List two observations that could be evidence of a chemical reaction.

1) Smell or odour

2) Colour change

Explanation:

1) Smell or odour

Example: Add HCl to a FeS solution. There will be a  strong odor of H2S

2) Colour change

Example : NaOH and KMnO4 solution to a river water sample. There will be  a colour change to brown.

in 1783 cavendish was working with his hydrogen gas and found that when he burned it and condensed the vapor it was water. this was very significant since it disproved the greek theory of elements completely since water was not a simple substance but the product of two gases. it was lavoisier who hearing of the experiment gave cavendish's gas the name hydrogen ("water-producer") and added that hydrogen burned by combining with oxygen and that water was a hydrogen-oxygen combination.

explanation: