Chemistry 2007 JAMB Past Questions - Question 45: when temporary hard water is boiled for some time in a kettle the inner surface of the kettle becomes coated with a deposit of

Correct Answer

B

Explanation

When temporary hard water is boiled for some time in a kettle, the inner surface of the kettle becomes coated with a deposit of **calcium carbonate (CaCO₃)**. This is what is commonly known as **kettle scale** or **scale deposit**.
Here's how it happens:Temporary hardness** is caused by dissolved **calcium bicarbonate (Ca(HCO₃)₂) and magnesium bicarbonate (Mg(HCO₃)₂) in the water.** These compounds are stable under normal conditions, but when the water is heated, they decompose.
* During boiling, the **bicarbonate (HCO₃)** breaks down, releasing carbon dioxide (CO₂) and leaving behind carbonate (CO₃)²⁻ ions. This causes the calcium and magnesium bicarbonates to become insoluble **calcium carbonate (CaCO₃) and magnesium carbonate (MgCO₃).**
* Since these **carbonates are less soluble in hot water**, they precipitate out of the solution and form a solid deposit on the kettle's inner surface. This is the scale build-up you see over time.

It's important to note that while **magnesium carbonate** also contributes to scale, **calcium carbonate** is usually the main culprit due to its higher concentration in most water sources.

This scale build-up can:

* **Reduce the efficiency of the kettle:** As the scale thickens, it acts as an insulator, making it take longer for the water to boil and increasing energy consumption.
* **Damage the kettle:** If the scale buildup becomes too thick, it can crack the heating element or damage other parts of the kettle.
* **Affect the taste of the water:** While considered generally safe, some people find the taste of water boiled in kettles with significant scale buildup unpleasant.

Therefore, it's recommended to clean your kettle regularly to remove scale deposits and ensure its optimal performance and lifespan.