The Basics of Acids and Bases

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The definitions of acids and bases
According to the Arrhenius definition, an acid is a substance that releases hydrogen ions (H⁺) when dissolved in water, and a base is a substance that releases hydroxide ions (OH⁻) when dissolved in water. Under the broader Brønsted-Lowry definition, the substance that donates a hydrogen ion (H⁺) to another substance is defined as the acid, and the one that accepts it as the base, which allows the concept to explain reactions that occur without water as well.
The concept of the pH scale
pH is a scale that indicates whether a solution is acidic or basic (alkaline), ranging from 0 to 14 — the lower the number, the more acidic, and the higher the number, the more basic. A pH of 7, like pure water, is neutral — neither acidic nor basic — and for every difference of 1 in pH value, the concentration of hydrogen ions differs by a factor of 10.
How neutralization reactions work
When an acid and a base meet, the acid's hydrogen ions (H⁺) combine with the base's hydroxide ions (OH⁻) to form water (H₂O), while the remaining ions combine to form a salt — this is called a neutralization reaction. For example, when hydrochloric acid (HCl) reacts with sodium hydroxide (NaOH), water and sodium chloride (NaCl) are produced; neutralizing acidic venom from an insect sting with a basic substance works on the same principle.
Acidic and basic substances in everyday life
Vinegar (acetic acid) is mildly acidic with a pH of about 2.5, while lemon juice, due to its citric acid content, is strongly acidic with a pH of about 2. Soap, by contrast, is formed through the reaction of fatty acids with a base and is mildly basic, with a pH of around 9–10; baking soda (sodium bicarbonate) becomes mildly basic when dissolved in water and is often used to neutralize acidic odors or stains.
The concept of acid rain
Acid rain refers to rain whose acidity has increased to a pH of 5.6 or lower because sulfur dioxide (SO₂) or nitrogen oxides (NOx) in the atmosphere react with rainwater to form sulfuric acid or nitric acid. Even natural rainwater is mildly acidic (about pH 5.6) from reacting with carbon dioxide in the air, but when pollutants increase from burning fossil fuels, even more strongly acidic rain can fall, causing damage to soil, buildings, and plants.
Distinguishing acids from bases using indicators
Indicators such as litmus paper or phenolphthalein change color depending on whether a solution is acidic or basic, making it easy to tell them apart visually. Blue litmus paper turns red in an acidic solution, red litmus paper turns blue in a basic solution, and phenolphthalein turns a reddish pink color only in a basic solution.

Why do acids and bases matter?

The properties of acids and bases are put to use in all corners of everyday life — cooking, cleaning, medicine, and soil management. Understanding that a substance's properties can be gauged on a single scale, pH, lets you logically explain familiar phenomena as well, such as why vinegar and baking soda fizz when mixed.

Take a look at the reaction that happens when acids and bases meet

Neutralization is just one of several types of chemical reactions. If you're curious about other reaction types, such as combination, decomposition, and combustion reactions, take a look at our complete overview of chemical reaction types as well.

Frequently Asked Questions

Does a pH of 7 always mean it's water?

No. A pH of 7 is characteristic of pure water, but other substances can also have a neutral pH. That said, among liquids commonly encountered in everyday life, pure water is the most representative neutral substance.

What's the difference between a strong acid and a weak acid?

A strong acid is one that releases nearly 100% of its hydrogen ions when dissolved in water (such as hydrochloric acid or sulfuric acid), while a weak acid only partially ionizes (such as acetic acid or carbonic acid). At the same concentration, a strong acid will show a lower pH (be more strongly acidic) than a weak acid.