Final answer:
Brewing coffee with water that is too cool causes an incomplete extraction, leading to a weak or sour-tasting coffee due to an imbalance of flavor compounds extracted from the beans.
Step-by-step explanation:
When brewing coffee, using water that is too cool causes extraction to happen incompletely. Ideal coffee brewing temperatures are typically between 195°F to 205°F (90°C to 96°C). When the water temperature is below this range, it does not have enough energy to fully dissolve all of the desirable compounds from the coffee grounds. As a result, cooler water leads to an incomplete extraction of the coffee, which can result in under-extracted coffee that might taste weak or sour due to the lack of balance in flavors that are extracted from the coffee beans.