affect (verb): to have an influence on, alter, cause a change in
Changing the pH of the solution may affect [alter for better or worse] the result.
affect (noun): observable feeling or emotion, or subjective experience of emotion associated with a thought
Most subjects in the trial showed a happy affect in response to the photos of the puppies.
effect (verb): to bring about, cause, produce, result in
Changing the pH of the solution can effect [bring about] a colour change.
effect (noun): the result of a change
The effect of changing the pH of the solution is ...