity.

7. Dilution — and the rule that prevents burns

Adding water to an acid or base lowers the concentration of ions — this is dilution. It is strongly exothermic.

8. Strength and the pH scale

How acidic or basic a solution is can be read from the pH scale, which runs from 0 to 14. It is a measure of the concentration of H⁺ ions.

Figure 4 — The pH scale. The pointer travels to show where everyday substances sit.

9. pH in everyday life

10. Try it yourself

Measure the pH of coffee, saliva, soap and milk below. Add water and watch what happens to the pH. Five minutes here beats half an hour of reading.

Simulation: PhET Interactive Simulations, University of Colorado Boulder, used under CC-BY 4.0.

11. Salts

A salt is the compound formed when an acid neutralises a base.

Families of salts

The pH of a salt solution

12. Chemicals from common salt

Sodium chloride is not just table salt — it is the raw material for much of the chemical industry.

Figure 5 — The chlor-alkali process. All three products are useful, hence the name: "chlor" for chlorine, "alkali" for sodium hydroxide.

(a) Bleaching powder — CaOCl₂

Uses: bleaching cotton, linen and wood pulp; disinfecting drinking water; as an oxidising agent in industry.

(b) Baking soda — NaHCO₃

Uses: in baking powder (the CO₂ released makes cakes rise), as an antacid, and in soda-acid fire extinguishers. It is a mild base.

(c) Washing soda — Na₂CO₃·10H₂O

Uses: glass, soap and paper industries; removing permanent hardness of water.

(d) Plaster of Paris — CaSO₄·½H₂O

Mixed with water it sets back into gypsum as a hard solid. Uses: casts for broken bones, statues, smoothing walls.

13. Water of crystallisation

Some salt crystals contain a fixed number of water molecules — the water of crystallisation. Salts like this are called hydrates.

14. Summary and equation bank

15. Misconceptions

16. Common exam mistakes

1. Missing the gas arrow. H₂↑ and CO₂↑ need it.
2. Forgetting the second limewater reaction — excess CO₂ removes the milkiness.
3. Omitting 373 K for plaster of Paris, or writing 1 instead of ½H₂O.
4. Dropping the water from hydrate formulae — Na₂CO₃ and Na₂CO₃·10H₂O are different substances.
5. Confusing strong with concentrated.
6. Reversing indicator colours. Phenolphthalein is pink in base and colourless in acid.
7. Giving pH a unit. pH has no unit — saying so can earn a mark.

17. Questions at three levels

Start at your own level. Green is foundation, purple is challenge.

Foundation

Q1 What colour is phenolphthalein in a base?

Q2 What is the pH of a neutral solution?

Q3 How do you test for hydrogen gas?

Exam level

Q4 [2 marks] Why does dry HCl gas not change the colour of dry blue litmus, while hydrochloric acid does?

Q5 [3 marks] Describe what happens when CO₂ is bubbled through limewater, and then in excess. Give both equations.

Q6 [3 marks] How is plaster of Paris made? Give the equation and two uses.

Q7 [4 marks] What is the chlor-alkali process? Name the three products and give one use of each.

Challenge

Q8 A student adds water to concentrated sulfuric acid instead of the other way round. What happens and why?

Q9 Fresh milk has pH 6. A milkman adds a little baking soda, raising it to pH 8. Why? Will it take longer or shorter to set into yoghurt?

Q10 A farmer's soil is too acidic. What should be added and why? What happens if too much is added?

18. Self-test — 15 questions

Write your answers on paper, then open the answers. 12 or more correct means you are ready.

1. What colour does blue litmus turn in acid?
2. Which ion does an acid release in water?
3. What is the pH of pure water?
4. Below what pH does tooth decay start?
5. What is applied to a bee sting?
6. Give the formula of washing soda.
7. Give the formula of bleaching powder.
8. At what temperature is plaster of Paris made?
9. What colour does blue copper sulfate turn when heated?
10. Which gas forms at the anode in the chlor-alkali process?
11. Is a solution of NH₄Cl acidic, basic or neutral?
12. Name two natural indicators found in a kitchen.
13. Below what pH is rain called acid rain?
14. Give one example of an olfactory indicator.
15. State one difference between a base and an alkali.

