Cracking Glass with hot water?


Question:

If you pour hot water into a glass (drinkware), what are the chances that the glass (drinkware) will crack? If so, what precautions can you take?

Answer:

Glass (drinkware) is made up of glass (material) with a certain amount of thickness. When you pour hot water into a glass (which is at room temperature), the inner layer of the glass will absorb heat. When a material absorbs heat, it expands. Glass is no exception to that. (i.e. The inner layer of the glass will expand.) Since glass is not a good conductor of heat, the conduction of heat through the glass layer is slow. Hence, the inner layer of the glass is hot while the outer layer is still relatively cold.

This temperature difference will cause the inner layer of the glass to expand significantly more than the outer layer. This difference in the amount of expansion exerts a tremendous amount of pressure on the outer layer. If the glass is unable to hold up against the large pressure, the glass will crack. The magnitude of the temperature difference between the inner and outer layers will determine the likelihood of the glass cracking. If the water is really hot, the glass will definitely crack.

If you want to prevent the glass from cracking, you should aim to reduce the temperature difference between the inner and outer layers. Here are some ways to do that:

  1. Pour the hot water (very) slowly into the glass.
  2. Put a metal spoon in the glass to conduct away some of the heat. The temperature of the water will drop and the inner glass layer will face a slightly cooler body of water.
  3. Warm up the glass gradually by filling it with 50% room temperature water and 50% hot water. Pour it out and replace with 60% room temperature water and 40% hot water. And so on.. (Well, you get the idea)

Note: The thinner the glass, the less likely it is to crack. (Why?) 🙂

 

Video of glass cracking with hot water:

Video of hot glass cracking in cold water:

https://www.youtube.com/watch?v=46G-K_0ZxUY

Since the probability of cracking the glass is only dependent on the temperature difference between the inner and outer layer of the glass, it is possible to crack a hot glass with cold water. (as shown by the video above)

 

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9 thoughts on “Cracking Glass with hot water?”

  1. I put a hot tea in my plastic cup and covered it with a mirror…. After sometime it cracked from the middle….
    please I want to know the physics behind that…

    Reply
    • The heated vapour from the hot tea has transferred heat energy to the side of the mirror closer to the hot tea. As there is a temperature difference between the two sides of the mirror, the expansion will be different on both side. This difference in the expansion will cause the mirror to crack.

      Note: This is the same as the process described in the post above.

      Reply
  2. What about the inside of the glass material, how can we heat it? Submerging the glass in a container of moderately hot water will penetrate heat from both sides of the glass but what about from the inside. I think it will still crack if the glass is too thick.

    Reply
  3. Since the temperature differential between the outside and inside of the glass is what breaks it, assuming that your goal is to not end up with a house full of broken glasses, there are several possible solutions:

    1) warm the glass up slowly first from the inside out, with moderately hot water (may take a few minutes as the heat has to pass slowly through the thick glass)

    or 2) submerge the entire glass in a container of moderately hot water for a couple of minutes (the heat will be penetrating at the same speed from both sides of the glass, so this would be a quicker method)

    Reply
    • What about the inside of the glass material, how can we heat it? Submerging the glass in a container of moderately hot water will penetrate heat from both sides of the glass but what about from the inside. I think it will still crack if the glass is too thick.

      Reply

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