Acid

An acid is a special kind of molecule or tiny particle that can give away a proton, which is a hydrogen cation, H⁺. This type of acid is called a Brønsted–Lowry acid. Another way acids can work is by forming a covalent bond with an electron pair; these are known as Lewis acids.

Acids in water, called aqueous solutions, have some easy-to-spot features. They often taste sour, can turn blue litmus paper red, and react with bases and certain metals like calcium to make salts. The word acid comes from the Latin word acidus, meaning "sour." A solution with acid has a pH lower than 7, which tells us how strong the acid is.

Common examples of acids include hydrochloric acid, found in your stomach to help digestion; acetic acid, which makes vinegar sour; sulfuric acid, used in car batteries; and citric acid, found in citrus fruits. Some strong acids can be dangerous because they can break down other materials, but not all acids are like this.

Definitions and concepts

Acids are special kinds of chemicals that can change how other substances behave. Most of the acids we see every day, like those in vinegar or lemon juice, work because they can mix with water. These acids can give away tiny parts called protons, which scientists write as H⁺. When an acid gives away a proton, it makes the water more sour or reactive.

There are different ways to think about acids. One way, called the Brønsted–Lowry definition, says that an acid is something that can give away a proton. For example, when vinegar (which has acetic acid) mixes with water, it gives away a proton and makes the water a little sour. This idea works even when acids mix with things other than water, like ammonia.

Another way to think about acids is the Arrhenius definition. It says that an acid makes more H⁺ parts when it mixes with water. These H⁺ parts actually join with water to make a group called a hydronium ion, H₃O⁺. For example, when hydrochloric acid (found in some cleaning supplies) mixes with water, it makes lots of these hydronium ions, which makes the water sour and reactive.

Main article: Brønsted–Lowry acid–base theory

Main article: Lewis acids and bases

Dissociation and equilibrium

Acids can break apart in water, releasing a particle called a proton (H⁺) and forming another particle called a conjugate base (A⁻). This process is called protolysis. The balance between the acid and its conjugate base in a solution is called equilibrium.

Scientists use a special number called the acid dissociation constant, written as K_a, to measure how much an acid breaks apart. A bigger K_a means the acid is stronger because it breaks apart more. To make these numbers easier to work with, scientists sometimes use p_K__a, which is just a way of flipping and simplifying the K_a value. Smaller p_K__a values mean stronger acids.

Nomenclature

Acids are named based on the part of them that is left after they give away a hydrogen atom. In the old way of naming, we drop part of the name and add a new ending. For example, if an acid has something called "chloride" in it, we call it hydrochloric acid.

There is also a newer way to name acids used by scientists. In this way, we just add the word "aqueous" to the name of the compound. So, for hydrogen chloride mixed with water, the name becomes aqueous hydrogen chloride.

Acid strength

The strength of an acid describes how easily it can give away a tiny part called a proton. A strong acid easily gives away its proton when mixed with water, turning completely into water-loving pieces. Examples of strong acids include hydrochloric acid, hydroiodic acid, hydrobromic acid, perchloric acid, nitric acid, and sulfuric acid. Weak acids only partly give away their protons, so both the acid and its changed form stay in the mixture.

Strong acids have a bigger number called an acid dissociation constant, shown as K_a, and a smaller p_K__a value than weaker acids. Some special very strong acids, known as superacids, are even stronger than common strong acids.

Lewis acid strength in non-aqueous solutions

Lewis acids can be organized using a special model called the ECW model. This model helps us understand how strong a Lewis acid is by looking at how it behaves with different substances. Scientists use special charts called C-B plots to compare the strength of Lewis acids. To really know how strong a Lewis acid is, we need to look at more than one property. Some scientists focus on two ideas: hardness and strength, while others use a method that looks at both electrostatic and covalent properties.

Chemical characteristics

Acids are special kinds of substances that can give away particles called protons. These protons are tiny parts of atoms called hydrogen ions (H+). When an acid gives away a proton, it changes into another form, called a conjugate base.

Some acids can give away just one proton per molecule. These are called monoprotic acids. Examples include hydrochloric acid (HCl) and acetic acid (CH₃COOH). Other acids can give away more than one proton per molecule. These are called polyprotic acids. For example, sulfuric acid (H₂SO₄) can give away two protons, and phosphoric acid (H₃PO₄) can give away three protons.

When an acid meets a base, they can neutralize each other, forming a salt and water. For example, hydrochloric acid and sodium hydroxide mix to make sodium chloride (table salt) and water. This process is important in many chemical reactions and measurements.

Main article: Henderson–Hasselbalch equation

Titration

To find out how strong an acid is in water, scientists use a method called acid–base titration. They add a strong base, like sodium hydroxide (NaOH), bit by bit to the acid. As they add the base, they watch for a color change in a special dye called an indicator. This color change tells them when the acid has been completely balanced out by the base.

In a graph showing this process, one line goes up and down with the amount of base added, and the other shows how sour or bitter the solution is (its pH). As more base is added, the solution becomes less sour and more bitter. For some acids that can give off more than one sour part at a time, there are special points where the acid is just halfway balanced, and other points where it is fully balanced. These points help scientists understand exactly how strong the acid is.

Applications of acids

Acids are very important in many parts of our world. In industry, they help make things like fertilizers, detergents, and batteries. One of the most common acids is sulfuric acid, which is used to clean metals and help create many products.

We also find acids in foods we eat every day. For example, citric acid gives lemons and oranges their sour taste, and tartaric acid is in tamarind. Acids like acetic acid, which is found in vinegar, help preserve foods and give them flavor. Our bodies also need acids — the hydrochloric acid in our stomach helps us digest food, and other acids are important for building proteins and keeping our body’s balance just right.

Biological occurrence

Many important molecules in living things are acids. For example, DNA and RNA are types of nucleic acids that carry the instructions for making proteins and other parts of our bodies. These molecules have special parts called phosphate groups that act as acids.

Another group of acids is called fatty acids. They help make up the outer layer of cells, called cell membranes, which keep everything inside a cell safe. In our stomachs, we also have an acid called hydrochloric acid that helps break down food. Some animals, like ants, make acids to defend themselves.

Common acids

Acids are special kinds of chemicals that can give away a tiny part called a proton or connect to certain other chemicals. There are many different types of acids.

Some common acids include hydrofluoric acid, hydrochloric acid, hydrobromic acid, and hydroiodic acid. Others are sulfuric acid, nitric acid, and phosphoric acid. You might also know acetic acid, which is found in vinegar, and citric acid, which is in many fruits and sodas. There are many more acids that scientists study and use in different ways.