For the reaction a → b, the rate law is: rate = k × [a]. what is the order of the reaction? second-order first-order zero-order k-order

Answer is: first order reaction.
First-order reaction is a reaction (unimolecular reaction) that rate depends linearly on only one reactant concentration (in this case A).  First order reaction give graph with a negative and linear slope.
k - rate constant, found through experiment.

Evaporation: this is when warmth from the sun causes water from oceans, lakes, streams, ice and soils to rise into the air and turn into water vapour (gas). water vapour droplets join together to make clouds! condensation: this is when water vapour in the air cools down and turns back into liquid water.