Gas tests — give the method and the result
Every gas test needs two parts: how you carry it out AND the positive result. Hydrogen: a lit (burning) splint gives a squeaky pop. Oxygen: a glowing splint relights. Carbon dioxide: bubbled through limewater, it turns milky/cloudy. Ammonia: damp red litmus turns blue (ammonia is alkaline). Chlorine: damp litmus is bleached white. For chlorine the paper must be DAMP, and it must be chlorine gas (), not chloride ions.
Flame colours and hydroxide precipitates
Flame tests (clean the nichrome wire in concentrated hydrochloric acid, then hold it in the edge of a blue flame): lithium red, sodium yellow, potassium lilac, calcium orange-red, copper blue-green. Adding sodium hydroxide solution to a cation: copper(II) gives a blue precipitate, iron(II) a green precipitate, iron(III) an orange-brown precipitate; an ammonium salt warmed with sodium hydroxide gives ammonia gas, which turns damp red litmus blue.
Anion tests, water and pure water
Halides: add dilute nitric acid then silver nitrate — chloride gives a white precipitate, bromide cream, iodide yellow. Sulfate: add dilute hydrochloric acid then barium chloride — a white precipitate. Carbonate: add dilute acid — fizzing, and the gas turns limewater milky. Test for WATER: anhydrous copper(II) sulfate turns white to blue. Test for PURE water: it boils at exactly 100 C (and freezes at 0 C) at normal pressure — a physical test, not a chemical one.
Drawn from real examiner reports.
Sodium flame is yellow, not red
The sodium flame is YELLOW, not red — red is the lithium flame — and a wrong colour loses the observation mark. A flame test (not red litmus) is how you identify a metal cation: an alkaline result only shows is present, not which metal ion. Learn the five exactly: lithium red, sodium yellow, potassium lilac, calcium orange-red, copper blue-green.
Digest: sodium flame is yellow but some candidates say red (Jun24 1CR Q4b).
Put the sample into the flame
The sample on the wire is held IN the flame (the edge of a blue Bunsen flame), not above it, and "it burns" or "burning" is not accepted as the method. Clean the nichrome wire in concentrated hydrochloric acid first so a previous sample does not carry its colour over. Describe cleaning, dipping and holding in the flame, then give the colour seen.
Digest: "burning" is not accepted and the sample is placed into the flame, not above it (Jun23 Q5b).
Halide ion is not the halogen
A chloride ion () is not chlorine (); a bromide ion () is not bromine (); an iodide ion () is not iodine (). The ION ends in -ide and sits in solution; the neutral ELEMENT does not. Examiners repeatedly see "chlorine" written where "chloride" is meant. Use the -ide word for the ion you are testing for.
Digest: halogens and halide ions are confused throughout (Jun23).
Name the precipitate, not the colour
A precipitate is an insoluble solid formed when two solutions react — so name the SOLID, not just a colour. "A white precipitate of silver chloride" scores; "it goes white" does not. In the silver-nitrate halide tests name silver chloride (white), silver bromide (cream) or silver iodide (yellow); in the sulfate test name barium sulfate (white).
Test for water vs test for pure water
These are two different tests. Anhydrous copper(II) sulfate turning white to blue shows only that water is PRESENT — tap water and salt solution contain water too. To show water is PURE, measure a physical property: it must boil at exactly 100 C (or freeze at 0 C) at normal pressure. Impurities raise the boiling point. Describe the heating, do not just quote 100 C.
Digest: candidates confuse the chemical test for water with the physical test for pure water (Jun23 Q9biii; Nov24 Q7b).
Test answers have three parts
Give three parts: (1) the reagent and how you add it (e.g. add nitric acid, then silver nitrate); (2) the observation — exact colour and any solid ("a cream precipitate forms"); (3) the conclusion ("a bromide ion is present"). A method without its result is only half a test.
Acidify before the precipitate test
Add dilute nitric acid before silver nitrate (halides), and dilute hydrochloric acid before barium chloride (sulfate): the acid removes carbonate ions that would give a false white precipitate. Both give white precipitates — the reagent, not the colour, tells them apart.
Carbonate test — gas into limewater
For the carbonate test, add dilute acid to the sample and pass the gas into limewater held in a SEPARATE test tube — do not add limewater to the acid mixture. Fizzing alone is not enough: confirm the gas is carbon dioxide by the limewater turning milky.
This Double-Award topic is almost entirely rote recall of tests, reagents, observations and colours. The marks come from precise wording, so learn the tables below exactly. A precipitate (ppt) is an insoluble solid that forms when two solutions are mixed; state symbols are solid , liquid , gas and aqueous .
Define the term precipitate.
A test tube contains a colourless gas thought to be hydrogen. Describe the test you would carry out and the positive result that would confirm the gas is hydrogen.
