Heat is added to a substance, but its temperature does not rise. Which one of the following statements provides the best explanation for this observation ?

Options

A. The substance must be a gas.
B. The substance must be a non-perfect solid.
C. The substance undergoes a change of phase.
D. The substance has unusual thermal properties.

Answer

To solve the question, we need to analyze the situation where heat is added to a substance but its temperature does not rise. Here’s the step-by-step solution:

### Step 1: Understand the Concept of Heat and Temperature
Heat is a form of energy that is transferred between systems or objects with different temperatures. Temperature is a measure of the average kinetic energy of the particles in a substance. When heat is added to a substance, we typically expect its temperature to rise.

**Hint:** Consider what happens to the energy of particles when heat is added.

### Step 2: Identify Conditions for Temperature Change
For a substance to experience a temperature change, the added heat must increase the kinetic energy of its particles. However, there are specific conditions under which this does not happen.

**Hint:** Think about physical changes that might occur when heat is added.

### Step 3: Consider Phase Changes
One key condition where heat can be added without a temperature change is during a phase change. When a substance undergoes a phase change (e.g., solid to liquid, liquid to gas), the added heat is used to break intermolecular bonds rather than increasing the kinetic energy of the particles.

**Hint:** Recall examples of phase changes, like melting or boiling.

### Step 4: Analyze the Given Options
Now, we need to evaluate the provided options to find the best explanation for the observation that heat is added but the temperature does not rise.

1. The substance must be a gas.
2. The substance is undergoing a chemical reaction.
3. The substance undergoes a change of phase.
4. The substance is a poor conductor of heat.

Among these options, option 3, "The substance undergoes a change of phase," is the most appropriate explanation for the situation described.

**Hint:** Focus on the concept of latent heat during phase changes.

### Conclusion
The best explanation for the observation that heat is added to a substance but its temperature does not rise is that the substance undergoes a change of phase.

**Final Answer:** The substance undergoes a change of phase (Option 3).

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