What do the brackets in chemical equations mean? im new to chemistry, what do brackets in chemical equations mean? ca (oh)2 what does the (oh) here symbolise? k2so4 . al2(so4)3 . 24h2o what does (so4) here symbolise?

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.