Chemical Reactions

To solve this, let’s carefully analyze each of the four statements about the reaction between a Group I metal (like lithium, sodium, or potassium) and cold water:

1. The metal melts:

• This is a physical change because the metal transitions from a solid state to a liquid state due to the heat produced in the exothermic reaction. The chemical composition of the metal remains the same; only the state changes.

2. Hydrogen is produced:

• This is a chemical change because it involves the production of hydrogen gas (H₂) from the reaction between the metal and water. A new substance (hydrogen gas) is formed.

3. Steam is produced:

• This is a physical change because steam is simply water in a gaseous state. The water changes its state from liquid to gas due to the heat produced during the reaction. The chemical composition of water doesn’t change; only the state changes.

4. The pH of the solution increases:

• This is a chemical change because it results from the formation of a metal hydroxide (e.g., lithium hydroxide, NaOH) in water. This increases the pH of the solution, making it more basic.

Now, let’s match the statements with the physical changes:

• Physical changes: 1 (metal melts) and 3 (steam is produced).
  Thus, the correct answer is A. 1 and 3.

Fundamental concepts:

• Oxidation: Loss of electrons, increase in oxidation state
• Reduction: Gain of electrons, decrease in oxidation state
• Oxidizing agent: Causes oxidation, gets reduced itself
• Reducing agent: Causes reduction, gets oxidized itself

Memory aid: “OIL RIG”

• Oxidation Is Loss (of electrons)
• Reduction Is Gain (of electrons)

Analyzing the Reaction

Given equation: 2CuO + C → 2Cu + CO₂

Step-by-Step Oxidation State Analysis

Step 1: Determine Oxidation States Before Reaction

CuO (Copper(II) oxide):

• Cu: +2 (given as copper(II))
• O: -2 (oxygen typically -2 in compounds)

C (Carbon):

• C: 0 (elemental form)

Step 2: Determine Oxidation States After Reaction

Cu (Copper metal):

• Cu: 0 (elemental form)

CO₂ (Carbon dioxide):

• C: +4 (since O is -2, and molecule is neutral: C + 2(-2) = 0, so C = +4)
• O: -2

Step 3: Identify Changes in Oxidation States

Copper changes:

• Cu: +2 → 0 (decrease of 2)
• This is REDUCTION (oxidation state decreases)

Carbon changes:

• C: 0 → +4 (increase of 4)
• This is OXIDATION (oxidation state increases)

Step 4: Analyze Each Option

A – CuO is reduced:

• CuO contains Cu²⁺ which goes to Cu⁰
• Oxidation state decreases (+2 → 0)
• CORRECT – CuO (specifically the Cu²⁺ ion) is reduced

B – CO₂ is oxidised:

• CO₂ is a product, not a reactant being oxidized
• The carbon in CO₂ has already been oxidized (from C⁰ to C⁴⁺)
• INCORRECT – CO₂ is the product of oxidation, not what gets oxidized

C – Cu is oxidised:

• Cu (metal) is a product with oxidation state 0
• The Cu²⁺ in CuO was actually reduced to form Cu⁰
• INCORRECT – Cu metal is the reduced product

D – C is reduced:

• Carbon goes from 0 to +4 oxidation state
• This is an increase, which means oxidation, not reduction

INCORRECT – C is oxidized, not reduced