Edexcel IGCSE Chemistry · Spec 2.35-2.38
Acids & Bases & Reactions of Acids
How acids react with bases, metals and carbonates to make salts.
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Acids & Bases & Reactions of Acids
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Explained
Acids, bases, and naming the salt
A hydrogen ion, H plus, is a hydrogen atom that has lost its electron, which leaves just a proton. That fact gives the deeper definition of acids and bases.
An acid is a proton donor: it releases hydrogen ions in solution. A base is a proton acceptor. An alkali is a base that dissolves in water, producing hydroxide ions, OH minus.
So every alkali is a base, but not every base is an alkali. Copper oxide is a base and does not dissolve; sodium hydroxide is a base that does, so it is also an alkali. Ammonia is a base because it accepts a proton, and it dissolves, so it is an alkali too.
The three reactions of acids
- Acid plus metal gives a salt plus hydrogen.
- Acid plus base, meaning a metal oxide or hydroxide, gives a salt plus water. This is neutralisation.
- Acid plus carbonate gives a salt plus water plus carbon dioxide.
The ionic equation for neutralisation is worth knowing on its own: hydrogen ions plus hydroxide ions give water. That is what neutralisation actually is, whichever acid and alkali you started with.
Naming the salt
The first part of the name comes from the metal, and the second from the acid.
Hydrochloric acid gives chlorides. Sulfuric acid gives sulfates. Nitric acid gives nitrates. Ethanoic acid gives ethanoates.
So magnesium plus sulfuric acid gives magnesium sulfate, and copper oxide plus nitric acid gives copper nitrate. Two halves of the name, one from each reactant, every time.
What the mark scheme accepts and rejects
An Edexcel International GCSE Chemistry mark scheme asks candidates to describe what universal indicator would show in an alkaline solution, and awards three separate marks: the colour being blue or purple, the pH being between 10 and 14, and the conclusion that the solution is alkaline. It allows indigo or violet for the colour and accepts basic for alkaline.
Three marks for one solution, and the third is the one people leave off. Giving the colour and the pH without stating the conclusion answers two thirds of the question.
Asked for the ion responsible, the same mark scheme wants OH minus and accepts HO minus, so the order of the two symbols is not policed, though the charge is.
On the test for hydrogen, another mark scheme credits a lighted or burning splint that pops, rejects a glowing splint, and ignores the squeaky pop test named on its own. A glowing splint is the test for oxygen, and naming a test without describing it demonstrates nothing.
Elsewhere the same series marks an equation in two parts, one for the equation being correct and balanced and one for the state symbols, so a neutralisation equation is worth writing out fully.
Testing for the gases
Hydrogen: a lighted splint burns with a squeaky pop.
Carbon dioxide: bubbled through limewater, it turns the limewater milky or cloudy.
Both are commonly asked as observations, so describe what happens rather than naming the test.
Strong and weak, concentrated and dilute
Strong means fully ionised in water, as hydrochloric, sulfuric and nitric acids are. Weak means only partially ionised, as ethanoic acid is.
Concentrated and dilute describe how much acid is dissolved in a given volume, which is a different property altogether. A dilute strong acid and a concentrated weak acid can have similar pH values, and questions rely on students assuming the four words come in two pairs when they do not.
Spec 2.35-2.38
What you need to know
- Define acids and bases by proton transfer
- Recall the reactions of acids
- Name the salt produced in each case
Active recall
Quick check
Answer each question before opening the answer.
What is produced when an acid reacts with a base or alkali?
A salt and water (neutralisation).
What is produced when an acid reacts with a carbonate?
A salt, water and carbon dioxide.
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