How can I represent a question in first-order logic?

Question

How can I represent a question such as:

Which colour turns into white when it is under the sun?

in first-order logic? I think that if it was the following sentence:

There is a colour that turns into white when it is under the sun.

I would represent it as:

$(\exists x) IsColour(x) \wedge TurnsInto(x,White) \wedge UnderTheSun(x)$

Is there a different approach because of the question? Thank you

Εdit: The original question is:

Which liquid boils when its temperature is 100°C?

I wanted to have a similar example so that I could do my homework. Thank you for your time.

Answer 1

I'm going to try to answer the actual question, that you finally added in comments:

Which liquid boils when its temperature is 100°C?"

First of all, "which" cannot be asked in first order logic. In general, first order logic cannot ask questions, although there is an implicit question for every sentence: "Is this true or false?" But you can definitely only ask "true/false" questions.

The best you can do is give an expression $S(x)$ in terms of a variable $x$ and then ask which $x$ satisfies it. The sentence:

$$\exists x(S(x))$$ simply asserts some value satisfies $S(x)$, which is hardly what we want.

I really think it is a broken question, and the best answer is that you can't state this in first order logic, but if you have a proposition $P(x,y)$ which means "$x$ is a liquid, $y$is a temperature, and $x$ boils at temperature $y$," then you can write:

$$P(x,100°C)$$

is the best you can do. And that is a very odd logic, because first order logic likes the universe to be of one "type."