The Acid-Base Properties of Water

Instructor: Amanda Robb
This lesson will explain one of the most unique properties of water: its ability to act as both an acid and a base. We'll discuss this property and look at examples of how it acts as each in different types of reactions.

Acids and Bases

What do the contents of your stomach, the battery of your car, and a glass of orange juice all have in common? These seemingly different liquids are all examples of acids. On the opposite end of the spectrum we have bases, which include examples that vary from household bleach to the baking soda you use when baking. These examples and even the concepts of acids and bases are most likely already familiar to you. But in science, what makes something an acid or a base?

There are a few definitions, but here we'll stick with the Bronsted Lowry definition. This definition says that acids are molecules that can donate a hydrogen ion, also called a proton. While bases are molecules that can accept a hydrogen ion. The hydrochloric acid in your stomach donates a proton to other molecules, helping to break up their structure, making it an acid. The sodium hydroxide in bleach can accept a hydrogen ion, making it a base.

Water

Water is a special molecule and has many unique properties that make it essential for all life on Earth. In fact, when scientists look for life on other planets, they first look for signs of water! Water is made of two hydrogen atoms bonded to an oxygen atom. They form these bonds by sharing electrons.

However, oxygen is not very good at sharing. Oxygen is extremely electronegative, meaning it likes electrons. So it pulls the electrons in the bond closer to itself, causing oxygen to get a partial negative charge and the hydrogen atoms to get partial positive charges. This allows water to form bonds between different molecules based on the charges, called hydrogen bonds. This gives water many of its unique properties that allow it to support life.

Water also has the important property of acting as both an acid or a base depending on the conditions. Water goes through autoionization, where one water molecule can donate a hydrogen atom to another. This forms hydroxide ions (OH-) and hydronium ions (H3O+). In this example, water acts both as the acid, donating hydrogen ions, and as the base, accepting hydrogen ions.

During autoionization, water acts as both an acid and a base
autoionization

Water as an Acid

When water acts as an acid, it donates a hydrogen ion to other molecules and becomes a hydroxide (OH-) molecule itself. Let's look at an example. Let's say you want to dilute a cleaning product made mostly of ammonia, NH3, with water. Ammonia is a base that accepts a hydrogen atom to become ammonium, or NH4+. When ammonia is mixed with water, ammonia is a stronger base, so water donates a hydrogen ion forming ammonium and hydroxide ions.

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