Acids
An acid is a substance with a pH of less than 7.
![seed driving a car that won't go faster than 7 miles per hour](/images/user/base/Chemistry/5. Acids, Alkali, Bases and Salts/2.5.4 Acids 7 or less on ph image Resized.jpg)
A seed (acid) is driving a car that won't go any faster than 7MpH (acids are less than 7 on the PH scale) because it is so small it can't push the accelerator pedal down any further (so an acid pH is from 1 to 7).