Identifying the Type of Chemical Reaction: CaO + H₂O

The question asks to identify the type of chemical reaction represented by the equation: CaO + H₂O → Ca(OH)₂.

Let's analyze the reactants and the product in this chemical equation:

• Reactant 1: CaO (Calcium Oxide, also known as quicklime)
• Reactant 2: H₂O (Water)
• Product: Ca(OH)₂ (Calcium Hydroxide, also known as slaked lime)

In this reaction, two substances, Calcium Oxide and Water, combine together to form a single new substance, Calcium Hydroxide.

Let's consider the definitions of the reaction types given in the options:

• Dissociation Reaction: A reaction where a compound breaks down into simpler substances or ions. This is the opposite of what is happening in the given reaction.
• Substitution Reaction: A reaction where one element replaces another element in a compound. This is not the case here; two entire molecules are combining.
• Combination Reaction: A reaction in which two or more reactants combine to form a single product. This definition perfectly matches the given reaction (CaO + H₂O → Ca(OH)₂), where two reactants form one product.
• Addition Reaction: In inorganic chemistry, this term is often used interchangeably with combination reaction, especially when simple molecules combine. However, "Combination Reaction" is the more general and precise term for the combination of two or more substances to form a single substance, regardless of whether the reactants are elements or compounds. In organic chemistry, "Addition Reaction" specifically refers to adding atoms across a multiple bond (like a double or triple bond). Given the options and the inorganic nature of the reactants, "Combination Reaction" is the most appropriate classification.

Based on the analysis, the reaction CaO + H₂O → Ca(OH)₂ is a reaction where two reactants combine to form a single product. This is the definition of a Combination Reaction.

The balanced chemical equation for this reaction is:

\(\text{CaO(s)} + \text{H}_2\text{O(l)} \rightarrow \text{Ca(OH)}_2\text{(aq)}\)

This reaction is also highly exothermic, meaning it releases a significant amount of heat.

Understanding Combination Reactions

A combination reaction is a fundamental type of chemical reaction. Its general form can be represented as:

\(\text{A} + \text{B} \rightarrow \text{AB}\)

Where A and B are reactants (elements or compounds) and AB is the single product formed.

Examples of combination reactions:

• Formation of water from hydrogen and oxygen: \(2\text{H}_2 + \text{O}_2 \rightarrow 2\text{H}_2\text{O}\)
• Formation of ammonia from nitrogen and hydrogen: \(\text{N}_2 + 3\text{H}_2 \rightarrow 2\text{NH}_3\)
• Formation of sulfur dioxide from sulfur and oxygen: \(\text{S} + \text{O}_2 \rightarrow \text{SO}_2\)

The reaction of Calcium Oxide with Water fits this pattern perfectly, with CaO being 'A', H₂O being 'B', and Ca(OH)₂ being 'AB'.

Revision Table: Types of Chemical Reactions

Additional Information: Slaking of Lime

The reaction between Calcium Oxide (quicklime) and Water is also specifically known as the "slaking of lime". This process is widely used in various applications, including construction (making mortar and plaster) and water treatment.

Key characteristics of the slaking of lime reaction:

• It is a combination reaction.
• It is highly exothermic, producing a significant amount of heat, which can even boil the water.
• The product, Calcium Hydroxide, is moderately soluble in water and forms an alkaline solution called "limewater".
• If excess water is added, it forms a suspension known as "milk of lime".

Understanding this specific reaction helps solidify the concept of combination reactions with a practical example.