Tests for elements, properties and chemicals

Hydrogen:

• Makes a squeaky pop with a lighted splint (noise comes from hydrogen burning with the oxygen in the air to become H₂O)

Chlorine

• Chlorine bleaches blue litmus paper, turning it white (may turn red for a moment as a solution of chlorine is acidic)

Oxygen

• Will relight a glowing splint

Carbon Dioxide

• Turns limewater cloudy, bubble gas through a test tube to do so

Ammonia

• Turns red litmus paper blue and has a strong smell

Cations: Regular Metal Ions

• Using a flame test, first clean a nichrome wire in dilute HCl and then holding it over a flame. When the fire burns colourlessly, dip it into the test sample and hold it over the fire again

Lithium flame Li⁺ : Red flame

Sodium Na⁺: Yellow flame

Potassium K⁺: Lilac flame

Calcium Ca⁺: Orange-red flame

Copper Cu⁺: Blue-green flame

Cations: Ammonium ions NH₄⁺

• Warm a solution of ammonium ions with sodium hydroxide solution, this releases ammonia gas which turns red litmus paper blue

Cations: Insoluble metal hydroxides

• Add a few drops of sodium hydroxide solution into the test metal hydroxide and it will precipitate or change colour

Copper (II) Cu²⁺: Blue precipitate, does not dissolve with excess solution

Iron (II) Fe²⁺ Green precipitate, does not dissolve with excess solution

Iron (III) Fe³⁺: Reddish brown precipitate, does not dissolve with excess solution

Anions: carbonate ions CO₃^2– 

• Add dilute hydrochloric acid to test sample, if carbonate is present then carbon dioxide will be released and fizzing will be seen as it is a gas. Limewater will turn cloudy as the gas is carbon dioxide

Carbonate ion test equation: CO3^2-(s) + 2H⁺(aq) → CO2(g) + H2O (l)

Anions: Halide ions (ions formed by group 7 elements)

• Acidify the sample with dilute nitric acid, and then add silver nitrate solution which will form a silver halide precipitate, colour of silver halide precipitate depends on type of halide ion

Halide ion test example (chloride): Ag⁺(aq) + Cl^-(aq) → AgCl(s)

Why is acid used in anion tests? → to get rid of carbonate or sulfite ions before the test as they both will also produce a precipitate which is not the one we are looking for

Chloride: White precipitate of silver chloride

Bromide: Cream precipitate of silver bromide

Iodide: Yellow precipitate of silver iodide

Anions: sulfate ions

• Acidify sample with dilute HCl and add a few drops of barium chloride/nitrate solution, a white precipitate of barium sulfate will be formed

Sulfate ion test equation: Ba²⁺ (aq) + SO₄^2- (aq) → BaSO₄ (s)

Water

• Using copper (II) sulfate, which turns blue when it is hydrated, and when heated the water evaporates, leaving it white in colour and it will be dry/anhydrous

Purity of water

• Pure substance means it is only made of one substance, meaning it will have defined boiling and freezing points. Pure water will always boil at 100degree and freeze at 0 degrees, sample will not be pure if temperature points are different

Reactive metals

• Metals interact with water to form metal hydroxide and hydrogen, as heard by a squeaky pop and bubbles of hydrogen gas. Very reactive metals e.g potassium and sodium will react vigorously

Less reactive metals

• They only react with steam and not cold water, use mineral wool soaked in water and the metal and place them inside a boiling tube. Heat up the tube where the cotton is with flame and hydrogen gas will be given off, while the metal turns into a metal oxide. Use a lighted splint to identify whether hydrogen was released or not.

Copper: less reactive metal

• Very unreactive that it won’t react with water nor steam

Magnesium oxide

• Burns a white flame in air (because of contact with oxygen) and forms a white powder called magnesium oxide.

Hydrogen (gases in the atmosphere)

• Burns very easily in oxygen and can be explosive. has a orange/yellow flame and the only product is water (as water vapour).

Sulfur

• Burns in air or oxygen with a pale blue flame and forms sulfur dioxide which is acidic when dissolved in water, if it is released in the air and enters a cloud, acidic rain will form

Group 7 displacement reactions

• When a more reactive element pushes out a less reactive element from a compound. E.g when chlorine water is added to potassium iodide, the chlorine will react with the potassium to form potassium chloride, displacing iodine from the salt and turns the solution brown

Displacement: Orange solution formed (Br2)

• Potassium bromide solution + chlorine water, chlorine reacts with potassium while bromide is left in the solution

Displacement: Brown solution formed (I2)

• Potassium iodide solution + chlorine water/bromine water

Group 1 metal oxides

• Group 1 metals will react with oxygen in the air to form metal oxides, oxide formed will depend on which group 1 metal was used