Solubility Rules and Proton Transfer

What this lesson covers in the specification

Incandio is aligned to this specification. It is not published by, endorsed by or affiliated with Pearson, and it reproduces none of Pearson's wording — the statement numbers are given so you can check every lesson against your own copy.

• 2.34 — The general solubility rules for ionic compounds in water
• 2.35 — Acids and bases in terms of proton transfer
• 2.36 — An acid as a proton donor and a base as a proton acceptor

Specification section 2(f) Acids, alkalis and titrations — also taught in Uses of Metals, and Alloys, Acids, Alkalis and the pH Scale, Hydrogen Ions and Neutralisation, Titrations.

1 · Understand it

No exam language yet. The only question this section answers is: do I actually understand what is happening?

Whether an ionic compound dissolves in water decides how it can be made, so the SOLUBILITY RULES are a practical tool rather than a list of trivia. They are used on every one of the salt preparation pages that follow.

Statement 2.34 — the rules worth learning

1. ALL sodium, potassium and ammonium salts are SOLUBLE. No exceptions.
2. ALL nitrates are SOLUBLE. No exceptions.
3. MOST chlorides are soluble, EXCEPT silver chloride and lead(II) chloride.
4. MOST sulfates are soluble, EXCEPT barium sulfate, calcium sulfate and lead(II) sulfate.
5. MOST carbonates are INSOLUBLE, except those of sodium, potassium and ammonium.
6. MOST hydroxides are INSOLUBLE, except those of sodium, potassium and ammonium — with calcium hydroxide slightly soluble.

The shape of that list is easier to hold than the individual entries. The first two rules have NO exceptions, and the middle two are 'soluble with a short list of exceptions'. The last two invert: mostly insoluble, with the same three cations always soluble. Sodium, potassium and ammonium turn up as exceptions to everything, which is worth noticing on its own — nothing containing them is ever insoluble.

Now the second half of the page, and it revisits acids from a broader angle. The previous lesson defined an acid as a source of hydrogen ions. A hydrogen atom is one proton and one electron, so a hydrogen ion, H⁺, having lost its electron, is simply A PROTON. That single observation is what statement 2.35 rests on.

So an ACID IS A PROTON DONOR and a BASE IS A PROTON ACCEPTOR. This is the same idea as before, expressed in terms of what is transferred rather than what is produced — and expressing it that way lets it cover cases the earlier definition could not.

Think of it like describing a gift by the giver or by the receiver

You can describe a present as 'something handed over' or you can name both parties: someone gave and someone received. The first description is fine and the second reveals that giving requires a receiver — there is no such thing as handing something over into a void. Proton transfer is the second description. Defining an acid as a proton donor immediately implies that a base must be present to accept it, which is why acids only behave as acids when something is there to react with. It also explains why ammonia is a base despite containing no hydroxide at all: it does not need to supply OH⁻, only to accept a proton, which it does readily.

What the proton-transfer definition explains that the earlier one could not

1. AMMONIA is a base. It contains no OH⁻ ions, so the earlier definition could not classify it — but it readily ACCEPTS a proton, becoming NH₄⁺, so it is a base by proton transfer.
2. REACTIONS WITHOUT WATER can be classified. Hydrogen chloride gas and ammonia gas react directly to form ammonium chloride, with a proton transferred and no solution anywhere.
3. The definitions do not conflict: producing H⁺ in water IS donating a proton to a water molecule.
4. It makes clear that acid and base behaviour is a RELATIONSHIP rather than a property held in isolation.
5. So the proton definition is broader, and contains the hydrogen-ion definition rather than replacing it.

The clearest case is HCl + NH₃ → NH₄Cl, in which a proton transfers directly from the hydrogen chloride to the ammonia. No water is present anywhere, and ammonia is acting as a base despite containing no hydroxide at all.

Using both halves of the page together

(a) Is barium sulfate soluble? (b) Is ammonium carbonate soluble? (c) In the reaction HCl + NH₃ → NH₄Cl, identify the proton donor and the proton acceptor.

