Raffles Chem FE Notes

1. General Experimental Design: Gas volumes measured using a graduated syringe Liquid volumes measured using a (i) burette - 2d.p. (ii) pipette - 1d.p. (iii) measuring cylinder - various precisions Use an digital mass balance to measure mass Name of gas Solubility in water Density relative to air Collection method Hydrogen Not soluble Less dense Displacement of water Carbon dioxide Slightly soluble Denser Displacement of water Oxygen Slightly soluble Slightly denser Displacement of water Chlorine Soluble Denser Downward delivery Ammonia Very soluble Less dense Upward delivery Sulfur dioxide Very soluble Denser Downward delivery Drying agents - Concentrated sulfuric acid (not for alkaline gases - will react) - Calcium oxide (not suitable for acidic gases - will react) - Anhydrous calcium chloride Anhydrous copper (II) sulfate is used as an indicator for water vapour It turns from white to blue when hydrated - CuSO4 → CuSO4 · 5H2O Universal Indicator

Gas Tests Gas Test Result if present Hydrogen Insert lighted splint into test tube Lighted splint extinguishes Oxygen Insert glowing splint into test tube Splint relights Carbon dioxide Bubble gas through limewater (aq calcium hydroxide) White ppt (calcium carbonate) forms Ammonia (NH3) Insert moist red litmus paper Moist red litmus paper turns blue, pungent smell Chlorine Insert moist blue litmus paper Most blue litmus paper turns red then bleaches Sulfur dioxide Bubble through aq acidified potassium manganate (VII) Aq acidified potassium manganate (VII) turns from purple to colourless Purification Techniques: If it’s solid + solid dissolved in water (solution), 1. Filter mixture and collect residue/filtrate 2. A. Wash residue with deionised water and pat dry B. Heat filtrate in an evaporating dish until all the water evaporates, collect remaining solid If solid dissolved in water is thermally unstable, do crystallisation instead: 1. Heat solution in evaporating dish, stop heating when there is about ⅓ of solution left 2. Let solution cool to allow crystals to form 3. Filter the mixture to obtain crystals, then pat dry To collect the solvent from a solution (where solute and solvent have different boiling points) - use simple distillation To separate a mixture of two miscible liquids, which have different boiling points - use fractional distillation As vapours go up the fractionating column, they condense when they reach a part of the column that is below their boiling point Liquid with the lower boiling point is collected as distillate Fractionating beads increase