1) The word “effect” is generally a noun. It means to be a result or a consequence of something.
2) The word “affect” is a verb (as in “to affect”). It means to influence.
In scientific writing, these two rules virtually always apply. Go ahead and memorize them. “Effect” is a noun, “affect” is a verb.
So as you’re writing and you’re not sure whether to use the word “effect” or “affect,” just ask yourself, are you trying to use the word to convey a thing (i.e., a noun) or an action (i.e., a verb)?
The addition of salt did not effect/affect the reaction.
In this instance, is the red text trying convey a thing or an action?
In this case, it’s an action. So the word needs to be a verb, which means you should use “affect." You can double-check by making the sure the word “influence” could also be inserted into the sentence without changing its meaning.
The addition of salt did not affect the reaction.
The addition of salt did not influence the reaction.
Bingo. “Affect” is definitely the correct word to use in this instance.
Let’s do another example:
UV light had no effect/affect on the reaction.
In this instance, should the word be a noun or verb? I can tell it’s a noun because of the verb “had” that comes before it, in which case the correct usage would be “effect.”
Still having some trouble?
Try this: could you in theory put either “the” or “an” before the word in question? If so, the word is a noun, and the correct usage is therefore "effect." To do this test, don’t let other words like adjectives in the sentence confuse you. In fact, let’s remove the adjective “no” from the sentence, and then try putting the word “the” or “an” in front of the red text.
UV light had an effect/affect on the reaction.
That sentence is grammatically correct, therefore we know the red text is being used as a noun, hence we would write “effect” in this case. (Let’s go ahead and put the word "effect" back into the original sentence.)
UV light had no effect on the reaction.
Note: In some contexts, largely outside of science, there are some exceptions to this noun/verb rule. For example, the word “effect” can occasionally to be used as a verb, in which case it means to “bring about” or “to cause.” However, using “effect” as a verb tends to sound overly technical and even pretentious, which is why I would suggest you avoid using it this way.
Just stick to the effect = noun and affect = verb rule and you’ll be good to go.