Two different liquids are mixed together. which observation indicates that an exothermic chemical reaction has taken place?
a. an increase in volume
b. a decrease in volume
c. a rise in temperature
d. a drop in temperature

The  observation  which indicate  that an exothermic reaction  has taken place when two different  liquid are mixed  together is  a rise  in temperature  (answer C)

Explanation

 Exothermic reaction  is a   chemical reaction which releases  energy  in form of heat  or light.

  Examples  of   exothermic reaction is  a combustion reaction,  for instance combustion of methane which occurs as below.

CH4 +O2→  CO2 + 2H2O + heat

other  examples  of exothermic reaction is   neutralization reaction,  rusting of iron  and reaction between water and calcium  chloride.

answers: to make sure that all of the broken pieces of tablet were added to the water, to allow for fair comparison of all three trials, with different particle sizes to avoid experimental error