It’s been a long time since we had the opportunity to explain the ‘science of being at home, but this week you have given it a solution: one of the questions you have sent us is if it helps to put a metal teaspoon in a glass cup empty before filling with water or other very hot liquid to prevent the container from cracking.
In general, when their temperature increases, materials expand, that is, their volume also increases. The opposite happens when they cool: they contract and their volume, this time, decreases. This is the reason why glass breaks when it suddenly comes into contact with a liquid at a much higher temperature.
What happens to the glass cup when you add hot water? In its natural state, glass causes the object to have a rather low temperature. “By adding hot water, it dilates, increasing its volume; but this dilation is not uniform“, Explain on its YouTube channel the Mirador Interactive Museum (MIM) dedicated to science, in Chile.
The part that comes into contact with the heat first is the inner layer, so that increases its volume (heats up quickly) before the outside does. This causes a ‘shock of pressures’ and causes the glass to break or crack. The reason is that glass is not a good conductor of heat, that is, “heat cannot easily pass through its structure”, as the MIM explains. Hence, it does not reach the external part as quickly as the internal part.
What we get by introducing a metal teaspoon inside is that, when pouring the hot liquid, this ‘pick up’ part of the temperature, as metal is a material that does conduct heat well, thus reducing the temperature that the glass ‘faces’ and, therefore, the difference in pressure between its internal and external layers.
No pressure difference, no crack: “Now we can continue to pour hot coffee without risk of breakage because the entire cup has had a chance to heat a little (or a lot). In short, putting the metal spoons in the glass slows down the heating jump, preventing the cup from breaking”, concludes the website Zientzia.