RGS 2025 Y4 QA Notes

1 QA Notes (I) – Introduction and Laboratory Techniques Qualitative analysis (QA) is the process of identifying the cations and anions present in unknown substances using chemical reactions and test reagents . Unlike quantitative analysis, where the amount of a particular chemical is measured, a qualitative analysis scheme simply confirms t he presence or absence of certain chemicals or ions. The chemical reactions used to identify the cations and anions mostly involve observable changes including: • production of a precipitate, • evolution of a gas, or • visible colour change 1.1 Preliminary examination Before carrying out any chemical reacti on, a preliminary examination to note the appearance, colour and solubility of the unknown substance often gives clues to the identity of the unknown chemical or the element present in the substance. (a) Colour • In general, compounds of transition metals have characteristic colours. • Group I, II and III metals generally form white salts that dissolve to form colourless solutions. • However, as can be seen from Table 1 , there are several possibilities for a give n appearance. The appearance is not conclusive and you should carry out further chemical tests to arrive at your final conclusion about the identity of the sample. Table 1: Characteristic colours of some chemicals Colour Inferences Colourless Dilute acids , alkalis and solutions of salts of Group I, II and III metals White Solid salts of Na+, K+, NH4+, Ca2+, Zn2+, Pb2+, Al3+ Black CuO, CuS, CoO, FeO, FeS, PbS, MnO2, I2 crystals Grey Metals e.g. Zn, Fe, Al in powder form Dark green Chromium salts Light green Iron(II) salts Blue or bluish green Hydrated copper(II) salts Yellow or brown Solutions of iron(III) salts (Fe3+), PbI2, AgI Pale pink Manganese(II) salts (Mn2+) Purple KMnO4 RAFFLES GIRLS’ SCHOOL (SECONDARY) RAFFLES PROGRAMME YEAR 4 CHEMISTRY Name : _________________________ ( ) Class : _______ Date : _____________ Established in 1879

2 • Crystalline salts sometimes show different colours when in hydrate d and anhydrous states. For example, CuSO4.5H2O is blue and anhydrous CuSO4 is white in colour. (b) Solubility of ionic compounds Below is a summary of the solubility of common substances in water. The following are soluble in water: 1. Salts of NH4+, K+, Na+ 2. Nitrates 3. Hydrogencarbonates 4. Sulfates except PbSO4, BaSO4, CaSO4 (sparingly soluble) 5. Halides including Chlorides Bromides Iodides except PbCl2, AgCl except PbBr2, AgBr except Pbl2, Agl The following are insoluble in water: 1. Carbonates except those of Na+, K+, NH4+ CaO, Ca(OH)2, CaS, CaSO3 are slightly soluble. 2. Hydroxides 3. Oxides 4. Sulfides (S2-) 5. Sulfites (SO32-) 1.2 Types of Reactions involved in QA After a preliminary examination of the app earance and solubility of the unknown substance, che