Acid

Some things taste sour.

These are called acids. You can find them in fruit. They are also in your tummy. They help you digest food.

Acids can do many things. They can turn blue paper red. They can also react with some metals. This can make new things called salts.

Some acids are very strong. They can eat through things. This is called being corrosive.

We find acids in many places. Vinegar is a type of acid. Citrus fruits have acid too. Some acid is used in car batteries.

Acids work by moving tiny parts. These tiny parts are called protons. Moving these parts is how they react.

What is an acid? The word comes from a Latin word meaning "sour." Many acids have a sour taste.

You can find acids in many places. Vinegar is a weak acid. Citrus fruits also have acid. Some acids are used in car batteries. Strong acids can be corrosive. This means they can eat through things.

Scientists use different ways to define an acid. One way is the Arrhenius definition. This way looks at acids in water. An Arrhenius acid adds hydrogen ions to water. These ions are tiny parts called protons.

When there are many protons, the pH is low. A pH less than 7 means it is an acid.

Another way is the Brønsted–Lowry definition. This way says an acid is a proton donor. A donor is something that gives a part away. The acid gives a proton to a base.

A third way is the Lewis definition. A Lewis acid is an electron pair acceptor. This means it takes a pair of electrons from something else. This happens even if no protons move.

An acid is a special kind of molecule or ion. The word itself comes from a Latin word meaning "sour." You might notice this sour taste in many things. For example, citric acid is found in citrus fruits. Vinegar is a weak acid called acetic acid. Other acids are much stronger and can be corrosive. This means they can eat through materials.

Some acids are used in car batteries. Others, like hydrochloric acid, help your stomach digest food.

Scientists have different ways to explain how acids work. One way is the Arrhenius definition. This focuses on what happens when an acid is in water. An Arrhenius acid increases the amount of hydrogen ions in the water. These tiny parts are often called protons. In water, these protons join with water molecules to form hydronium ions.

Another way is the Brønsted–Lowry definition. This says an acid is a proton donor. This means the acid gives a proton away to a base.

There is also a third way to look at acids. This is called the Lewis definition. A Lewis acid is an electron pair acceptor. This means it takes a pair of electrons from another substance. This can happen even if no protons are moved at all.

While most acids people talk about are Brønsted acids, Lewis acids are still very important. For example, boron trifluoride is a Lewis acid. It works by accepting an electron pair from a base like ammonia.

History helps us understand how these ideas grew. In 1884, a scientist named Svante Arrhenius shared his ideas. He linked acidity to the presence of hydrogen ions in water. Later, in 1923, two chemists worked on a new idea. Johannes Nicolaus Brønsted and Thomas Martin Lowry both realized that acids donate protons. They did this work independently. Then, in that same year, Gilbert N. Lewis proposed his own idea. He focused on how electrons move between substances.

We can measure how acidic a liquid is using a scale called pH. A solution is acidic if its pH is less than 7. A lower number means the liquid is more acidic. This happens because there are more hydrogen ions in the liquid. Scientists also use a number called Ka to show acid strength. A stronger acid has a higher Ka value. This means it is better at losing its proton.

Understanding acids helps us know how things like vinegar or stomach acid behave. It even helps us understand how life works at a tiny level.

An acid is a specific type of molecule or ion. It is defined by its ability to interact with other substances through chemical reactions. In a strict sense, an acid refers to the solute itself. However, people often use the word to describe an aqueous solution. These solutions are characterized by a sour taste and the ability to turn blue litmus paper red. Acids can also react with bases and certain metals, such as calcium, to form salts.

Chemists use different frameworks to define how acids function. The first is the Arrhenius definition, which focuses on aqueous solutions. An Arrhenius acid is a substance that increases the concentration of hydrogen cations, or H+ ions, when added to water. In these solutions, these protons do not exist alone. Instead, they join with water to form hydronium ions, written as H3O+. Because of this, an Arrhenius acid can also be described as a substance that increases the concentration of hydronium ions in a liquid.

A second, more general framework is the Brønsted–Lowry theory. In 1923, Johannes Nicolaus Brønsted and Thomas Martin Lowry independently developed this idea. They defined an acid as a proton donor. This means the acid is a species that gives a proton to a base. This theory is more versatile than the Arrhenius model. It can describe reactions that happen in non-aqueous solvents or even in the gas phase. For example, hydrogen chloride and ammonia can react to form solid ammonium chloride without any water present.

A third category is known as a Lewis acid. This concept was also proposed in 1923 by Gilbert N. Lewis. A Lewis acid is an electron pair acceptor. Unlike the other definitions, this does not require the movement of a proton. Instead, the acid forms a covalent bond by accepting a pair of electrons from another species. An example is boron trifluoride. Its boron atom has a vacant orbital that allows it to accept an electron pair from a base like ammonia.

Acids vary greatly in their strength and chemical behavior. We measure this strength using the acid dissociation constant, known as Ka. This constant represents the equilibrium between an acid and its conjugate base. A stronger acid has a higher Ka value. This indicates the acid has a greater tendency to lose its proton. Because these values can span many orders of magnitude, scientists often use pKa. The pKa is the negative logarithm of Ka, so a smaller pKa indicates a stronger acid.

We also use the pH scale to describe acidity in solutions. A solution is considered acidic if its pH is less than 7. The pH value is the negative logarithm of the hydronium ion concentration. A lower pH number signifies a higher concentration of hydrogen cations. This means the solution is more acidic. In an acidic solution, the concentration of hydronium ions is greater than 10−7 moles per liter.

Many different substances act as acids in our daily lives. Common examples include acetic acid, which is the main component in vinegar. Citric acid is found in many citrus fruits. In the human body, hydrochloric acid is found in gastric acid within the stomach. This acid is essential because it activates digestive enzymes. Other acids, like sulfuric acid, are used in car batteries. While many strong acids are corrosive, there are exceptions like boric acid.