Ice in blanket

Classical Mechanics Level 1

When we take a blanket at room temperature and wrap it around ourselves, we become warmer than if we are not covered by the blanket.

When we take a blanket at room temperature and wrap it around an ice cube, would the ice melt faster or slower than if it was not covered by the blanket?

Details and Assumptions:

Assume that the ice does not touch the blanket due to an air gap between the blanket and the ice.
The initial temperatures of the blanket and the room are the same.

The ice will melt faster in the blanket The ice will melt slower in the blanket

7 solutions

Kev S
Feb 16, 2017

Due to the air-trapping characteristic of a blanket, it acts as an insulator that separates outside and inside.

This is the reason we use a blanket on a cold day. Blanket traps air and that air inside the blanket are warmed by our body. The air inside can't escape so it stays inside the blanket, so, after a while, we feel warmer in the blanket.

The same apply here. The blanket traps cold air and prevents ice cube from absorbing heat energy from the surrounding. So, the ice cube wrapped in the blanket will melt slower .

Yes, the blanket being a bad conductor of heat slows the inflow of surrounding heat energy into the blanket. Hence, the ice cube won't get the heat energy readily and will melt slowly.

Rohit Gupta - 4 years, 3 months ago

upvote me! haha

Kev S - 4 years, 3 months ago

Done and thanks for the solution.

Rohit Gupta - 4 years, 3 months ago

@Rohit Gupta – no need, your question is interesting! It made me think so so long, haha

Kev S - 4 years, 3 months ago

@Kev S – I am glad you find my problem interesting. :)

Rohit Gupta - 4 years, 3 months ago

I believe that there are too many unspecified variables in this problem. The assumption that the blanket traps cold air depends, in part, on the volume of blanket. However, the heat capacity of the blanket material is greater than the heat capacity of air, and the total heat content depends on the volume as well. Also, it is well known that wet insulation loses its R value because water is a better conductor of heat. Also, comparing the effectiveness of a blanket at trapping heat from a body which is generating heat to a blanket wrapping an ice cube is suspect. After all, people have demonstrated the fact that hot water freezes faster than cold water, even though it must lose more heat. I suspect that with the right combination of parameters, either answer could be correct.

Tom Capizzi - 4 years, 3 months ago

Blanket traps air, it will become either cold or warm depending on what the blanket is wrapped around.

The case of hot water freezing faster than the cold one is altogether a different case as we are taking the identical ice cubes, one wrapped and the other is opened to the atmosphere.

Moreover, if due to melting the blanket becomes wet then the water will spread and evaporate which will further reduce the temperature.

Rohit Gupta - 4 years, 3 months ago

No blanket is 100% effective at trapping air, and even if it were, it could not be 100% effective at trapping energy. The effectiveness is quantified by a material's R-value, and even good insulation is compromised by wetness. This could be compensated for by putting the ice cube in a glass, but this is a variable that is not specified, hence my original point.

Tom Capizzi - 4 years, 3 months ago

@Tom Capizzi – The blanket provides better insulation when it is dry, so the ice in the blanket will melt slower than ice without blanket initially and faster when it becomes wet. It would be difficult to provide insulation without getting the blanket wet.

Thermocol could be a better option since it has a higher R value and absorbs less water. However it does not degrade easily, which fills up landfills very quickly.

Pranshu Gaba - 4 years, 3 months ago

@Pranshu Gaba – I do not agree with this. Even if the blanket gets wet ice won't melt faster.

This is due to two reasons. First, the conduction is a slower process than the convection.

Second, for the transfer of heat across the blanket a temperature difference must set up. So, first, the temperature of the outer layer of water( which is exposed to the surrounding) will increase. As the specific capacity of water is double than ice, its temperature will rise very slowly. Also, as its temperature rises it will tend to evaporate that will further cool the blanket.

Moreover, consider a situation, a hot bowl to be picked up by a dry cloth or by a wet cloth. I think the wet cloth will take much more time to get warm and you will be able to hold the bowl for a longer duration.

Rohit Gupta - 4 years, 3 months ago

Is this answer also valid if outside temperature is -10 Celsius? Basically below freezing

Salil Divekar - 4 years, 3 months ago

If the outside temperature is below the freezing temperature then none of the ice will melt. Therefore, to assume such a temperature for this problem is not correct.

