In basic physics, why does a block of ice that is covered with a layer of sawdust not melt quickly even when it is kept in relatively warm surroundings?

Introduction / Context:
When we study basic heat transfer in physics, we often come across examples that show how certain materials slow down the process of heating or cooling. One such example is the observation that ice covered with sawdust melts much more slowly than bare ice kept in the same environment. This question checks your understanding of thermal conductivity and the concept of insulation using a very simple real life situation involving ice and sawdust.

Given Data / Assumptions:

• There is a block or lump of ice.
• The ice is covered with a layer of sawdust from outside.
• The surroundings are warmer than the ice, so heat can potentially flow from the surroundings to the ice.
• We compare how fast the ice melts when it is covered with sawdust and when it is left uncovered.
• Sawdust is assumed to be dry and loosely packed, as in common practical situations.

Concept / Approach:
Heat flows from a hotter region to a colder region mainly by three modes: conduction, convection and radiation. For a solid object in contact with air, conduction through materials touching the object and convection of air around it are very important. Good conductors of heat allow heat to flow quickly, while bad conductors of heat, called insulators, slow down the flow of heat. Sawdust contains a lot of trapped air and is itself a poor conductor of heat. Because of this, a layer of sawdust around the ice acts like a thermal blanket or insulating jacket. It reduces the rate at which heat from the warm surroundings can reach the cold ice, so the ice melts more slowly.

Step-by-Step Solution:
Step 1: Recognise that for ice to melt, it must receive heat from its surroundings until it reaches its melting point and then absorb latent heat of fusion. Step 2: Notice that when ice is left uncovered, the surrounding warm air is in direct contact with the ice surface. Heat can flow relatively quickly by conduction at the surface and by convection of air. Step 3: When sawdust is placed over the ice, it forms a thick, loose layer that contains many air pockets. Both dry wood particles and still air are poor conductors of heat. Step 4: Because sawdust is a bad conductor of heat, it reduces conduction from the warmer environment to the ice, and it also reduces air movement directly over the ice surface, lowering convective heat transfer. Step 5: As a result, the rate at which heat reaches the ice decreases, so the ice melts more slowly when covered with sawdust. Therefore, the correct explanation is that sawdust is a bad conductor of heat and acts as an insulator.

Verification / Alternative check:
You can verify this idea by thinking of similar insulating materials such as wool, thermocol, foam and glass wool. All of them trap air and are used in jackets, flasks and building insulation to slow down heat gain or heat loss. In cold regions, people cover ice blocks with straw, sawdust or other insulating materials to store ice longer. The underlying principle is always the same: low thermal conductivity. No special chemical reaction is happening between sawdust and ice. The effect can also be reversed in summer when ice cream vendors use insulating materials to keep ice cream cold for longer periods. These everyday examples support the explanation that sawdust acts as a thermal insulator.

Why Other Options Are Wrong:
Option a suggests that heat only comes through air touching the ice, but even if direct air contact is reduced, this is not the main scientific reason given in textbooks. The key idea is poor thermal conductivity, not simply blocking air by itself.

Option b says that water is absorbed by sawdust. Even if some meltwater is absorbed, that does not stop heat from reaching the ice or significantly slow melting compared to the effect of insulation.

Option d claims that sawdust is a good conductor of heat, which is the opposite of the truth. A good conductor would make the ice melt faster, not slower.

Common Pitfalls:
A common mistake is to think that only direct contact with air matters for melting, or to believe that any material touching the ice will automatically make it melt slower. In reality, a metal cover would speed up melting because metal is a good conductor of heat. Another pitfall is to assume that the absorption of meltwater is the main reason, which confuses the role of water with the actual physics of heat flow. Always link such questions to the concept of thermal conductivity and remember that materials that trap air and conduct heat poorly, such as sawdust, wool and foam, act as insulators and slow down temperature change.

Final Answer:
Ice covered in sawdust does not melt quickly because sawdust is a bad conductor of heat and acts as an insulating layer, reducing the flow of heat from the warm surroundings to the cold ice.