Which one of the following substances does not have a sharp and definite melting point?

Introduction / Context:
This question examines your knowledge of the physical properties of crystalline versus amorphous solids. In particular, it asks which substance lacks a sharp melting point, a key diagnostic feature that distinguishes amorphous materials like glass from crystalline solids such as salts and metals.

Given Data / Assumptions:

• Options include bromine, sodium chloride, mercury, glass and ice.
• We are asked to identify the substance that does not have a single, well defined melting point.
• We assume standard atmospheric pressure and ordinary laboratory conditions.
• Crystalline solids typically have sharp melting points, whereas amorphous solids soften over a range.

Concept / Approach:
Crystalline solids have long range order and a highly organised lattice structure, so they undergo a sharp phase transition at a specific temperature, called the melting point. Amorphous solids lack this long range order and gradually soften over a temperature range instead of melting sharply. Glass is a classic amorphous material and shows such behaviour. Liquids like bromine and mercury have well defined freezing points corresponding to their melting points, and ionic crystals like sodium chloride and molecular solids like ice also melt sharply at characteristic temperatures.

Step-by-Step Solution:
1) Sodium chloride is an ionic crystalline solid with a high melting point around 801 degrees Celsius and melts sharply at that temperature.2) Ice is the crystalline form of water with a sharp melting point at 0 degrees Celsius under normal pressure.3) Metals and simple molecular substances such as mercury and bromine have specific freezing or melting points, even though some are liquids at room temperature.4) Glass, on the other hand, is an amorphous solid with disordered molecular arrangement and does not have a fixed melting point.5) When heated, glass gradually softens and flows over a range of temperatures instead of changing suddenly from solid to liquid at one specific value.

Verification / Alternative check:
In materials science, glass is often described as a supercooled liquid or amorphous solid. It exhibits a glass transition temperature region rather than a single melting point. When you heat glass, there is no sharp phase change; instead, viscosity decreases smoothly. Crystalline materials like sodium chloride and ice are used as calibration standards precisely because their melting points are well defined, which confirms that glass is the correct answer here.

Why Other Options Are Wrong:
Bromine: It is a liquid at room temperature but has a definite freezing and melting point when it turns to solid.
Sodium chloride: A typical crystalline salt with a sharp and high melting point.
Mercury: Although a liquid at ordinary conditions, it has a fixed freezing point around minus 39 degrees Celsius, equivalent to a sharp melting point.
Ice: The solid form of water, which melts sharply at 0 degrees Celsius at one atmosphere pressure.

Common Pitfalls:
Students sometimes think that liquids like bromine and mercury have no melting points because they are already liquid. In reality, every pure substance has a well defined temperature at which it changes between solid and liquid under given pressure. Another confusion arises from the everyday idea that glass is a solid like a crystal, but in physics and chemistry it is classified as amorphous and does not have a sharp melting point. Remember that the key clue is the term amorphous versus crystalline.

Final Answer:
The substance that does not possess a sharp, definite melting point is glass.