1. (a) Most sulfates are soluble, but barium sulfate is one of the named exceptions. It is INSOLUBLE.
2. (b) Most carbonates are insoluble — but ammonium is one of the three cations whose salts are always soluble. It is SOLUBLE.
3. (b) is the more instructive case, because two rules appear to conflict and the cation rule wins.
4. (c) The HCl gives away a proton, so hydrogen chloride is the PROTON DONOR and therefore the acid.
5. The NH₃ takes that proton, becoming NH₄⁺, so ammonia is the PROTON ACCEPTOR and therefore the base.

Answer: (a) Insoluble. (b) Soluble — the cation rule overrides. (c) HCl the acid, NH₃ the base.

That conflict in part (b) is worth a rule of its own. When the cation rule and the anion rule disagree, the SODIUM, POTASSIUM AND AMMONIUM rule always wins, because it has no exceptions at all. Ammonium carbonate is soluble despite carbonates generally not being.

2 · Grade 9 Notes

A different job from the section above. You have already understood it; this is the precise set of things to LEARN — definitions to reproduce word for word, processes in order, equations with units, and the answers that score full marks.

Statement 2.34 — the solubility rules

• ALL sodium, potassium and ammonium salts — SOLUBLE, no exceptions
• ALL nitrates — SOLUBLE, no exceptions
• Chlorides — SOLUBLE except SILVER and LEAD(II)
• Sulfates — SOLUBLE except BARIUM, CALCIUM and LEAD(II)
• Carbonates — INSOLUBLE except sodium, potassium and ammonium
• Hydroxides — INSOLUBLE except sodium, potassium and ammonium; calcium hydroxide is slightly soluble
• When rules conflict, the sodium/potassium/ammonium rule WINS — it has no exceptions

Statements 2.35 and 2.36 — proton transfer

Proton

A hydrogen ion, H⁺. A hydrogen atom that has lost its single electron is just a proton.

Acid

A PROTON DONOR — a substance that gives away a hydrogen ion.

Base

A PROTON ACCEPTOR — a substance that takes a hydrogen ion.

Proton transfer

The movement of a hydrogen ion from the acid to the base. Every acid–base reaction is one.

Deciding whether a salt is soluble

1. Check the CATION first: sodium, potassium or ammonium means SOLUBLE, whatever the anion.
2. Check for NITRATE: always soluble.
3. Chloride or sulfate: soluble unless it is one of the named exceptions.
4. Carbonate or hydroxide: insoluble unless the cation was one of the three above.
5. If two rules seem to conflict, the sodium/potassium/ammonium rule wins.

The two definitions of an acid

A. SOURCE OF HYDROGEN IONS — a substance producing H⁺ in aqueous solution. Correct, but only applies in water.

B. PROTON DONOR — a substance that gives away a hydrogen ion. Covers reactions with no water at all, and makes ammonia a base.

Model answer [4 marks]

State whether each of these is soluble in water: barium sulfate, ammonium carbonate, silver chloride, potassium nitrate. [4]

Barium sulfate is insoluble, because although most sulfates are soluble, barium sulfate is one of the named exceptions. Ammonium carbonate is soluble, because although most carbonates are insoluble, all ammonium salts are soluble and that rule has no exceptions. Silver chloride is insoluble, because although most chlorides are soluble, silver chloride is one of the exceptions. Potassium nitrate is soluble, both because all nitrates are soluble and because all potassium salts are soluble.

Model answer [3 marks]

Explain why ammonia is classified as a base, even though it contains no hydroxide ions. [3]

A base is defined more generally as a proton acceptor, meaning a substance that accepts a hydrogen ion. Ammonia, NH₃, readily accepts a hydrogen ion to become the ammonium ion, NH₄⁺, so it acts as a proton acceptor and is therefore a base. This definition is broader than defining a base as a source of hydroxide ions, and it correctly classifies ammonia even though ammonia contains no hydroxide.

