When two substances are mixed and light is produced is it a chemical or physical change?

Is 343 in the sequence -5, -2, 1, 4, 7

Which best describes the activation energy of a chemical reaction? a. the combined energy of all the reactants b. the amount of energy required for a reaction to occur c. the difference in energy between products and reactants d. the potential energy stored in the bonds of reactants and products

What percentage of energy used in the u.s is produced from fossil fuels