In thermodynamics and material science, which of the following is not an intensive property of a substance?

Introduction / Context:
Physical properties of matter are often classified as intensive or extensive. Intensive properties do not depend on the amount of substance present, while extensive properties change when the quantity of material changes. Understanding this difference is important in thermodynamics, chemistry, and engineering. This question asks you to identify which property in the list is not intensive, meaning that it does depend on the total quantity of substance rather than being independent of it.

Given Data / Assumptions:
- Intensive properties remain the same for a small sample and a large sample of the same substance, under identical conditions.
- Extensive properties scale up or down when you change the amount of material.
- The options include weight, density, refractive index, melting point, and specific heat capacity.
- We assume normal laboratory conditions where these properties are defined for a homogeneous sample.

Concept / Approach:
Weight is the force due to gravity on a mass and clearly depends on how much material is present; if you double the amount of substance, the weight doubles. Therefore, weight is an extensive property. In contrast, density is mass per unit volume, refractive index describes the bending of light in a material, melting point is the temperature at which solid becomes liquid, and specific heat capacity is heat needed per unit mass to raise temperature by one degree; all these are defined per unit and do not change simply because you have more or less of the substance. Hence, they are intensive properties. The correct answer is the one property in the list that depends directly on quantity of matter.

Step-by-Step Solution:
Step 1: Recall that intensive properties are independent of the amount of substance, while extensive properties depend on it. Step 2: Examine weight, which is proportional to mass; if you double the sample, weight doubles, so it is extensive. Step 3: Examine density, which is mass divided by volume; for a pure material at given conditions, density remains constant regardless of sample size, so it is intensive. Step 4: Examine refractive index, which characterizes how light travels through a medium; it is an intrinsic property that does not change with the quantity of material, so it is intensive. Step 5: Examine melting point, which is a characteristic temperature of a substance; again, it does not change with sample size under the same pressure, so it is intensive. Step 6: Examine specific heat capacity, which is defined per unit mass; it remains the same for large and small samples of the same material under identical conditions. Step 7: Conclude that weight is the only property listed that is not intensive.

Verification / Alternative check:
A simple check is to imagine cutting a block of material exactly in half. The density of each half remains the same as the original block, the melting point remains the same, and the refractive index remains the same. However, the weight of each half becomes half of the original block. This thought experiment makes it clear that weight depends on the quantity of matter while the other properties do not. Therefore, weight must be the extensive property and the correct answer to the question asking which is not intensive.

Why Other Options Are Wrong:
Density, option B, is independent of sample size for a uniform material and is therefore a classic example of an intensive property.
Refractive index, option C, is determined by the material's structure and does not change with the amount of material under the same conditions, making it intensive.
Melting point, option D, is a characteristic temperature and remains the same for a small piece and a large piece of the same pure substance, so it is intensive.
Specific heat capacity, option E, is defined per unit mass and stays constant for a given substance under specified conditions, so it is intensive as well.

Common Pitfalls:
Students sometimes confuse specific heat capacity with total heat capacity, which can depend on mass. Another pitfall is thinking that any property related to heat or temperature is automatically extensive, when in fact many thermal properties are defined per unit mass or volume and are intensive. Mixing up mass and weight can also cause confusion, but in physics weight is clearly the force due to gravity and grows with the amount of material. Keeping the definitions of intensive and extensive properties clear helps prevent these errors.

Final Answer:
Weight is not an intensive property, because it depends directly on the amount of substance present, whereas properties like density, refractive index, melting point, and specific heat capacity do not change with sample size for a uniform material.