When hot water is poured into an empty water bottle and the bottle keeps its shape without softening, the plastic of the bottle is most likely which type of material?

Introduction / Context:
This question tests basic knowledge of materials science and polymers, especially how different plastics behave when heated. Common plastic items like bottles are made from various polymer types, and their reaction to heat depends on whether they are thermoplastics or thermosetting plastics. The scenario describes a water bottle that does not soften even when hot water is added, and we must identify the correct category of plastic that explains this behaviour.

Given Data / Assumptions:

• Hot water is placed in an empty bottle, so the plastic is exposed to elevated temperature from the inside.
• The bottle keeps its shape and does not soften or deform.
• The options given include thermoplastic, PVC, polyurethane, and thermosetting.
• We assume that the hot water temperature is high enough to soften thermoplastics if they are near or above their softening point.

Concept / Approach:
Plastics are broadly classified into thermoplastics and thermosetting plastics. Thermoplastics soften on heating and can be remoulded multiple times. Thermosetting plastics, once set by heat or chemical reaction, do not soften again on heating; instead, they tend to retain their shape or may char at very high temperatures. To answer the question, we must recall that a material which maintains its structural rigidity when exposed to heat is characteristic of a thermosetting plastic rather than a typical thermoplastic.

Step-by-Step Solution:
Step 1: Understand that thermoplastics such as polyethylene and polyethylene terephthalate soften gradually when heated close to their softening or glass transition temperatures. Step 2: Realise that if the bottle were made of a thermoplastic, very hot water might cause some softening or deformation, especially if the plastic is thin. Step 3: Recall the definition of thermosetting plastics. These plastics cross link during curing and form a rigid three dimensional structure that does not soften upon reheating. Step 4: The question states that the bottle keeps its shape and does not soften, which is consistent with the behaviour of thermosetting plastics. Step 5: Among the given options, thermosetting is the correct general category that matches this behaviour better than the other specific materials listed.

Verification / Alternative check:
As a conceptual check, one can recall everyday examples. Thermoplastic items like disposable water bottles often deform with very hot liquids, whereas thermosetting plastics such as those used in some electrical fittings or cookware handles remain rigid at higher temperatures. Although in real life many commercial water bottles are made from thermoplastics designed to withstand moderate heat, the typical textbook style reasoning for a material that does not soften under hot water favours the thermosetting category. This consistency between theory and the described behaviour supports the chosen answer.

Why Other Options Are Wrong:

• Thermoplastic: By definition, this type softens when heated sufficiently, so it is not the best match for a bottle that does not soften with hot water.
• PVC: Polyvinyl chloride is a thermoplastic polymer. It can soften or deform under the action of heat, especially in thin sections.
• Polyurethane: Polyurethane can appear in many forms, but the option is not as precise as the general classification of thermosetting plastics requested in the question.

Common Pitfalls:
Learners sometimes confuse real world commercial products with ideal textbook examples. In actual practice, some thick or specially formulated thermoplastics may withstand hot water without obvious deformation. However, exam questions usually rely on simple definitions: thermoplastics soften on heating, while thermosetting plastics remain rigid. Another common error is to focus on brand names or particular polymers rather than the broad behaviour based classification that the question is targeting.

Final Answer:
The water bottle that does not soften when hot water is added is best described as being made from a thermosetting plastic.