Which of the following can be repeatedly soften on heating?
(i) Polystyrene
(ii) Melamine
(iii) Polyesters
(iv) Polyethylene
(v) Neoprene

Options

A. (i) and (iii)
B. (i) and (iv)
C. (iii), (iv) and (v)
D. (ii) and (iv

Answer

To determine which of the given materials can be repeatedly softened on heating, we need to classify each material based on its properties as either thermoplastic or thermosetting.

### Step-by-Step Solution:

1. **Identify the Materials**:
- (i) Polystyrene
- (ii) Melamine
- (iii) Polyesters
- (iv) Polyethylene
- (v) Neoprene

2. **Classify Each Material**:
- **Polystyrene**: This is a thermoplastic polymer. Thermoplastics can be softened upon heating and hardened upon cooling.
- **Melamine**: This is a thermosetting polymer. Thermosetting polymers do not soften upon heating; instead, they harden and cannot be remolded.
- **Polyesters**: Generally, polyesters can be thermoplastic or thermosetting depending on their structure. However, many common polyesters (like PET) are thermoplastic.
- **Polyethylene**: This is also a thermoplastic polymer. It can be softened upon heating and hardened upon cooling.
- **Neoprene**: This is classified as an elastomer, which has elastic properties but does not soften on heating like thermoplastics.

3. **Determine Which Materials Can Be Repeatedly Softened**:
- From the classifications:
- **Polystyrene** (thermoplastic) - can be softened on heating.
- **Melamine** (thermosetting) - cannot be softened on heating.
- **Polyesters** (thermoplastic) - can be softened on heating (if we consider common thermoplastic polyesters).
- **Polyethylene** (thermoplastic) - can be softened on heating.
- **Neoprene** (elastomer) - does not soften on heating.

4. **Final Conclusion**:
- The materials that can be repeatedly softened on heating are **Polystyrene** and **Polyethylene**. If we consider common polyesters as thermoplastics, they can also be included.

### Answer:
The correct options are **(i) Polystyrene** and **(iv) Polyethylene**.

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