Because “affect” and “effect” sound almost the same, they are easily confused.
Affect is usually a verb meaning “to influence” or have an effect on something.
Example: Pressure affects the solubility of gas in oil.
Effect is usually a noun meaning “a result.”
Example: Pressure has a direct effect on gas solubility.
Fun Fact: These two examples illustrate Henry’s Law. Who was Henry?
William Henry (1774-1836) was an English chemist who discovered that the amount of gas that dissolves in a liquid is proportional to the partial pressure of the gas over the liquid (provided no chemical reaction takes place).
Now, back to our discussion of affect vs. effect.
Just because affect is usually a verb and effect is usually a noun, that doesn’t mean that affect can’t be a noun and effect can’t be a verb. It’s just a rare occasion.
Affect as a noun is pronounced with the accent on the first syllable: AFF-ect. It is usually used in psychology to mean “an emotion” or “feeling.” Think: affection.
Example: The change in her affect must have been caused by falling in love.
Effect as a verb means “to bring about” or “to accomplish.”
Example: I ran for the school board in 1992 because I honestly thought I could effect some positive changes in our school district, but I was only one of seven board members.
Now let’s venture into the realm of adjectives.
Affective, the adjective, means “relating to or arousing an emotional reaction” or “acting with a certain feeling.”
Example: His winter melancholy was diagnosed as seasonal affective disorder (SAD).
Effective, the adjective, means “has the right effect,” or “goes into effect.”
Crosswell tomography is an effective way to monitor steam front movement.
The new social media policy is effective immediately.