The pH scale

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

• pH below 7 — acidic (the lower the number, the more acidic).
• pH 7 — neutral (e.g. pure water).
• pH above 7 — alkaline (the higher the number, the more alkaline).

Watch out: a lower pH means more acidic — pH 1 is far more acidic than pH 6. And pH 7 is neutral, not "no acid at all everywhere"; it's the middle of the scale.

Single indicators

Unlike universal indicator, single indicators give just one colour change — useful for telling acid from alkali, and for spotting the end-point of a titration.

Tip: litmus is good for a quick acid/alkali test; phenolphthalein and methyl orange give sharp colour changes that make them ideal for titrations.

Read the indicator

It's all about the ions

In water (aqueous solution):

• an acid releases hydrogen ions, H⁺ — these are what make it acidic;
• an alkali (a soluble base) releases hydroxide ions, OH⁻.

When an acid and an alkali react, those ions combine — this is neutralisation:

Why neutral? Once the H⁺ and OH⁻ have paired up to make water, neither excess remains, so the solution sits at pH 7.

The neutralisation equation

Strong vs weak acids

This compares how completely an acid ionises in water — its degree of ionisation:

• a strong acid (e.g. hydrochloric, nitric, sulfuric) is fully ionised — nearly all molecules split into H⁺ ions;
• a weak acid (e.g. ethanoic/citric/carbonic) is only partially ionised — most molecules stay whole, so it sits in an equilibrium.

Higher-tier link: a higher H⁺(aq) concentration means a lower pH. As pH is a log scale, each pH unit is a 10× change in H⁺ concentration.

Strong ≠ concentrated

These two words describe different things — a classic exam trap:

• Strong / weak = the degree of ionisation (how much of the acid splits into ions).
• Concentrated / dilute = the amount of acid dissolved in a given volume of water (mol per dm³).

So you can have a dilute strong acid (little acid, but fully ionised) or a concentrated weak acid (lots of acid, but only partly ionised). They are independent ideas.

Watch out: "strong" tells you how readily it ionises, not how much is there. Vinegar can be concentrated and still be a weak acid.

Strong, weak, or both?

Acid + metal

A reactive metal reacts with an acid to give a salt and hydrogen gas:

You can test for the hydrogen with a lit splint — it gives a squeaky pop.

Naming the salt: the second word comes from the acid. Hydrochloric → chloride, sulfuric → sulfate, nitric → nitrate. The first word comes from the metal/base.

Acid + base, and acid + carbonate

Two more reactions complete the set:

Watch out: only carbonates give off a gas (CO₂). Acid + a metal oxide or hydroxide makes just salt + water — no fizzing.

Name the salt

What does the acid make?

Tap the correct set of products for each reaction of an acid.

Preparing soluble salts

To make a pure, dry sample of a soluble salt you start from an acid and choose a route based on solubility:

• Acid + insoluble base (a metal oxide, hydroxide or carbonate that doesn't dissolve): add it in excess, then filter off the leftover solid.
• Acid + alkali (a soluble base — there's no solid to filter): you can't see when it's neutral, so you use a titration with an indicator.

Either way, you then crystallise: evaporate some water and leave the solution to form crystals.

Solubility steer: use the insoluble-base route when the base won't dissolve (excess can be filtered out); use the titration route when the base is an alkali (soluble), because excess can't be filtered.

Soluble salt from an insoluble base

To make copper sulfate crystals from copper oxide (insoluble) and sulfuric acid:

• Warm the acid, then add copper oxide a little at a time until excess (some stays undissolved) — this makes sure all the acid has reacted.
• Filter to remove the unreacted copper oxide, leaving blue copper sulfate solution.
• Evaporate some water, then leave to crystallise; pat the crystals dry.

Which preparation method?

Titration — the technique

A titration finds exactly what volume of acid neutralises a known volume of alkali (or vice versa):

• measure a fixed volume of alkali into a conical flask with a pipette; add a few drops of indicator;
• run acid in from a burette, swirling, until the indicator just changes colour — the end-point;
• record the titre (volume added) and repeat for concordant results.

Indicator choice: use a single indicator with a sharp change (phenolphthalein or methyl orange) — not universal indicator, whose gradual colours make the end-point hard to judge.

Spotting the end-point

Which acid made this salt?

Tap a salt, then tap the acid it must have been made from.

The key ideas

You've covered all of Eduqas Topic 7 — Chemistry of acids. Press Finish to see your score.