The question asked, how can you tell the difference between acids and bases, is written for the basic understanding of acids and bases. Describing in detail about chemical bonding, molecular structures and the forces acting upon those structures, it is best left for another article. A reader having the ability to understand such details has acquired a higher knowledge than this article provides. Lets take a look at what an acid and a base is.
Our world is governed by laws and forces of nature. Our specie forever, will quest to discover and learn what they are. Science provides us with the knowledge and means to make these discoveries, so that we may better understand who and what we are. How science expresses these laws of this physical world, is by the use of mathematics.
There are physical forces that act upon us in everything we do. Sir Issac Newton (born January 4, 1643 – died March 31, 1727) is considered to be one of the most influential scholar in history. His fascination for how this world works, has led him into laying the ground work for classical mechanics. Classical mechanics is one of the two major sub-fields of study in the science of mechanics, which is concerned with the set of physical laws governing and mathematically describing the motions of bodies and aggregates of bodies, geometrically distributed within a certain boundary under the action of a system of forces. The other sub-field is quantum mechanics.
Quantum mechanics (QM) is a set of principles describing the physical reality at the atomic level of matter (molecules and atoms) and the subatomic (electrons, protons, and even smaller particles). A basic knowledge of the principals of matter (at least pertaining to the question), is required to understand the tools used to measure, and tell the difference between acids and bases. It is this principal of particles, which determine what an acid and what is a base.
A molecule as defined, is an electrically neutral group of at least two atoms in a definite arrangement held together by very strong (covalent) chemical bonds. The atom is a basic unit of matter consisting of a dense, central nucleus surrounded by negatively charged electrons. The atomic nucleus contains a mix of positively charged protons and electrically neutral neutrons. The electrons of an atom are bound to the nucleus by the electromagnetic force. Likewise, a group of atoms can remain bound to each other, forming a molecule. An acid is a compound when dissolved in water, will give the solution a higher hydrogen ion activity greater than that of pure water. A base is a compound that when dissolved in water, will give the solution a hydrogen ion activity lesser than that of pure water.
To express hydrogen ion activity the letters pH are used. To put it simply, pH is the strength of the hydrogen ion activity that binds the hydrogen molecule to compounds. The pH of pure water is neutral and has a pH of 7, where a solution with a pH lower than 7 is an acid, a solution with a pH higher than 7 a base. Any pH higher than 12 or lower than 2 is hazardous to humans and is classified as a strong acid or a strong base and is a hazardous and is generally regulated.
To identify or measure pH , an instrument or pH meter is used. Some cases for immediate testing by emergency responders and safety personnel, a pH test strip or Litmus Paper is used. Note: pH paper is only good for identification and not measurement.
In conclusion, to tell or determine an acid or base, a pH meter is used. A pH meter works by measuring the hydrogen ion activity through an electrode placed into the solution. Test paper or Litmus is measures the pH or hydrogen ion activity by having several chemicals or reagents soaked onto the paper. What color forms is particular from a specific reagent. Only one reagent will turn color on contact with a pH specific for that reagent.