There are quite a few ways to tell an acid from a base with an experiment. The simplest is to use color indicators, like litmus paper. Litmus paper has a mixture of different dyes in it. (Interestingly, the dyes are often taken from lichens.) The paper is then placed in a solution, and will turn blue if it is basic or red if it is acidic. If it does not change color, it is fairly neutral (pH in the 5-8 range). Here, you could combine this with a titration. Titration is where you slowly add base or acid until a solution becomes basic or acidic. If you have a solution at pH 6, it will take very little acid to get to pH 5 and turn the paper red. Therefore, the solution will be an acid. If it is pH 8 (but the paper still is white), it would take a lot of acid to get to pH 5 and turn the paper red. That way, you can easily tell if something is an acid or a base.

There are many different ways to tell if something is acidic or basic. However, some of the simplest involve the use of dyes that change color when something is acidic or basic. One of these simple tools is called litmus paper, which is paper infused with several dyes. These dyes change color to indicate whether something is acidic or basic. Generally, acids make litmus paper turn red, and bases make litmus paper turn blue.

One of the best known ways to distinguish between an acid and a base is using litmus paper. Litmus contains chemicals from a group of organisms called a lichen. These chemicals reflect different colors of light in acidic and basic solutions. Therefore, when a solution is added to a litmus strip, the strip will change color based on how basic or acidic the solution is. A more modern way to distinguish whether a solution is acidic or basic is using a pH meter. A pH meter gives a number based on the electrical properties of a solution and if it’s below 7, the solution is acidic, and if it’s above 7 then it’s basic.

Acids react with bases. Placing an acid into contact with a known base will cause a chemical reaction, as will putting a base in contact with a known acid. For example, sodium bicarbonate (baking soda) is a known base that produces carbon dioxide gas when it reacts with an acid. Acetic acid (vinegar) is a known acid. Putting the two together makes bubbles of carbon dioxide. The same will be true if you combine sodium bicarbonate with any other acid.

(Note - not all bases make carbon dioxide when combined with acid - this is a peculiar feature of sodium bicarbonate).

The safest test is to add a few drops of you unknown solution to a pile of baking soda (sodium bicarbonate, NaHCO₃). When the baking soda reacts with an acid, it will fizz because carbon dioxide is created via the following reactions:

NaHCO₃ + H⁺ → Na⁺ + H₂CO₃ and then H₂CO₃ → H₂O + CO₂(g)

When baking soda reacts with a base, you will not see a strong fizz:

NaHCO₃ + ^-OH → NaCO₃^- + H₂O

Another test: See if heat is produced when your unknown is mixed with a known strong acid or base. The neutralization reaction (when strong acid reacts with a strong base) is very exothermic, meaning it will release heat to the environment. You should be careful only to combine small amounts in a well-ventilated space and choose your known acid/base to avoid combinations that are dangerous, however. For example, perchlorate (a base found in bleach) reacts with both nitric acid and ammonia (a base) to produce toxic chlorine gas. Dilute hydrochloric acid (HCl) or sodium hydroxide (NaOH) are safer choices as your known reactants.

Finally, you can use an indicating dye that changes color in the presence of an acid or base. These can be found as a solution or coated on strips of paper for easy dipping (called litmus paper). “Universal” litmus paper (which can be found in pool supply stores and some hardware store) has a blend of dyes that will turn a unique color when exposed to a certain level of basic/acidic solution.