Chemistry 11–12 · Year 12

Solubility rules from mixing ionic solutions, and predicting precipitates from Ksp

Module 5: Equilibrium and Acid Reactions

Practical, model not builtMedium risk

School laboratory, not for home

In a school laboratory, with a teacher supervising, under the school's risk assessment. Not for home.

This site has no interactive model of its own. Where a step or a material names a Concept Studio model, simulation or tool, it has not been built; an external simulation a step names (for example PhET) is not part of this site.

The idea

Whether two ionic solutions give a precipitate follows a small set of solubility patterns, and quantitatively from comparing the ion product with the solubility product.

Safety card

Medium riskLearners carry it out, with a teacher supervising

Setting: In a school laboratory, with a teacher supervising, under the school's risk assessment. Not for home.

Hazards

lead(II) nitrate is toxic and a reproductive hazard
silver nitrate corrosive and staining
barium nitrate toxic
sodium hydroxide corrosive
copper(II) sulfate harmful

Controls

drop scale only
the lead(II) nitrate dropper bottle, made up by the teacher or technician from the solid, stays with the teacher, who adds the lead(II) drops to each group's row and dilution tubes; benches washed down with dilute detergent solution afterwards
gloves for lead, silver and barium bottles
well plates rinsed into a labelled heavy-metal waste container, never the sink
eye protection
wash hands

Note

NSW Department of Education Chemical Safety in Schools (CSIS) package, 2021 Technical Update: Section 1.7 (risk assessment) and Volume 2 Appendix D (generic assessment advice and DoE chemical categories); record a RiskAssess (riskassess.com.au) risk assessment before the lesson and check the school's hazardous chemical register (CSIS Section 1.9) for local restrictions. CSIS Volume 2 Appendix F lists lead and lead salts (solid) among the chemicals to be used with caution, largely by science teachers, and says lead salts are toxic by inhalation and absorbed through the skin, so the teacher makes up and dispenses the lead(II) nitrate and learners never handle the solid.

What you need

0.1 mol/L solutions in dropper bottles: potassium chloride, potassium iodide, sodium sulfate, sodium carbonate, sodium hydroxide, silver nitrate, lead(II) nitrate, barium nitrate, calcium nitrate, magnesium nitrate, copper(II) sulfate
Well plate (6 by 6) on a printed grid, white paper
Deionised water, wash bottle; centrifuge or filter paper for one confirmatory separation

How to do it

Put two drops of each cation solution (Ag+, Pb2+, Ba2+, Ca2+, Mg2+, Cu2+) in the wells of one row each.
Add two drops of each anion solution (Cl-, I-, SO4 2-, CO3 2-, OH-) down the columns; record precipitate (colour) or no change in the grid.
Test the three named pairs specially: potassium chloride with silver nitrate, potassium iodide with lead(II) nitrate, sodium sulfate with barium nitrate; write the net ionic equation for each.
From the grid write the solubility rules for nitrates, chlorides, iodides, sulfates, carbonates and hydroxides.
In the simulation choose the same pair and volumes, compare the ion product Q with Ksp and confirm the prediction; then find the dilution at which lead(II) iodide stops precipitating.
Dilute the lead(II) nitrate and potassium iodide stepwise in tubes to test that dilution limit.

What you should see

Silver: chloride gives a white curdy precipitate (AgCl), iodide a yellow one (AgI), carbonate a pale yellow one (Ag2CO3) and hydroxide a brown one (silver oxide); with sulfate the mixture is close to saturation for silver sulfate (0.83 g per 100 mL at 25 degrees Celsius), so at most a faint white precipitate forms slowly. Lead(II): iodide gives bright yellow lead(II) iodide, and sulfate, carbonate and hydroxide give white precipitates; chloride gives none at these concentrations, because the mixture holds less than a saturated lead(II) chloride solution (1.08 g per 100 mL of water at 20 degrees Celsius), although the model's Ksp comparison, which uses concentrations and ignores ion activities, predicts one (Q = 1.25e-04 against 1.7e-05); the RSC method uses 0.5 mol/L solutions to show that white precipitate, which dissolves on warming. Barium: sulfate and carbonate give white precipitates; chloride, iodide and hydroxide none (barium hydroxide octahydrate dissolves to 3.89 g per 100 mL of water at 20 degrees Celsius). Calcium: carbonate white; sulfate a white cloud that can take a minute or more to appear (Q = 2.5e-3 against Ksp 4.93e-05, but calcium sulfate stays supersaturated for a while); hydroxide a white precipitate or cloudiness (calcium hydroxide dissolves to only 0.173 g per 100 mL of water at 20 degrees Celsius). Magnesium: hydroxide and carbonate white. Copper(II): hydroxide pale blue, carbonate blue-green, and iodide a brown solution of iodine over an off-white precipitate of copper(I) iodide (2 Cu2+ + 4 I- gives 2 CuI + I2), a redox reaction, so the iodide rule names copper(II) as an exception. All nitrates, and all potassium and sodium salts, stay in solution. For 10 mL of 0.10 mol/L lead(II) nitrate with 10 mL of 0.20 mol/L potassium iodide, Q = 5.0e-04 against Ksp 9.8e-09, so precipitation is predicted; the molar solubility of PbI2 computed from Ksp is 1.35 mmol/L and of AgCl 13.3 micromol/L.

What changes

What you change: the pair of ions mixed (and, in the last step, their concentrations)
What you measure: whether a precipitate forms and its colour
What you keep the same: drop volume
concentrations at 0.1 mol/L
temperature
clean wells

Common misconceptions

Each of these ideas is wrong, and the activity is a chance to test it.

Insoluble means none dissolves (every salt has a Ksp; barium sulfate dissolves at about 10 micromol/L).
A precipitate forms whenever two ionic solutions are mixed (only when Q exceeds Ksp for one of the possible salts).
Diluting a mixture cannot stop a precipitate (below the concentration where Q equals Ksp none forms).

Curriculum references

The NSW syllabus outcomes and Australian Curriculum v9 codes this activity supports. They are references, not a verified or complete curriculum alignment.

Chemistry Stage 6 Syllabus (2017), NESA; the current syllabus, taught in 2026 (codes read from the syllabus document)CH12-12 CH11/12-4 CH11/12-5 CH11/12-6
Chemistry 11-12 Syllabus (2025), NESA; implemented from 2028, not yet taughtCH-12-01 CH-12WS-04 CH-12WS-06
Australian Curriculum v9No Australian Curriculum v9 code is listed.

Sources

The pages the author read to write this activity.

All Concept Studio activities