GCSE · Chemistry · Edexcel · Spec 1CH0

Solubility rules for common salts

Pour one clear solution into another and a white solid suddenly appears. Could you have predicted it before you poured?

Sort the salts

Soluble or insoluble?

Pick a salt, then pick the column you think it belongs in. Commit first, then read the rule that decides it.

Still to sort

Soluble: (aq) (0)

Dissolves in water, so it's in solution: aqueous, written (aq).

Where the line is: Any sodium, potassium or ammonium salt, and any nitrate, lands here. Most chlorides and sulfates do too, unless they're one of the named exceptions.

Insoluble: (s) (0)

Doesn't easily dissolve in water, so it stays a solid, written (s).

Where the line is: Most carbonates and hydroxides land here, unless the partner is sodium, potassium or ammonium. A chloride or sulfate only lands here if it's a named exception.

12 of 12 still to sort.

Here's the trick: check the quick wins first. Is there sodium, potassium or ammonium in it, or is it a nitrate? Then it dissolves, every time. Only if the answer is no do you need the chloride, sulfate, carbonate and hydroxide rules, and their exceptions.

Predict, then check

Now use the rules on a mixture. Both starting solutions are clear, so every salt in them is dissolved.

Iron(II) chloride solution is poured into sodium hydroxide solution. What happens?

Read the rules onto the equation

Remember the white solid from the start? This is that reaction: silver nitrate solution poured into sodium chloride solution. Tap each substance to see which rule gives its state symbol.

Tap a molecule to see what it does in the reaction.

Work it backwards

Choose the two solutions

You want to make a sample of insoluble barium sulfate. Which two solutions should you mix?

  1. Split the salt you want into its two parts: a positive barium ion and a negative sulfate ion. You need one solution to bring each.
  2. missing step
Which line is step 2?

What do you really think?

Lead chloride: in or out?

A spatula of lead chloride is stirred into a beaker of water.

Which is closest to what you think will happen?
How sure are you?

WHAT YOU'VE LEARNED

A quick recap of today's lesson.

Pour one clear solution into another and a solid can appear out of nowhere. A handful of rules, plus a few exceptions, tells you when it will happen and which solid it is.

What you need to know

  • Soluble means it dissolves in water, written (aq). Insoluble means it doesn't easily dissolve, written (s).
  • Always soluble: every sodium, potassium and ammonium salt, and every nitrate.
  • Most chlorides are soluble. Exceptions: silver chloride and lead chloride.
  • Most sulfates are soluble. Exceptions: lead sulfate, barium sulfate and calcium sulfate.
  • Most carbonates and hydroxides are insoluble. Exceptions: sodium, potassium and ammonium carbonates and hydroxides.
  • A precipitation reaction makes an insoluble product; the solid formed is the precipitate.

The big picture

Soluble salts dissolve in water and are written (aq); insoluble ones don't easily dissolve and are written (s). All sodium, potassium and ammonium salts dissolve, and so do all nitrates. Most chlorides and sulfates dissolve, apart from a few named exceptions; most carbonates and hydroxides don't, unless the partner is sodium, potassium or ammonium. When two solutions are mixed, the ions swap partners, and any insoluble new pairing appears as a solid precipitate. Run that backwards to choose two solutions that make an insoluble salt.

Key points

1To predict a precipitate, swap the partners and test each new pairing against the rules. The starting salts being soluble tells you nothing.
2To make an insoluble salt, mix a soluble salt containing its positive ion with a soluble salt containing its negative ion.
3Silver nitrate + sodium chloride and barium chloride + sodium sulfate each give a white precipitate; iron(II) chloride + sodium hydroxide gives a green one.
4Solubility depends on the pairing of ions, not on one ion alone: lead nitrate dissolves, lead chloride doesn't.

Worked example

Problem

Barium nitrate solution is mixed with sodium carbonate solution. Does a precipitate form? If so, name it, and give the state symbol for each product.

⚠ Watch out

Deciding there's no precipitate because both starting solutions are soluble. Of course they are: they're solutions! The question is whether one of the NEW pairings, made when the ions swap partners, is insoluble.

🧠

Memory hook

SNAP always dissolves: Sodium, Nitrate, Ammonium, Potassium. Carbonates and hydroxides only dissolve with a SNAP partner. Chlorides lose silver and lead; sulfates lose lead, barium and calcium.

✓

Check yourself

From memory, write out the rules and their exceptions. Then: silver nitrate solution meets potassium chloride solution. Name the precipitate and give both products' state symbols.

Flashcards

(13)
What does it mean for a substance to be soluble?
It dissolves in a solvent, such as water. An insoluble substance doesn't dissolve.
Does 'insoluble' mean none of the salt dissolves?
No. All salts have some solubility. Insoluble salts are ones that don't easily dissolve in water.
Sodium, potassium, ammonium, nitrate: what do salts containing these have in common?
They are always soluble in water, whatever they're paired with.
Chlorides: the rule and its exceptions?
Most chlorides are soluble. Silver chloride and lead chloride are insoluble.
Sulfates: the rule and its exceptions?
Most sulfates are soluble. Lead sulfate, barium sulfate and calcium sulfate are insoluble.
Carbonates and hydroxides: the rule and its exceptions?
Most are insoluble. Sodium, potassium and ammonium carbonates and hydroxides are soluble.
State symbol for a soluble salt in water?
(aq), aqueous: it's dissolved, in solution.
State symbol for an insoluble salt formed in water?
(s): it's a solid.
What is a precipitation reaction?
A reaction that forms an insoluble product. The solid formed is called the precipitate.
Silver nitrate solution + sodium chloride solution gives…?
A white precipitate of silver chloride.
Barium chloride solution + sodium sulfate solution gives…?
A white precipitate of barium sulfate.
Iron(II) chloride solution + sodium hydroxide solution gives…?
A green precipitate of iron(II) hydroxide.
How do you choose two solutions to make an insoluble salt?
One soluble salt containing the positive ion you want, and one soluble salt containing the negative ion you want.

Tap any card to flip it, or use Study as deck to go through them one at a time. In the full lesson these run as a spaced-repetition deck — you rate each card Hard, Good or Easy and the tricky ones keep coming back until they stick.

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