Not this: All carbonates are insoluble.

This: Most are, but sodium, potassium and AMMONIUM carbonates are all soluble. Those three cations are exceptions to every rule, and when two rules seem to conflict, that one wins.

Mark-losing trap. SODIUM, POTASSIUM and AMMONIUM salts are ALWAYS soluble. This rule beats every other rule.

Mark-losing trap. ALL nitrates are soluble — there is no exception to learn.

Mark-losing trap. A proton IS a hydrogen ion. Acid = proton DONOR, base = proton ACCEPTOR.

3 · Prove it — the five questions

The five questions climb Grade 6 → Grade 7 → Grade 8 → Grade 9 → Grade 9 challenge, and are marked inside Incandio on your own device, by rule, with an authored diagnosis of the mistake you actually made. The mark schemes stay in the app so that the practice is worth doing; the questions themselves are here.

1. Grade 6 · State [1 mark] — Are all nitrates soluble in water?
2. Grade 7 · Deduce [2 marks] — Is barium sulfate soluble or insoluble in water?
3. Grade 8 · Identify [3 marks] — In the reaction HCl + NH₃ → NH₄Cl, identify the proton donor and explain your answer.
4. Grade 9 · Explain [6 marks] — Select every statement that belongs in a full-mark explanation of why defining acids and bases in terms of proton transfer is more useful than defining them by the ions they produce in water.
5. 9+ · Analyse [6 marks] — A student is asked whether ammonium carbonate is soluble. They reason that carbonates are generally insoluble, so it must be insoluble. Select every statement that belongs in a full-mark analysis.

The people behind this science

Two ways into the same idea — the one who removed the wrong answer and pointed at hydrogen, and the one who insisted a definition must cover the awkward cases too. Inside Incandio each of them answers knowing exactly which lesson you have just finished.

Humphry Davy — the one who removed the wrong answer and pointed at hydrogen

The proton-transfer definition is the endpoint of a long argument about what acids have in common, and Davy delivered its decisive step. By showing hydrochloric acid contains no oxygen, he removed Lavoisier's answer and pointed instead at hydrogen — the element that would eventually be recognised as the thing transferred. He could not have known that a hydrogen ion is a bare proton, since the proton was a century away, but the identification he made is the one this page is built on.

• “How did you decide hydrogen was what acids had in common?”
• “What made hydrochloric acid the decisive case?”
• “How do you replace one definition with another?”
• “What did you think a hydrogen ion actually was?”
• “Why did the older definition last so long?”

Amedeo Avogadro — the one who insisted a definition must cover the awkward cases too

This page replaces a working definition with a broader one that covers cases the first could not, and Avogadro's career is the model for that move. His hypothesis reconciled two sets of results that appeared to contradict each other, at the cost of accepting something his contemporaries found unacceptable — that elementary gases exist as pairs. He is the right person to ask why a definition should be judged by the range it covers rather than by how comfortable it feels, and what it costs to propose one that is right and unwelcome.

• “Why should a definition cover the awkward cases too?”
• “What did your hypothesis require people to accept?”
• “How do you judge whether a broader idea is really better?”
• “Why did chemists resist it for so long?”
• “Was it worth publishing something nobody would use?”

Carry on through the course

• Previous lesson: Titrations
• Next lesson: The Reactions of Acids
• Group 1 — the Alkali Metals — Metals soft enough to cut with a knife, which set fire to water — and get worse as you go down.
• Group 7 — the Halogens — A green gas, a brown liquid and a grey solid — and a trend that runs the opposite way to Group 1.
• The Gases in the Air — Four gases, one of which is doing almost all the chemistry — and how to measure it without ever touching it.
• Combustion and Carbonates — Three things burning in oxygen, one carbonate breaking apart, and the 0.04% gas that changes the climate.
• The Reactivity Series — Putting eleven metals in order by watching what each one will and will not do.
• All of Chemistry · Chemistry key terms · required practicals · equations · Incandio Science · The 4CH1 specification