Why does a tea pot have a tiny hole on its cap

A tea pot is a vessel with a narrow spout coming from the lower portion of the pot which holds and keeps hot steaming tea till it is served into cups.

 

Normally the tea pots are made up of insulating materials like porcelain so that while the heat of the tea is not lost to the surrounding, the pot can be handled for pouring the tea when desired.

The pot also has a top cap which is useful for pouring the hot tea prepared in another utensil. This cap is made to size which fairly closely matches the mouth of the pot; so, when the cap is placed on the mouth of the pot, it leaves hardly any gap, making a narrow line contact with the pot.

However, as the tea in the pot is hot and steaming, the steam condenses on the relatively cooler portions of the inside of the pot, which happens to be the place near the cap.

Thus the line contact gap gets filled with the condensed water which sticks on there due to surface tension and viscosity of water. It makes a sealing, soft though.

If tea is now poured from the spout, a partial low pressure would be created inside the pot until a big enough bubble of air forms at the tip of the spout and travels into the pot. Thus, the tea gets poured in jerky draughts rather than as a continuous stream.

This desired continuous stream mode of pouring can be achieved if the low pressure inside the pot is not created.

This precisely happens by providing the cap with a hole which allows the required air to fill the space of the poured amount of tea.

The hole, therefore, enables the tea to be poured in a continuous stream, rather than in a jerky flow into the cups.