The temperature at which a solid substance melts and becomes a liquid at standard atmospheric pressure is called its ___________.

Introduction / Context:
This question concerns basic physical properties of substances, specifically the temperature at which a solid changes to a liquid. Understanding terms like melting point and boiling point is fundamental in chemistry and physics and helps you describe phase changes and identify substances based on characteristic temperatures.

Given Data / Assumptions:

• The process described is a solid melting to form a liquid.
• It occurs at a defined temperature under atmospheric pressure.
• Options include crystallisation, melting point, evaporation, galvanisation and boiling point.
• We assume standard atmospheric pressure, typically one atmosphere.

Concept / Approach:
The temperature at which a solid and its liquid form coexist in equilibrium at a given pressure is called the melting point. At this temperature, the solid absorbs heat and changes into liquid without any further increase in temperature until the phase change is complete. Crystallisation is the process of forming crystals, evaporation is liquid to gas, galvanisation is coating iron with zinc, and boiling point is the temperature at which liquid turns to gas. Only melting point refers specifically to the temperature where solid becomes liquid.

Step-by-Step Solution:
Step 1: Identify the direction of change: solid to liquid on heating. Step 2: Recall the definition of melting point: it is the temperature at which a solid turns into a liquid at a specified pressure, usually atmospheric pressure. Step 3: Compare this with boiling point, which is the temperature at which a liquid turns into vapour (gas) at atmospheric pressure. Step 4: Crystallisation describes the process of forming a crystalline solid from a liquid or solution, not the specific temperature of melting. Step 5: Evaporation is a gradual surface phenomenon where liquid molecules escape into the gas phase at temperatures below boiling point. Step 6: Galvanisation is a method of protecting iron by coating it with zinc and is not related to phase changes. Step 7: Therefore, the only option that correctly names the temperature where solid becomes liquid is melting point.

Verification / Alternative check:
Examples from daily life support this definition. Ice melts to water at 0 degrees Celsius under standard atmospheric pressure, so 0 degrees Celsius is called the melting point of ice. Similarly, pure iron has a melting point of about 1538 degrees Celsius, which is used in metallurgical processes. Boiling points are reported separately, such as water boiling at 100 degrees Celsius. The consistent use of the term melting point in tables of physical constants confirms that this is the correct term for the temperature in question.

Why Other Options Are Wrong:
Crystallisation refers to the process, not a fixed temperature, and more often describes liquid or solution turning into a solid. Evaporation and boiling point focus on liquid to gas changes, not solid to liquid. Galvanisation is a surface treatment process and has nothing to do with melting. Therefore, none of these terms correctly describe the temperature at which a solid melts to become a liquid at atmospheric pressure.

Common Pitfalls:
Students sometimes confuse melting point and boiling point because both involve phase changes and are listed in data tables. A helpful tip is to remember that melting always refers to solid to liquid, while boiling refers to liquid to gas. Another pitfall is to think of crystallisation as synonymous with freezing, but crystallisation can occur from solutions as well and does not refer to a single temperature in the same precise way that melting point does.

Final Answer:
The temperature at which a solid melts and becomes a liquid at atmospheric pressure is called its melting point.