Why would it be difficult to test a cup of coffee with universal indicator?

It would be difficult to test a cup of coffee with universal indicator for a few reasons:

* Coffee is acidic. Universal indicator changes color based on the pH of a solution. Coffee is acidic, meaning it has a pH lower than 7. This means the indicator will likely turn red or orange, which doesn't provide much specific information about the coffee's acidity.

* Coffee is colored. The brown color of coffee will interfere with the color changes of the universal indicator, making it difficult to determine the exact color shift and thus the pH.

* Coffee contains other compounds. Coffee isn't just a simple acidic solution. It contains numerous other compounds that can affect the indicator's reaction, making the results less reliable.

In summary: While you could technically use universal indicator with coffee, it wouldn't give you a clear or accurate reading due to the complex nature of coffee and its acidic nature. It's not the ideal tool for testing the pH of coffee.