Hey, so a redox reaction is defined as an electron transfer reaction. As we learn in the HSC course, the two types of redox reactions are oxidation (loss of electrons) and reduction (gain of electrons). These two reactions can't occur by themselves i.e. If oxidation occurs then reduction WILL also occur simultaneously. So what you have to look for to prove something is a redox reaction is to justify the definition I.e. An electron transfer. So you have to show a decrease in the oxidation state for one element (reduction) and an increase in the oxidation state of another element (oxidation). So first off you should write down all the oxidation states of reactants so in Hcl: hydrogen is 1+ and chlorine is 1- and in ca(oh)2: calcium is 2+ and the oxygen is 2- and hydrogen is 1+. In the products for cacl2: calcium is 2+ and chlorine is 1- and in h2o: hydrogen is 1+ and oxygen is 2-. Thus as we can see there is no change in the oxidation states and thus is NOT a redox reaction.