S2 Acids and Bases

Almost every fact you will meet in this topic can be traced back to one of two charged particles. An acid puts hydrogen ions, H⁺, into water; an alkali puts hydroxide ions, OH⁻, into water. Nothing else about the substance matters as much as which of these two ions it sets free.

Drop an acid into water and its molecules split apart, releasing H⁺ ions. Take hydrogen chloride as the standard example:

The particle written H⁺ is a bare proton — a hydrogen nucleus that has lost its one electron. It is far too small and too positively charged to sit alone among water molecules, so it attaches itself to the nearest one and travels as the hydronium ion:

Chemists write H⁺(aq) for short, and so will any exam paper, but you should be able to explain that the ion really exists as H₃O⁺ in solution. And whenever a question asks why an acid behaves the way it does — sour, corrosive, able to turn litmus red — the mark is for naming the H⁺ ion as the cause. An alkali's behaviour is explained in exactly the same way, with OH⁻ doing the work.

How much of an acid breaks up is what separates a strong acid from a weak one.

Strong does not mean concentrated, and weak does not mean dilute. Strength is a fixed property of the substance — how completely it splits — while concentration tells you how much of it you dissolved. A very dilute solution of a strong acid is still a solution of a strong acid.

Two broad families are worth naming. Mineral acids come from rocks and industrial chemistry rather than from living things — the three strong acids in the table above all belong here, along with phosphoric acid. Organic acids are the ones produced by plants and animals, and they are weak: ethanoic acid in vinegar, citric acid in citrus fruit, carbonic acid in any carbonated drink.

A base is anything that can wipe out the acidity of an acid, so the two react and leave salt and water behind. Some bases dissolve in water and some do not, and the soluble ones get their own name.

So every alkali is a base, but plenty of bases are not alkalis. Solubility in water is the single test that separates one group from the other — and a question about "why is CuO not an alkali?" is really asking whether you can apply that test.

Four alkalis account for most of the questions you will see, and a handful of insoluble bases fill in the rest.

Because the same ion is released every time, acids behave alike in a recognisable set of ways. These are the observations a question is usually fishing for.

The pH scale is simply a way of reporting how the two ions are balanced in a solution. It runs from 0 to 14, with the midpoint telling you that neither ion is winning.

Each single step along the scale represents a tenfold change in the concentration of H⁺ ions. A solution of pH 2 therefore holds ten times as many H⁺ ions as one of pH 3, and a hundred times as many as one of pH 4. Two solutions can look equally clear and colourless while differing by a factor of a thousand in acidity, which is exactly why the scale is a number line rather than a set of labels.

An indicator is a substance that takes on a different colour as the pH around it changes, so it reports acidity or alkalinity by eye. Plain litmus is a two-answer indicator; universal indicator is a spectrum; phenolphthalein is the one that changes in the alkaline region only.

Some indicators are useful precisely because they switch colour only across a narrow band of pH. That band is called the transition range, and below it the indicator shows one colour, above it another, with a third colour in between.

Read the ranges rather than memorising them, and note how they line up with what you already know: bromothymol blue changes right across the neutral point, which is why it is the natural choice when you are watching an acid being neutralised, whereas phenolphthalein says nothing at all until the solution is already mildly alkaline.

Coloured plant pigments behave the same way, which makes them a good kitchen demonstration:

• Red cabbage water is red in acid, stays purple in a neutral solution and turns green to yellow as the solution becomes more alkaline.
• Hibiscus extract starts pink in acid and shifts towards blue-green in alkali.

Metals that sit towards the top of the reactivity series — potassium, sodium, calcium, magnesium, aluminium, zinc and iron among them — will attack an acid. Two products appear:

Where the metal sits in the reactivity series decides how dramatic the reaction is. Magnesium fizzes furiously, zinc and iron bubble more gently, and the metals below hydrogen — copper, silver, gold and platinum — are simply left untouched. Drop a copper coin into dilute acid and nothing happens at all: no bubbles, no heat, no change.

Mix an acid with a base of any kind and both characters disappear. This is neutralisation, and it is the reaction that ties the chapter together.

The wording matters. A base may be a soluble alkali such as sodium hydroxide or an insoluble powder such as copper(II) oxide, and the products are the same in either case. What changes is the beginning of the experiment: an alkali dissolves into the acid solution immediately, while an insoluble base has to be stirred in and will leave a clear solution only once enough of it has reacted. That difference is what makes insoluble bases useful for making pure salts — any excess can be filtered off.

Whatever the acid and whatever the base, the same event is doing the work: a hydrogen ion from the acid meets a hydroxide ion from the base, and together they form water.

This is the net ionic equation for the reaction, and it is the same line for every neutralisation you will ever write. The remaining ions simply pair up into the salt: the cation that came from the base joins the anion that came from the acid. That is the whole story of why the pH of the mixture climbs towards 7.

Carbonates react with acids in a way you can hear from across the lab, because one of the products is a gas.

Warm an alkali with an ammonium salt and a third gas enters the picture. The alkali pulls the ammonium group apart and ammonia gas escapes from the mixture.

Two of these three gases look identical in the tube, so the confirming test is what actually earns the mark. Set out the observations side by side:

The white solid that appears in limewater is calcium carbonate, and the limewater itself is a dilute solution of calcium hydroxide. Once enough carbon dioxide has been bubbled through, the solution stays milky — that permanent cloudiness is the sign the test has worked.

Every salt has two halves: a metal (or ammonium) part from the base, and an acid part from the acid. The acid decides the second half every time.

A concentrated strong acid or strong alkali attacks living tissue directly. Because both are corrosive, the sensible habit is to treat every bottle on that shelf with the same respect, whether the label says acid or alkali.

• Put on safety goggles and gloves before you pick up any bottle of acid or alkali, and keep them on until the practical is finished.
• If any acid or alkali reaches your skin, rinse the area under running water at once and keep rinsing — a few seconds is not enough. Tell your teacher every time, even if the splash seemed trivial.
• Never add water to a concentrated acid. Tip the acid slowly into the water, stirring as you go.
• Point test tubes away from yourself and your neighbours whenever a reaction could spit, and never look straight down into a tube you are heating.
• Do not taste or smell anything deliberately, and never taste a chemical to find out what it is.

Once you can see neutralisation as a reaction that cancels out an unwanted acid or alkali, a whole list of everyday problems becomes solvable. Each case below is the same chemistry wearing different clothes.

Textbooks and exam papers use both terms, and they describe two different events. Ionisation is the process in which neutral molecules are pulled apart by water into ions that did not exist beforehand — a hydrogen chloride molecule becomes H⁺ and Cl⁻, and the ions are newly made. Dissociation is the separation of ions that were already there: the Na⁺ and OH⁻ in solid sodium hydroxide exist as ions inside the crystal lattice, and water simply lets them drift apart and move freely.

This also explains a phrase you will meet about acids: an acid that can donate only one H⁺ ion per molecule, such as HCl or HNO₃, is called monoprotic, whereas sulphuric acid has two to offer and ethanoic acid offers one from the COOH group. You will come back to this in more detail later; at this level it is enough to know that the number of acidic hydrogens in a formula is what the word counts.