CBSE Class 10 Science 2024: question 21(a)

Hints

Open one hint at a time and try again before the next.

Step-by-step solution

Each step shows the marks it earns, as an examiner would award them. Checked by an independent re-solve from the question paper.

1. A precipitate is an insoluble salt formed when ions from two reactants swap partners. This happens in a double displacement reaction, so a precipitate cannot form unless ions are exchanged.

   ½ mark

2. Example: sodium sulphate solution is mixed with barium chloride solution.

3. $\mathrm{N}{\mathrm{a}}_2\mathrm{S}{\mathrm{O}}_4(\mathrm{aq})+\mathrm{BaC}{\mathrm{l}}_2(\mathrm{aq})\to \mathrm{BaS}{\mathrm{O}}_4(\mathrm{s})+2NaCl(\mathrm{aq})$

   1 mark

4. $\mathrm{B}{\mathrm{a}}^{2+}$ and $\mathrm{S}{\mathrm{O}}_4^{2-}$ ions combine to give the white precipitate of $\mathrm{BaS}{\mathrm{O}}_4$, while $\mathrm{N}{\mathrm{a}}^{+}$ and $\mathrm{C}{\mathrm{l}}^{-}$ stay in solution. This exchange of ions is what produces the precipitate.

   ½ mark

Answer: A precipitate forms only by exchange of ions (double displacement), e.g. $\mathrm{N}{\mathrm{a}}_2\mathrm{S}{\mathrm{O}}_4+\mathrm{BaC}{\mathrm{l}}_2\to \mathrm{BaS}{\mathrm{O}}_4\downarrow +2NaCl$

Revise the chapter

• Chapter: Chemical Reactions and Equations (notes, formulas, where marks are lost and every past question)