Question

When an element 'X' reacts with water, it starts floating. Identify the element 'X':

• A. Potassium
• B. Calcium
• C. Sodium
• D. Iron

Hint:

Reaction of metals with water:

• K, Na: Violent, float (low density), catches fire
• Ca: Less violent, floats due to H₂ bubbles
• Mg: Reacts with hot water
• Fe, Al: React with steam

Answer 1

The Correct Option is B

Step 1: Understand reaction of metals with water.
Different metals react with water with varying degrees of vigor:

• Potassium (K): Reacts violently with water, catches fire, and floats.
• Sodium (Na): Reacts vigorously, melts into a silvery ball, and floats.
• Calcium (Ca): Reacts less violently, bubbles of hydrogen are formed, and it starts floating due to bubbles sticking to its surface.
• Iron (Fe): Does not react with cold water; reacts with steam.

Step 2: Identify the key characteristic - "starts floating".
The question specifically mentions that the element "starts floating" when it reacts with water.

• Sodium and potassium float because they are less dense than water and immediately float.
• Calcium starts floating because bubbles of hydrogen gas stick to its surface, making it buoyant.

Step 3: Analyze each option.

• (A) Potassium: Reacts violently, floats immediately due to low density.
• (B) Calcium: Correct. Calcium reacts with water to form calcium hydroxide and hydrogen gas. The hydrogen bubbles stick to the surface of calcium, causing it to float.
• (C) Sodium: Reacts vigorously, floats immediately due to low density.
• (D) Iron: Does not react with cold water; no floating observed.

Step 4: Why Calcium is the best answer.
While both sodium and potassium float, they do so primarily because of their low density. Calcium specifically "starts floating" due to hydrogen bubbles adhering to its surface, which is a distinctive characteristic mentioned in many textbooks. Final Answer: (B) Calcium