What is the pH Scale?

The pH scale is a rating system used to classify substances based on how acidic they are. This scale is a useful tool in testing the quality of water, and has many other universal applications. To understand how to use the pH scale, we must first cover a bit of background on what ions, acids and bases are.

Ions

All substances on the Earth, such as the food you eat, the air you breathe in and the chair you are sitting on, are made up of atoms. Atoms are just a group of protons, neutrons and electrons (collectively known as subatomic particles) that like to hang out together.
You might be familiar with the periodic table, which shows all elements arranged in different rows and columns depending on their number of subatomic particles.

Atoms themselves often like to hang out with other atoms, binding together to form molecules. For example, two atoms of hydrogen (H) and one atom of oxygen (O) like joining forces to make a water molecule (H2O).

When molecules are formed, they are usually quite stable by themselves and don’t react much to their surroundings. However, when you add these substances to water, the molecules often change a bit and break apart (or dissociate) to form ions. Ions are just the same individual atoms we started with to make the molecule, but with a slightly different number of electrons. Depending on whether an atom lost or gained electrons when the molecule dissociated, ions are classified as either positive (lost electrons) or negative (gained elections).

For example, when you add salt (sodium chloride or NaCl) to water (H2O), the salt dissociates to form sodium ions (Na) that are positive and chloride (Cl) ions that are negative.

Acids and bases

Now that we know what ions are, figuring out what acids and bases are should be pretty easy—it’s all about the hydrogen ion (H+). Generally speaking, any substance that breaks apart in water to produce a hydrogen ion is known as an acid. A good example of this is hydrochloric acid or HCl, which breaks apart to form hydrogen ions (H+) and chloride ions (Cl-) in solution.

On the flipside, a base is generally any substance that breaks apart in water to produce a negative ‘hydroxide’ ion (OH-).

In special cases, some substances don’t produce any H+ or OH- when they are added to water, but are still classified as either an acid or a base. They are categorised as acids or bases by whether they donate or accept a hydrogen ion or a pair of electrons. A more detailed explanation of how these substances are classified along with some examples can be found here.

The pH scale ranges from 0 (acidic) to 14 (basic). The pH of some common substances is shown on the right. Image: Edward Stevens/Wikimedia Commons

The pH scale

Now, onto the most important bit: how do we actually use the pH scale?

As mentioned above, the pH scale is used to classify whether a substance is an acid or a base. The scale itself runs between 0-14, with each number representing a different level of acidity or basicity. Anything that is an acid will have a pH below 7, and anything that is a base will have a pH above 7. Strong acids have a pH close to 0, whilst strong bases have a pH close to 14. Substances with pH’s closer to 7 are known as weak acids or weak bases. When there are equal numbers of H+ and OH- ions, the pH of that solution is 7 (neutral) as the charges on the ions cancel each other out. In other words, if you add H+ and OH- together, you get H2O (water!) that is neither an acid nor a base.

Although you may not realise it, a number of everyday foods and other household items are actually classified as acids or bases. Some common examples are shown in the picture on the left.

Measuring pH

Knowing how the pH scale works is really only useful if you can measure the pH of a substance. The pH is usually measured in one of three ways. The oldest method of measuring pH is by using the litmus test. Litmus is just a molecule that reacts with both acids and bases, and results in a colour change. When litmus reacts with an acid it turns red, and when it reacts with a base it turns blue.

Other ways to measure pH include using a pH meter, or an indicator (similar to the litmus test). A simple experiment you can try in the classroom or at home involves using cabbage to test the pH of household substances. Cabbage also changes colour depending on the pH of the solution it is added to.

Remember to take extra care when handling acids and bases, as they can burn your skin, but other than that – have some serious science pHun! —By Lucy Weaver