Read the following and answer the questions:
A chemistry teacher demonstrates two reactions to the class. In the first, she adds dilute HCl to a test tube containing sodium carbonate powder and passes the gas produced through lime water — the lime water first turns milky. She then passes excess gas and the milkiness disappears. In the second demonstration, she places copper oxide powder into a beaker and slowly adds dilute HCl with stirring, and the contents gradually turn blue-green.
(i) Name the gas produced in the first reaction and write the balanced equation for its reaction with lime water to produce the white precipitate. (1 mark)
(ii) Why does the milky white precipitate disappear when excess gas is passed? Write the equation for this reaction. (1 mark)
(iii) In the second reaction, what causes the blue-green colour of the solution? Write the balanced equation for the reaction. (1 mark)
(iv) Based on the second reaction, explain why copper oxide is classified as a basic oxide. (1 mark)
Generated by claude-sonnet-4-6 · 2026-06-26 01:05 · grounding rag
Model Answer
(i) The gas produced is carbon dioxide (CO₂).
$$\text{Ca(OH)}_2\text{(aq)} + \text{CO}_2\text{(g)} \rightarrow \text{CaCO}_3\text{(s)} + \text{H}_2\text{O(l)}$$
The white precipitate is calcium carbonate (CaCO₃), which makes lime water milky.
(ii) When excess CO₂ is passed, the insoluble CaCO₃ reacts with water and excess CO₂ to form soluble calcium hydrogencarbonate, causing the milkiness to disappear.
$$\text{CaCO}_3\text{(s)} + \text{H}_2\text{O(l)} + \text{CO}_2\text{(g)} \rightarrow \text{Ca(HCO}_3)_2\text{(aq)}$$
(iii) The blue-green colour is due to the formation of copper(II) chloride (CuCl₂). Balanced equation:
$$\text{CuO(s)} + 2\text{HCl(aq)} \rightarrow \text{CuCl}_2\text{(aq)} + \text{H}_2\text{O(l)}$$
(iv) Copper oxide reacts with hydrochloric acid (an acid) to produce a salt and water — this is similar to the reaction of a base with an acid. Since metallic oxides react with acids to give salt and water, they are classified as basic oxides.
Source: Chapter 2, Sections 2.1.3 and 2.1.5
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Explanation
- (i) The reaction of Na₂CO₃ with HCl releases CO₂; always write the lime water equation with state symbols.
- (ii) Examinees often forget the reason — stress that Ca(HCO₃)₂ is soluble, which is why milkiness disappears.
- (iii) "Blue-green" is the key observation cue; always name the compound responsible (CuCl₂) and balance the equation.
- (iv) The definition-based justification is essential: link "metal oxide + acid → salt + water" directly to the definition of a basic oxide. One sentence of reasoning scores the mark.