Rohit Gupta - 4 years, 3 months ago

Initially the ice will melt a bit faster from contact with the room temperature blanket, which will become damp as well, but as the part of the blanket in contact with the ice cools, heat transfer will slow down the ice cube will melt slower. I would have liked "Faster initially and then slower".

Ed Sirett - 4 years, 3 months ago

Can you elaborate on why should it melt faster initially?

Rohit Gupta - 4 years, 3 months ago

The surface (and nearby to the surface ) of the blanket (at say 20 C) will have to loose a certain amount of thermal energy to the ice cube, so that those parts of the blanket cool to 0 C. The amount of energy needed will depend on the density, heat capacity, thermal conductivity of the blanket and the effect on those quantities by how much water is absorbed by the blanket in contact with the cube. I estimate that initially the blanket at 20 C will be much better at melting the cube than air at 20 C even allowing for the removal of convection effects in the air. Probably the most effective blanket would be a solid block of expanded polystyrene with a small drain hole for melt water, having low density, low conductivity and water proof.

Ed Sirett - 4 years, 3 months ago

Hah! Our bodies generate the heat, not the blanket. The blanket traps the heat.

Hunter Edwards - 4 years, 2 months ago

Daniel Saddawi-Konefka
Feb 19, 2017

We must assume there is some air circulation which causes heat transfer due to convection. The blanket decreases convective transfer. Therefore, the ice will melt more slowly.

What if you cover yourself with a bed sheet instead of a blanket/quilt. Bedsheet will also trap the air and stop the convection but you won't feel much warm. I think as the blanket is a bad conductor of heat and it does not allow the heat to escape through conduction (as well as convection), therefore, the heat is trapped and you feel warm.

Both the factors of convection and conduction should be responsible for making one warm in the blanket.

Rohit Gupta - 4 years, 3 months ago

Naren Bhandari
Feb 23, 2017

Its blanket that makes slow temperature differenence between surrounding and inside of blanket. (ice cube) If the same ice cube is kept in plams the rate of melting is greater as some heat is transfered from our body to ice but in case of blanket neither it radiate heat nor it let absorbed to ice cubes

"Blanket traps heat energy" would be more accurate when talking about humans wearing blanket to remain warm. In case of ice cubes, it would be more appropriate to say that blanket reduces heat absorbed by the ice cube.

Pranshu Gaba - 4 years, 3 months ago

ohh! I see, if it traps the heat energy than it will be absorbing it as well and melting process will be faster.

Naren Bhandari - 4 years, 3 months ago

Abhiram Bondada
Apr 6, 2017

A blanket doesnt allow heat transfer at a fast rate as it is an insulator

Now if we cover ourselves with blanket the heat produced by our body doesnt escape so we feel better

But ice doesnt produce heat as it doesnt have muscles and stuff we have to produce heat ( sorry ice ) so no heat is produced and less heat is transmitted by blanket from outside so it doesnt melt

Odinrawo201 Rom
Mar 20, 2017

The blanket makes us warmer because it reflects our own body heat back at us. if an ice cube was wrapped in a blanket, it would be giving if waves of cold air instead of warm, which the blanket would reflect back at the ice.

Peter Watkins
Mar 4, 2017

To melt ice, you need the ice to absorb heat from outside. When the blanket is on a cold person, your body is still generating heat that is trapped by the blanket while a cold ice cube is being deprived of the heat it needs to melt faster than rif uncovered.

Yes, I agree with you. Blanket works as an insulation and does not let the heat from the surrounding to enter. It also cuts the convection of air as the air is trapped.

Rohit Gupta - 4 years, 3 months ago

Ozanoz44 Uysal
Feb 19, 2017

Even we think blanket doesn't have any resistance, it would need more energy to melt down an ice with blanket.

Can you elaborate more on what you want to say here? It will help others understand your solution better.

Rohit Gupta - 4 years, 3 months ago

I agree with Rohit, I believed that there are some missing gaps.

What resistance are you referring to? How do you know that it follows that it needs more energy to melt the ice?

Pi Han Goh - 4 years, 3 months ago

Ice in blanket

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