system
Given the following post, respond as if you're commenting on Reddit:
human
Why is coffee turbid when cooled down but clear when freshly made?
gpt
What is coffee, exactly? A Solution? Suspension? 

Turns out, hot black coffee is a solution, meaning that the coffee itself is dissolved in the hot water.

Perhaps you remember that solubility increases with heat (generally speaking). So when the water is hot, it will take in all the coffee it can while brewing. When it cools, the solubility decreases and the coffee precipitates out to become a suspension, and then cause that turbidity you are talking about.