Chemistry • Year 12 • Module 5 • Lesson 16
Solubility Rules & Precipitation
Lock in the NAGSAG framework, solubility vocabulary, and the three NESA-specified precipitation reactions before moving to prediction and equation writing.
1. Term–definition match
The definitions below are shuffled. In the right-hand column write the matching term from this list: precipitate, solubility rules, net ionic equation, spectator ions, double displacement reaction, NAGSAG, lattice energy, hydration energy. 8 marks
| # | Definition (shuffled) | Matching term |
|---|---|---|
| 1.1 | An insoluble solid that forms when two aqueous ionic solutions are mixed, appearing as a cloudy or coloured suspension. | |
| 1.2 | Empirical guidelines that allow chemists to predict whether an ionic compound will dissolve in water at room temperature. | |
| 1.3 | An equation showing only the ions that actually react, omitting all spectator ions. | |
| 1.4 | Ions that are present in solution before and after a reaction, unchanged in charge and formula — they do not participate in the net reaction. | |
| 1.5 | A reaction in which two ionic compounds in solution exchange anions to potentially form a precipitate or weak electrolyte. | |
| 1.6 | An acronym used to recall a priority-ordered set of solubility rules: Nitrates Always, Group 1 and Ammonium Always, Sulfates Generally, Acetates Generally, Generally insoluble. | |
| 1.7 | The energy released when the ionic lattice of a solid compound is broken apart into its gaseous ions — higher values favour insolubility. | |
| 1.8 | The energy released when water molecules surround and stabilise individual ions in solution — higher values favour solubility. |
2. Complete the NAGSAG solubility reference table
Fill in the missing cells. Write Soluble, Mostly soluble, or Mostly insoluble, and list the key exceptions where indicated. 10 marks
| Letter | Ion / group | Solubility rule | Key insoluble exceptions (if any) |
|---|---|---|---|
| N | Nitrates (NO3−) | ||
| A | Ammonium (NH4+) | ||
| G | Group 1 metals (Li+, Na+, K+, Rb+, Cs+) | ||
| S | Sulfates (SO42−) | ||
| A | Acetates (CH3COO−) | ||
| G | Carbonates (CO32−), Phosphates (PO43−), Hydroxides (OH−), Sulfides (S2−) | ||
| Halide rule — Cl−, Br−, I− | |||
3. True or false — with correction
Circle T or F. If false, write the corrected version on the line below. 10 marks (1 T/F + 1 correction each)
3.1 All ionic compounds dissolve readily in water because water is a polar solvent. T / F
3.2 Barium sulfate (BaSO4) is insoluble in water even though sulfates are generally soluble. T / F
3.3 Spectator ions appear on the left side of the equation but are consumed during the precipitation reaction. T / F
3.4 The precipitate formed when KI(aq) reacts with Pb(NO3)2(aq) is yellow PbI2. T / F
3.5 In the full ionic equation, the precipitate must be split into its constituent ions because it is an ionic compound. T / F
4. Function recall
Answer each in 1–2 sentences. 8 marks (2 each)
4.1 What is the purpose of writing a net ionic equation rather than a molecular equation?
4.2 Why must the NAGSAG rules be applied to both possible products when two solutions are mixed?
4.3 What is the function of adding a soluble sodium salt (e.g. Na2CO3) to water containing dissolved Pb2+ ions in a treatment plant?
4.4 What is the significance of the bright yellow colour of PbI2 in an analytical context?
5. Cloze — fill in the blanks
Complete the paragraph by selecting from the word bank below. Use each word once only. 10 marks
When two aqueous ionic solutions are mixed, chemists use __________ to predict whether a solid will form. The acronym __________ summarises the most commonly tested rules for HSC. If the reaction produces a solid, that solid is called a __________. To write the equation for such a reaction, three levels of detail are used. The __________ equation shows intact formula units on both sides. The __________ equation separates all aqueous species into their constituent ions, but leaves the solid product in its formula form because it is __________. Ions that appear identically on both sides are called __________ and are removed to give the __________. A classic example is barium sulfate, __________, which forms a white precipitate that is used in medical imaging precisely because of its extreme insolubility. In every net ionic equation, students must verify __________ on both sides before accepting the equation.
Q1 — Term–definition matches
1.1 precipitate • 1.2 solubility rules • 1.3 net ionic equation • 1.4 spectator ions • 1.5 double displacement reaction • 1.6 NAGSAG • 1.7 lattice energy • 1.8 hydration energy.
Q2 — NAGSAG table
N (Nitrates): Soluble — no exceptions. A (Ammonium): Soluble — no exceptions. G (Group 1): Soluble — no exceptions. S (Sulfates): Mostly soluble; exceptions: BaSO4 (insoluble), PbSO4 (insoluble), CaSO4 and Ag2SO4 (sparingly soluble). A (Acetates): Soluble — no exceptions. G (Generally insoluble): Mostly insoluble; exception — soluble when paired with Group 1 or NH4+; Ba(OH)2 soluble, Ca(OH)2 sparingly soluble. Halides: Mostly soluble; insoluble exceptions: all Ag+ halides (AgCl, AgBr, AgI) and PbCl2, PbBr2, PbI2.
Q3 — True / false with corrections
3.1 False. Not all ionic compounds dissolve in water. Solubility depends on the balance between lattice energy and hydration energy; compounds with very high lattice energy (e.g. AgCl, BaSO4) are insoluble despite water’s polarity.
3.2 True. Ba2+ is one of the named exceptions to the general sulfate solubility rule.
3.3 False. Spectator ions appear identically (same formula, charge, and state) on both sides of the full ionic equation. They are not consumed — they remain dissolved throughout the reaction.
3.4 True. PbI2 is a bright yellow precipitate — one of the three NESA-specified reactions.
3.5 False. The precipitate is an insoluble solid; it must be written as the formula unit with state symbol (s), never split into ions. Only aqueous (aq) species are split into ions.
Q4.1 — Purpose of the net ionic equation
The net ionic equation shows only the species that actually participate in the reaction, removing spectator ions. This reveals the essential chemical event (e.g. Ag+(aq) + Cl−(aq) → AgCl(s)) regardless of what counter-ions are present, making it the most transferable and concise representation of the precipitation.
Q4.2 — Why apply NAGSAG to both products
Mixing two solutions produces two possible new ionic combinations. Both must be assessed because either product could be insoluble. A common error is checking only one product and incorrectly concluding no precipitate forms when the other product is actually insoluble. The precipitate is always the insoluble product; the other remains dissolved.
Q4.3 — Function of Na2CO3 in water treatment
Adding Na2CO3 introduces CO32− ions, which combine with Pb2+ to form lead(II) carbonate (PbCO3), an insoluble precipitate. This converts the soluble, toxic Pb2+ ions into a filterable solid that can be physically removed, dramatically lowering the concentration of dissolved lead in the treated water.
Q4.4 — Significance of yellow colour of PbI2
The distinctive bright yellow colour of PbI2 can be used as a confirmatory test for either Pb2+ ions or I− ions in solution: adding iodide to an unknown sample and observing a yellow precipitate confirms the presence of lead(II) ions (or vice versa). Colour is a diagnostic observation used in qualitative analysis.
Q5 — Cloze answers (in order)
solubility rules • NAGSAG • precipitate • molecular • full ionic • insoluble • spectator ions • net ionic equation • BaSO4 • charge balance.