When analyzing two different isotopes of the same element, what distinguishes one isotope from the other, i. e., what are isotopes of an element?

Which of the following is an example of physical change not a chemical change? a) a log gives off heat and light as it burns. b) a tree stores energy from the sun in its fruit. c) a penny lost in the grass slowly changes color. d) a water pipe freezes and cracks on a cold night.