Acid–base reaction

Some things are acids.

Some things are bases. They can mix together. This makes a change.

When they mix, they make a salt and water. This is called a neutralization. It can even make bubbles in cake!

Baking powder uses this. It has acid salts and baking soda. When you add water, they react. This makes gas bubbles. These bubbles help the cake rise. It is very cool to see it work!

An acid-base reaction is a special way that chemicals change.

Acids and bases can react in many ways. Scientists use different ideas to explain how this happens.

One idea comes from Svante Arrhenius.

He said acids make hydrogen ions in water. A base makes hydroxide ions in water. When they mix, they make a salt and water. This is called a neutralization reaction. Baking powder uses this to make cakes rise. It mixes baking soda with acid salts. When you add water, they make gas bubbles.

Another idea is the Brønsted–Lowry theory.

This theory says acids give away a proton. A proton is a tiny part of an atom. Bases are things that take those protons. This idea works even without water. It can happen in gases or other liquids.

Finally, there is the Lewis model. This is the broadest way to think about it. It does not require any hydrogen at all. These different ideas all help us understand the world.

An acid-base reaction is a special way that chemicals interact.

Scientists use these reactions to find the pH of a liquid. pH is a way to measure how acidic or basic a solution is. There are several different theories to explain how this works. These theories help us understand how things react in water or gas. Each idea adds a new piece to the puzzle. They work together to help us solve chemistry problems.

One way to look at it is through the Arrhenius theory.

This theory focuses on what happens when substances are in water. An Arrhenius acid increases the amount of hydrogen ions in a solution. A base increases the amount of hydroxide ions in a solution. When they mix, they undergo a neutralization reaction. This process creates a salt and water. For example, hydrochloric acid and sodium hydroxide make sodium chloride and water.

People have studied these reactions for a very long time.

In 1754, Guillaume-François Rouelle used the word "base" for the first time. Around 1776, Antoine Lavoisier proposed a theory about acids and oxygen. Later, in 1838, Justus von Liebig suggested acids contain hydrogen. Svante Arrhenius created his modern definition in 1884. He later won the Nobel Prize in Chemistry in 1903. These scientists helped us move from old ideas to modern ones.

Another important idea is the Brønsted–Lowry theory.

This was made in 1923 by Johannes Brønsted and Martin Lowry. They said acids are things that donate a proton. A proton is a tiny part of an atom called a hydrogen cation. Bases are things that accept these protons. This theory is great because it works in gases and other liquids. It does not only rely on water like the Arrhenius model does.

You can see these reactions in your own kitchen.

Baking powder is a common example of this science. It is a mixture of baking soda and acidic salts. When you add water, the acid and base react together. This reaction produces carbon dioxide gas. The gas creates millions of tiny bubbles in the dough. These bubbles are what make your bread or cake rise.

An acid–base reaction is a fundamental chemical process occurring between an acid and a base. These reactions are essential for understanding chemical behavior across many different environments. Scientists use these interactions to determine pH, which measures how acidic or basic a solution is, often through a method called titration. Because these reactions can happen in gases, liquids, or solids, chemists use several different theoretical frameworks to explain them. These frameworks, known as acid–base theories, complement one another to provide a complete picture of how molecules interact.

One way to understand these reactions is through the Arrhenius theory. Developed by Svante Arrhenius, this model focuses specifically on substances dissolved in water, or aqueous solutions.

An Arrhenius acid is a substance that increases the concentration of hydrogen ions (H+) in water. When these ions enter water, they create hydronium ions (H3O+). An Arrhenius base is a substance that increases the concentration of hydroxide ions (OH-) in a solution. When an Arrhenius acid and base react, they undergo a neutralization reaction. This process typically produces a salt and water as the final products. For example, mixing hydrochloric acid (HCl) with sodium hydroxide (NaOH) produces sodium chloride (NaCl) and water.

While the Arrhenius model is useful, it is quite restrictive because it only applies to water. It cannot explain reactions in non-aqueous solutions, such as substances dissolved in toluene or liquid ammonia. To solve this, the Brønsted–Lowry theory was introduced in 1923 by Johannes Brønsted and Martin Lowry.

This theory defines an acid as a proton donor and a base as a proton acceptor. A proton is another name for a hydrogen cation (H+). In this model, the reaction is viewed as the transfer of a proton from one molecule to another. This process creates what are called conjugate acids and conjugate bases. The acid becomes a conjugate base after losing its proton, while the base becomes a conjugate acid after gaining one.

Because the Brønsted–Lowry theory focuses on proton transfer rather than water, it works in many different environments. It can explain reactions in gases, liquids, and solids. This model even explains why water is amphoteric, meaning it can act as both an acid and a base. In one instance, a water molecule can donate a proton to become a hydroxide ion. In another instance, a water molecule can accept a proton to become a hydronium ion. This flexibility allows chemists to study complex systems, such as acetic acid dissolving in liquid ammonia, which the Arrhenius model cannot fully describe.

Our understanding of these substances has evolved through centuries of discovery. In 1754, Guillaume-François Rouelle first used the term "base" to describe substances that react with acids to form salts. Around 1776, Antoine Lavoisier proposed that acids were defined by the presence of oxygen. This "oxygen theory" lasted for decades until Sir Humphry Davy proved in the 1810s that some acids, like hydrohalic acids, do not contain oxygen. Later, in 1838, Justus von Liebig proposed a hydrogen theory, suggesting acids were compounds where hydrogen could be replaced by a metal. These shifting perspectives allowed scientists to move from simple observations to the sophisticated molecular models used today.

We can see these chemical principles in action in our own homes, specifically in baking powder.

Baking powder is not the same as baking soda, which is sodium bicarbonate (NaHCO3). Instead, baking powder is a mixture of sodium bicarbonate and various acidic salts. When baking powder meets water, the acid and the base react to produce carbon dioxide gas. This gas creates millions of tiny bubbles within dough. These bubbles are responsible for making breads and cakes rise. A common acidic component used in these mixtures is monocalcium phosphate, or calcium dihydrogen phosphate.

Finally, the most inclusive way to view these reactions is through the Lewis definition. Proposed by Gilbert N. Lewis in 1938, this theory removes the requirement for a substance to contain hydrogen. While the Brønsted–Lowry theory is a subset of the Lewis model, the Lewis model is the broadest. It defines acids and bases based on how they handle electron pairs rather than just protons. This allows chemists to classify a much wider range of substances as acids or bases, ensuring that the rules of chemistry apply to almost every molecular interaction encountered in science.