Edit: Thanks so much for the silver kind stranger!
Edit 2: And the others! You’ve made my day! Glad I dropped my biscuit in my tea and decided I needed answers
In: Chemistry
By hot liquid, you probably mean hot water. Water dissolves simple carbohydrates (sucrose being the most common sugar in a cookie; sucrose is a disaccharide, formed of two different sugar molecules joined together in a solid, covalent bond), which help the cookie stick together. Hot water dissolves them more readily (quicker) and at higher concentration (more sugar dissolved in a given volume of water) than does cold water. Keep in mind that this is not melting, which is a common misconception. Sugars do not melt in water – they dissolve. People think they melt because a higher temperature allows the sugar to dissolve faster and in greater amounts. Again, this is not melting.
Thinking of non-aqueous liquids, hot vegetable oil would probably cause a cookie to fall apart faster than cold vegetable oil as well, but because in this case the oil is dissolving the fats, which would be congealed and sticky in the cookie, helping it hold together as well. It’s a general rule that the solubility (amount you can dissolve, per volume of liquid) of a solid increases in a liquid as the temperature is increased, although some solids are not soluble in certain liquids at all. For example, table salt is not soluble in vegetable oil or mineral oil at all.
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