c) The reaction between lead (II) nitrate and concentrated sulphuric acid starts but stops immediately. Explain. 7. a) i) Potassium in fertilizers may be in the form of potassium nitrate. Describe how

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c) The reaction between lead (II) nitrate and concentrated sulphuric acid starts but stops immediately. Explain. (2 marks)
The reaction produces insoluble lead (II) sulphate, which forms a protective coating on the surface of the unreacted lead (II) nitrate. This coating prevents further contact between the reactants, thus stopping the reaction.

7. a) The elements nitrogen, phosphorous and potassium are essential for plant growth. i) Potassium in fertilizers may be in the form of potassium nitrate. Describe how a sample of a fertilizer may be tested to find out if it contained potassium ions. (2 marks)
   • Dissolve a small amount of the fertilizer in distilled water.
   • Dip a clean platinum or nichrome wire into the solution.
   • Introduce the wire into a non-luminous Bunsen flame. A lilac (or pale purple) flame indicates the presence of potassium ions.