19. Glossary

20. For parents and teachers

A. Three definitions of an acid

The definition in Part 1 — an acid gives H⁺ ions in water — is the Arrhenius definition, from the Swedish chemist Svante Arrhenius in the 1880s. It works well, but it has a gap: it only describes what happens in water. ^[2]

Ammonia is the classic puzzle. Its solution turns red litmus blue and neutralises acids — it clearly behaves as a base — yet NH₃ contains no hydroxide ion. ^[2]

In 1923, Johannes Brønsted in Denmark and Thomas Lowry in England independently proposed a broader idea: an acid is a proton donor and a base is a proton acceptor. ^[2] Under this definition, ammonia is a base because it accepts a proton:

This reaction even happens between two gases, with no water present — something the Arrhenius view cannot describe. ^[2] A third, still wider definition — the Lewis definition, based on electron pairs — is taught at university level. ^[2]

Figure 6 — Each definition contains the one before it. Nothing you learned in Part 1 becomes wrong; it simply becomes one case of a larger picture.

B. The mathematics of pH

The pH scale is defined by a logarithm: ^[2]

where [H⁺] is the concentration of hydrogen ions in moles per litre. The consequence is the single most important idea in this section: each step of one pH unit is a tenfold change in acidity.

Figure 7 — pH 3 is not "a bit more acidic" than pH 5. It is a hundred times more acidic.

Pure water also ionises very slightly. At 25°C the product of the two ion concentrations is a constant, written K_w: ^[2]

That is why the scale runs from 0 to 14, and why pure water, with equal amounts of both ions, sits exactly in the middle at 7.

C. Why "strong" is not "concentrated"

Part 1 warned that these words mean different things. Here is the reason.

Strength describes what fraction of the acid molecules give up their proton. A strong acid such as HCl ionises essentially completely. A weak acid such as ethanoic acid (vinegar) ionises only partly — most of its molecules stay intact. ^[2]

Concentration describes how much acid is dissolved in a given volume. So a small amount of HCl in a large volume of water is strong but dilute; a large amount of vinegar in little water is weak but concentrated. The two ideas are independent.

D. Buffers — why your blood stays at pH 7.4

Part 1 noted that the body works only in a narrow pH range. Blood holds close to pH 7.4 even though muscles constantly release acids and we eat acidic foods.

It manages this with a buffer — a mixture of a weak acid and its partner base that absorbs added H⁺ or OH⁻ without the pH shifting much. ^[2] In blood the main pair is carbonic acid and the hydrogencarbonate ion — the same HCO₃⁻ ion as in the baking soda of Part 1. Chemistry and biology meet here.

E. Ocean acidification — acid rain's larger cousin

The ocean absorbs carbon dioxide from the air. When CO₂ dissolves in seawater it forms a weak acid, and the pH falls. ^[1]

This is section B in action. A change of 0.1 sounds trivial until you remember that pH is logarithmic. The falling pH makes it harder for corals, oysters and shelled plankton to build their calcium carbonate shells — the same CaCO₃ that turns limewater milky in Part 1. ^[1]

F. Where this leads

References

Only government bodies and universities are cited. Each link was checked when this guide was written.

1. National Oceanic and Atmospheric Administration (NOAA), United States Department of Commerce. Ocean acidification — education resource collection. noaa.gov
2. Chemistry LibreTexts — open textbook project hosted by the University of California, Davis, with contributing universities. Chapters on Arrhenius and Brønsted–Lowry theory, self-ionisation of water and the pH scale, weak acids, buffers and Lewis acids. chem.libretexts.org
3. PhET Interactive Simulations, University of Colorado Boulder. pH Scale and Acid–Base Solutions simulations, CC-BY 4.0. phet.colorado.edu
4. Royal Society of Chemistry, United Kingdom. Practical activities and teaching resources in acids, bases and salts. edu.rsc.org
5. National Council of Educational Research and Training (NCERT), Government of India. Science — Textbook for Class X. ncert.nic.in
6. ePathshala, Ministry of Education, Government of India. Official e-textbooks and audio-visual resources. epathshala.nic.in

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