Final answer:
An increase in temperature from a chemical reaction indicates that energy was released (exothermic), related to the rearrangement of chemical bonds and the resulting transfer of energy.
Step-by-step explanation:
If a test tube increases in temperature from a chemical reaction then energy was released (exothermic). This increase in temperature is evidence that the chemical reaction is releasing heat into its surroundings, indicating that the reaction is exothermic. Examples of exothermic reactions include combustion, such as the burning of paper, where heat is clearly being produced and released into the environment.
An important point to understand is that during a chemical reaction, chemical bonds are broken and new ones are formed, and this bond rearrangement involves the transfer of energy. If the total energy of the products' bonds is less than the energy of the reactants' bonds, the excess energy is released as heat, leading to an increase in temperature. Conversely, if more energy is required to break the bonds of the reactants than is released when new bonds are formed in the products, the reaction absorbs heat from the surroundings, thus making it endothermic and causing a decrease in temperature.
The law of conservation of energy stipulates that energy cannot be created or destroyed, so the heat generated in an exothermic reaction is simply the transformation of the potential energy stored in chemical bonds into thermal energy.