Why Does Hot Water Break Glass? Uncover the Truth!


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?


The likelihood of a glass cracking when hot water is poured into it is influenced by the temperature difference between the inner and outer layers of the glass. Understanding the physics behind this phenomenon allows for the implementation of precautions to prevent potential cracks.

Thermal Dynamics of Glass:

  1. Glass Composition and Thickness:
    • Material Properties: Glass drinkware comprises a certain thickness of glass material. The thickness is a crucial factor in determining the glass’s resilience to temperature changes.
  2. Heat Absorption and Expansion:
    • Inner Layer’s Interaction with Hot Water: When hot water is poured into a room-temperature glass, the inner layer absorbs heat, causing it to expand. The heat absorption and expansion are intrinsic characteristics of glass.
  3. Conduction and Slow Heat Transfer:
    • Glass as a Poor Conductor: Glass is a poor conductor of heat, leading to slow heat transfer through the glass layers. Consequently, the inner layer becomes significantly hotter than the outer layer due to the delayed conduction.
  4. Pressure Buildup and Cracking:
    • Differential Expansion Pressure: The temperature difference between the inner and outer layers results in the inner layer expanding more than the outer layer. This creates a significant pressure differential, and if the glass is unable to withstand this pressure, it may crack.
  5. Temperature Magnitude Matters:
    • Hot Water Influence: The magnitude of the temperature difference, particularly when hot water is very hot, increases the likelihood of glass cracking.

Precautions to Prevent Cracking:

  1. Slow Pouring:
    • Gradual Introduction of Hot Water: Pour hot water into the glass slowly to mitigate the rapid temperature difference between the inner and outer layers.
  2. Metal Spoon Heat Conduction:
    • Heat Conductance Aid: Place a metal spoon in the glass to conduct away some heat, lowering the water temperature and reducing the thermal stress on the inner glass layer.
  3. Gradual Warming:
    • Stepwise Temperature Adjustment: Warm up the glass gradually by first filling it with a mixture of room temperature and hot water. Gradually increase the proportion of hot water, allowing the glass to adjust to temperature changes.

Glass Thickness as a Factor:

  1. Thin Glass and Crack Resistance:
    • Inverse Relationship: Thinner glass is less likely to crack under the same conditions. This is due to the reduced stress caused by the smaller temperature differential in thinner glass.


Understanding the thermal dynamics of glass during the pouring of hot water unveils the intricacies that may lead to cracking. By implementing precautions such as slow pouring, heat conduction with a metal spoon, and gradual warming, individuals can minimize the temperature difference and reduce the risk of glass cracking. Additionally, opting for thinner glass can be a preventive measure due to its inherent resistance to stress caused by temperature differentials.

Video of glass cracking with hot water:

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