A student added white crystals to a clear liquid. The crystals dissolved in the liquid. After a few seconds, the liquid became quite warm and then turned bright red in color. The first indication that a chemical change was taking place could be observed when

Is when there is an increase in the temperature of the liquid

Explanation:

Increase in temperature of the liquid is an indication that chemical change occur because chemical change occur through process of chemical reaction and it involve when two or more substances combine together to form new substances which is as a result of change in chemical composition. The crystals dissolved in water to form another new solution and it became warm and this indicate a chemical change, new substances are formed as a result of increase in temperature.