CBSE Class 10 Science 2019 · Paper 31/3/1 · Question 7(a)

Activity (displacement reactions): A more reactive metal displaces a less reactive metal from a solution of its salt. 1. Clean strips of P, Q and R with sandpaper. Take salt (for example, sulphate) solutions of P and Q in three test tubes. 2. Tube 1: put a strip of R in the salt solution of Q. Tube 2: put a strip of R in the salt solution of P. Tube 3: put a strip of P in the salt solution of Q. Leave them for some time. Observations and inferences: Tubes 1 and 2: a coating of the displaced metal forms on R and the colour of the solution changes (for coloured salts, such as those of iron or copper), so R displaces both Q and P; R is more reactive than both. Tube 3: no change, so P cannot displace Q; P is less reactive than Q. (For example, with R as zinc, Q as iron and P as copper, zinc turns both iron(II) sulphate and copper sulphate solutions colourless, while copper leaves iron(II) sulphate unchanged.) Order of decreasing reactivity: $\text{R}>\text{Q}>\text{P}$.