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QLDChemistryQuick questions
Unit 2: Molecular interactions and reactions
Quick questions on Solubility rules and precipitation reactions (QCE Chemistry Unit 2)
14short Q&A pairs drawn directly from our worked dot-point answer. For full context and worked exam questions, read the parent dot-point page.
What is qCAA solubility rules (memorise this set)?Show answer
QCAA data booklets sometimes include a solubility table; verify which version applies to your cohort. The rules above are the most common 2026 set.
What is predicting whether a precipitate forms?Show answer
1. Identify the cations and anions in each starting solution. 2. List the two possible new ionic combinations (the "swap partners" products).
What is writing the three equation forms?Show answer
Once you know a precipitate forms, you can write the reaction in three equivalent ways. Each makes different information explicit.
What is common precipitate colours?Show answer
Worth knowing for IA stimulus and EA short response:
What is worked example?Show answer
A solution contains either Na_2SO_4 or NaCl. Adding a few drops of AgNO_3 solution produces no precipitate from one sample and a white precipitate from the other. Identify each.
What is limitations of solubility rules?Show answer
"Insoluble" in the rules really means "very slightly soluble". Even AgCl has a tiny solubility (about 1.3 x 10^-5 mol/L at 25 degrees C). The solubility product Ksp formalises this at Unit 3 / 4 level.
What is molecular equation?Show answer
Shows the formulas of the reactants and products. Always include state symbols.
What is complete ionic equation?Show answer
All aqueous strong electrolytes (soluble ionic compounds and strong acids/bases) are written as separated ions. Insoluble solids, weak electrolytes, gases and molecular liquids stay together.
What is net ionic equation?Show answer
Cancel the spectator ions (those that appear unchanged on both sides).
What is forgetting state symbols?Show answer
A "precipitation reaction" requires (s) for the precipitate and (aq) for the dissolved ions. Equations without state symbols typically lose marks.
What is writing precipitates as separated ions?Show answer
AgCl(s) is a solid lattice; do not split into Ag+ + Cl- on the product side.
What is forgetting to cancel spectators in the net ionic equation?Show answer
The whole point of the net ionic equation is to show only the chemistry that happens. Leaving spectators in loses the conceptual mark.
What is mis-balancing for ion charges?Show answer
When NaCl reacts with Pb(NO_3)_2, the equation is 2NaCl + Pb(NO_3)_2 -> PbCl_2 + 2NaNO_3 (two chlorides per lead). Always balance both atoms and charges.
What is confusing "no reaction" with "no precipitate I expected"?Show answer
If the rules say both possible products are soluble, the correct answer is "no reaction"; do not invent a precipitate.