Yes, it is possible for a substance to increase its volume when left open, although this is not directly related to volatility. Volatility refers to the tendency of a substance to evaporate or vaporize at a given temperature and pressure.
The opposite of volatility is stability, which generally implies that a substance does not readily evaporate or change its physical state. However, the opposite of volatility does not necessarily mean that a substance will increase its volume when left open. A substance can remain stable and maintain its volume or undergo other physical changes.
If you're looking for an example of a substance that can increase its volume when left open, one possibility is certain chemical reactions that produce gases as byproducts. When these reactions occur, the gases generated can occupy a larger volume than the original substance. For instance, if you mix baking soda (sodium bicarbonate) and vinegar (acetic acid), it produces carbon dioxide gas, which will cause an increase in volume if the reaction occurs in an open container.
It's important to note that substances can exhibit different behaviors depending on their properties, so the opposite of volatility, in terms of volume change, will vary from case to case.