I am going to a visit day on the 11th of February too.
I'm also wondering what to expect from the informal interview!
According to the chemistry dept. website they may ask some chemistry related questions; such as your understanding of Avogadro's constant, converting grams of a substance into mols and explaining the difference between acidic and basic solutions.
There's conflicting information though, as some people have said that they will probably only ask you questions about your personal statement??