Solubility Rules

Students systematically mix aqueous cations and anions in a well plate to observe when precipitates form, then use patterns in the results to draft practical solubility rules and write net ionic equations.

1. Prepare an 8×12 well plate or test tubes and label columns for cations (as nitrate solutions) and rows for anions (as sodium salts).
2. Dispense about three drops of each cation into its column wells and three drops of each anion into its row wells, creating all pairwise combinations without letting droppers touch solutions.
3. Observe each mixture for evidence of precipitation (cloudiness, solid, color, gel-like texture) and record Y (precipitate) or N (no precipitate) in a grid; add brief notes on appearance.
4. For wells that form a precipitate, deduce and write the formula of the insoluble compound by balancing ionic charges (total positive charge equals total negative charge).
5. Write a net ionic equation for several representative precipitating pairs (e.g., Pb²⁺ + 2 I⁻ → PbI₂(s)).
6. If a result is unclear in the plate, repeat that pair in a clean small test tube to confirm.
7. Extension (Part B): Specifically test I⁻ with Ag⁺ and I⁻ with Pb²⁺ in clean wells to explore known exceptions.
8. Develop a set of solubility rules from your grid (e.g., “all nitrates soluble,” “most carbonates insoluble except with group 1 and ammonium,” etc.).
9. Collect all liquids in a labeled waste beaker, then transfer to the designated waste container. Rinse the plate and test tubes and return equipment.

📄 Solubility Rules - NC State University Chemistry Department: https://www.webassign.net/question_assets/ncsugenchem102labv1/lab_3/manual.html

• Compare results using different anion sets (e.g., acetate, chloride, bromide) to expand your rule set.
• Investigate concentration effects by repeating one cation–anion pair at two different molarities.
• Explore temperature effects on borderline precipitates by gently warming or cooling matched samples (record any reversals or increased turbidity).
• Use a smartphone light and dark background to improve detection of slight cloudiness; compare inter-rater judgments for reliability.

• Wear gloves, and a lab coat; avoid ingestion and skin contact with all solutions, especially silver nitrate and lead(II) nitrate (toxic if ingested).
• Silver nitrate can stain skin dark after ~24 hours; stains fade but avoid contact.
• Do not cross-contaminate reagent bottles - keep droppers out of wells.
• Work with small volumes only; clean spills immediately with water and towels, then wash hands.
• Collect all liquid wastes (including rinses) in the heavy-metal/aqueous waste container as directed.

• Which ions were always soluble in your grid? (Group 1 cations and ammonium; nitrate.)
• Which anions were usually insoluble, and with what exceptions? (Carbonate and phosphate are generally insoluble except with group 1 and ammonium.)
• What was the general trend for sulfate salts? (Often soluble; notable insoluble exceptions include PbSO₄, BaSO₄, and frequently SrSO₄.)
• What halide exceptions did you observe? (Ag⁺ and Pb²⁺ commonly form insoluble halides such as AgCl and PbI₂.)
• How do ion charges relate to precipitation likelihood? (Higher charge magnitude increases lattice energy, favoring precipitation.)
• In your observations, which rule takes precedence when rules seem to conflict—group 1/ammonium or anion insolubility? (Group 1 and ammonium solubility generally overrides anion insolubility.)
• Write the net ionic equation for a magnesium precipitate you observed. (Example: Mg²⁺(aq) + CO₃²⁻(aq) → MgCO₃(s), if that pair precipitated in your data.)
• Why are nitrate salts used as cation sources and sodium salts as anion sources? (Both nitrate and sodium/ammonium salts are typically soluble, ensuring the reacting ions are available in solution.)
• What is the role of spectator ions in your total ionic equations? (They remain unchanged and are omitted from the net ionic equation.)
• How could you reduce uncertainty in borderline cases? (Replicate trials, use clearer backgrounds, allow more settling time, or confirm in a test tube.)