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River water in a chalky agricultural area contains Ca<sup>2+</sup>, Mg<sup>2+</sup>, CO<sub>3</sub><sup>2-</sup>, HCO<sub>3</sub><sup>-</sup>, Cl<sup>-</sup> and NO<sub>3</sub><sup>-</sup> ions - CIE - A-Level Chemistry - Question 18 - 2016 - Paper 1
Question 18
River water in a chalky agricultural area contains Ca<sup>2+</sup>, Mg<sup>2+</sup>, CO<sub>3</sub><sup>2-</sup>, HCO<sub>3</sub><sup>-</sup>, Cl<sup>-</sup> and NO<... show full transcript
Worked Solution & Example Answer:River water in a chalky agricultural area contains Ca<sup>2+</sup>, Mg<sup>2+</sup>, CO<sub>3</sub><sup>2-</sup>, HCO<sub>3</sub><sup>-</sup>, Cl<sup>-</sup> and NO<sub>3</sub><sup>-</sup> ions - CIE - A-Level Chemistry - Question 18 - 2016 - Paper 1
Step 1
What will be precipitated from the river water following the addition of calcium hydroxide?
Answer
When calcium hydroxide (Ca(OH)₂) is added to the river water, it will react with the carbonate ions (CO₃²⁻) present in the water. This leads to the formation of calcium carbonate (CaCO₃), which is a poorly soluble salt and will precipitate out of the solution.
The reaction can be summarized as follows:
ightarrow ext{CaCO}_3 (s) + 2 ext{OH}^-$$ Therefore, the precipitate formed is calcium carbonate (CaCO₃), corresponding to option B.