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VICChemistryQuick questions
Unit 2: How do chemical reactions shape the natural world?
Quick questions on Properties of water, dissolving and precipitation: VCE Chemistry Unit 2
12short 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 why water has unusual properties?Show answer
Water is a small, bent, polar molecule with two O-H bonds and two lone pairs on the oxygen. Each H2O molecule can hydrogen bond to up to four neighbours (donating two H atoms and accepting two via its lone pairs). The resulting hydrogen-bond network produces several anomalous properties:
What is water as a solvent?Show answer
Ionic compounds. When an ionic solid (e.g. NaCl) is added to water, the dipoles of water orient around the ions. The partial negative O attracts cations (Na^+), the partial positive H atoms attract anions (Cl^-). Each ion is surrounded by a hydration shell of water molecules.
What is solubility rules?Show answer
A workable VCE-level set of solubility rules for ionic compounds in water:
What is predicting precipitation and writing equations?Show answer
When two solutions are mixed, swap the partners and check each potential product against the solubility rules. If either product is insoluble, a precipitate forms.
What is worked example?Show answer
A student mixes 50 mL of 0.10 mol L^-1 lead(II) nitrate solution with 50 mL of 0.10 mol L^-1 potassium iodide solution. Predict the precipitate and write the net ionic equation.
What is ionic compounds?Show answer
When an ionic solid (e.g. NaCl) is added to water, the dipoles of water orient around the ions. The partial negative O attracts cations (Na^+), the partial positive H atoms attract anions (Cl^-).
What is polar molecular substances?Show answer
Substances such as sugar, ethanol or ammonia dissolve in water because they can hydrogen bond (or at least form dipole-dipole interactions) with water. They do not dissociate into ions; they dissolve as intact molecules.
What is writing H2O as the reactant for dissolving?Show answer
Water surrounds the ions but is not consumed. Use (aq) on the ions and (s) on the solid; do not put H2O in the equation unless it is genuinely a reactant or product.
What is forgetting states?Show answer
A net ionic equation without (aq), (s), (l), (g) is incomplete.
What is cancelling ions that have changed in number?Show answer
A K^+ on both sides at 2 each cancels; a Pb^2+ that appears as 1 on the left and is now inside the PbI2 precipitate is not a spectator.
What is listing PbI2 as soluble because most iodides are?Show answer
Read the exceptions: Ag, Pb and Hg(I) halides are insoluble. Always check both sides of the rule.
What is saying ice is less dense because it is colder?Show answer
Most substances are denser when solid. Ice is the standout exception because its open hydrogen-bonded lattice traps empty space.