iGCSE Chemistry (0620) Core:7.3 Preparation of salts: Exam Style Questions Paper 1- New Syllabus
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Detailed solution:
For a precipitation reaction to occur, both starting reactants must be soluble in water, and when their solutions are mixed, an insoluble product (the precipitate) must form. We can use the solubility rules to work this out. In option D, lead(II) nitrate is soluble (all nitrates are soluble), and potassium chloride is also soluble (all potassium salts and most chlorides are soluble). When mixed, the ions can recombine: $\mathrm{Pb^{2+}}$ from the lead nitrate meets $\mathrm{Cl^-}$ from the potassium chloride, forming lead(II) chloride ($\mathrm{PbCl_2}$), which is one of the few insoluble chlorides. This precipitates out as a white solid. The other options either contain an already insoluble reactant (like $\mathrm{BaSO_4}$ in B) or produce only soluble products (A gives $\mathrm{MgCl_2}$ and $\mathrm{NaNO_3}$, both soluble; C gives $\mathrm{Ca(NO_3)_2}$ and $\mathrm{NaCl}$, also both soluble), so no precipitate forms.
▶️ Answer/Explanation
Detailed solution:
Copper(II) sulfate is a soluble salt that can be prepared by reacting dilute sulfuric acid with an excess of copper(II) oxide (a base) or copper(II) carbonate (a metal carbonate). Copper metal is not reactive enough to directly displace hydrogen from dilute acids. Copper(II) hydroxide (alkali) could be used but is less common. The reaction between an acid and a metal carbonate is a standard method that produces the salt, water, and carbon dioxide gas.
▶️ Answer/Explanation
Detailed solution:
A hydrated salt is a solid that contains water of crystallisation—water molecules that are chemically bound within its crystal structure. This is different from being dissolved in water (an aqueous solution) or being just a dry powder. Anhydrous salts contain no water. So, a hydrated salt is a solid chemically combined with water.