What do the brackets in chemical equations mean?

what do brackets in chemical equations mean? ca (oh)2

what does the (oh) symbolise?

k2so4 . al2(so4)3 . 24h2o
what does (so4) here symbolise?

In which of these cases are the two wave points considered to be in phase with each other?

The fact that the total amount of energy in a system remains constant is a(n)

What amount of heat is exchanged when 106.2 grams of substance y goes from a liquid at 35 degrees celsius to a solid at the same temperature? melting point of substance y = 35 degrees c; δhvaporization = 3.67 j/mol; δhfusion = 3.30 j/mol. mwsubstance y = 28.22 g/mol. −12.4 j −3.51 x 102 j 1.24 x 101 j 351 j