Purity, Formulations and Gas Tests

Pure substances contain only one type of compound or element and have fixed melting and boiling points. Water always boils at exactly 100°C when it's pure. If you mix in impurities, substances will melt and boil over a range of temperatures instead.

Formulations are mixtures made using specific recipes with precise amounts of different components. Think paint, medicines, cleaning products - each ingredient has a particular job to do. The exact proportions matter because each component contributes different properties.

Testing for gases is surprisingly straightforward. Chlorine bleaches damp blue litmus paper white (be careful - it's poisonous!). Oxygen will relight a glowing splint, making it burst back into flame.

Quick Tip: Always do gas tests in a fume cupboard or well-ventilated area for safety!

More Gas Tests and Anion Detection

Hydrogen makes a distinctive squeaky pop sound when you put a burning splint into it. Carbon dioxide turns lime water cloudy - this is your go-to test for CO₂.

Anion tests (negative ions) follow clear patterns. For carbonates, add dilute HCl and bubble any gas produced through lime water. If it goes cloudy, you've got a carbonate because it's producing CO₂.

Sulfate testing needs two steps. First, add dilute HCl to remove interfering ions. Then add barium chloride - a white precipitate of barium sulfate confirms sulfates are present.

Halide ions (chlorine, bromine, iodine) also need dilute nitric acid first, then silver nitrate. You'll get different coloured precipitates: white for chloride, cream for bromide, and yellow for iodide.

Remember: Always add acid first in anion tests to remove interfering substances!

Cation Tests - Flame Tests and Metal Hydroxides

Flame tests are brilliant for identifying metal ions through their characteristic colours. Clean your nichrome wire with dilute HCl first, then dip it in your sample and hold it in the blue part of the Bunsen flame.

Each metal gives a distinctive colour: lithium burns crimson, sodium bright yellow, potassium lilac, calcium orange-red, and copper green. These colours are like fingerprints for metals.

Metal hydroxide tests use sodium hydroxide solution to create coloured precipitates. Copper gives blue, iron(II) gives green, iron(III) gives brown, while calcium and magnesium both give white precipitates.

The clever bit with aluminium is that it initially forms a white precipitate, but if you add excess sodium hydroxide, it redissolves to form a colourless solution. This unique behaviour helps distinguish it from other metals.

Pro Tip: Learn the flame test colours using memory tricks - "Little Suzy Plays Carefully Outside" for Lithium-crimson, Sodium-yellow, Potassium-lilac, Calcium-orange, Copper-green!