Understanding Calcium's Reaction with Water

The question asks us to identify the statement that is NOT true regarding the reaction of Calcium with water. Calcium is an alkaline earth metal. When Calcium reacts with water, it forms Calcium hydroxide and hydrogen gas. The chemical equation for this reaction is:

$$ \text{Ca(s)} + 2\text{H}_{2}\text{O(l)} \rightarrow \text{Ca(OH)}_{2}\text{(aq)} + \text{H}_{2}\text{(g)} $$

Let's analyze each statement provided in the options based on the known properties of this reaction.

Analyzing Statements about Calcium-Water Reaction

• Statement 1: Heat evolved is less

  Compared to the reactions of alkali metals (like Sodium or Potassium) with water, the reaction of Calcium with water is less vigorous. While it is still an exothermic reaction (heat is evolved), the amount of heat produced is significantly less than that produced by alkali metals reacting with water. This lower heat evolution is why the reaction is less violent. Therefore, this statement is true for the reaction of Calcium with water.

• Statement 2: The reaction is more violent

  As mentioned above, the reaction of Calcium with water is less vigorous and less exothermic than the reaction of alkali metals with water. It is a relatively gentle reaction, especially with cold water, and becomes more vigorous with hot water. Stating that the reaction is "more violent" implies it is more violent than a standard reference, often implied to be the highly violent reactions of alkali metals. Compared to those, Calcium's reaction is much less violent. Therefore, this statement is NOT true for the reaction of Calcium with water.

• Statement 3: H₂ gas is evolved

  Looking at the chemical equation ($\text{Ca(s)} + 2\text{H}_{2}\text{O(l)} \rightarrow \text{Ca(OH)}_{2}\text{(aq)} + \text{H}_{2}\text{(g)}$), Hydrogen gas ($\text{H}_{2}$) is clearly shown as a product. The bubbles observed during the reaction are due to the evolution of Hydrogen gas. Therefore, this statement is true for the reaction of Calcium with water.

• Statement 4: Calcium starts floating

  Hydrogen gas produced during the reaction forms small bubbles that adhere to the surface of the Calcium metal. These bubbles effectively decrease the overall density of the Calcium-gas combination. If enough bubbles attach, the effective density becomes less than the density of water, causing the Calcium metal to float on the surface. Therefore, this statement is true for the reaction of Calcium with water.

Based on the analysis of each statement, the statement that is NOT true for the reaction of Calcium with water is "The reaction is more violent".

Revision Table: Calcium vs. Alkali Metal Reaction

Additional Information on Metal Reactions with Water

The reactivity of metals with water varies significantly across the periodic table.

• Highly reactive metals like alkali metals (Group 1: Li, Na, K, Rb, Cs) react vigorously or explosively with cold water, producing hydrogen gas and metal hydroxides. Reactivity increases down the group.

• Alkaline earth metals (Group 2) are less reactive than alkali metals. Calcium reacts steadily with cold water and more vigorously with hot water. Magnesium reacts very slowly with cold water but readily with steam. Beryllium does not react with water.

• Less reactive metals like Zinc or Iron react with steam, but not with cold or hot water.

• Unreactive metals like Copper, Silver, Gold, and Platinum do not react with water or steam.

The reactivity series of metals helps predict whether a metal will react with water or steam and the relative vigor of the reaction.