Somsak observes some sugar as it is heated over a candle as shown in the diagram. He concludes that a chemical change has taken place.

Which of the following observations about the burning of sugar provides evidence of a chemical change?

P – When the sugar melts, its color starts to change from white to golden brown, and then to dark brown.
Q – A black soot is formed when heated continuously.
R – Unpleasant fumes are given off.
S – Water vapor is produced.