The Chemistry of Pop

When you first open a can of Coca-Cola, what’s the first thing you notice? Maybe it’s that fizzing sound. Maybe it’s the sight or sound of bubbles rising and popping. But did you know that fizz is actually a chemical process? In fact, inside every can or bottle of pop, there is some pretty cool chemistry going on.

Did you know?

Coca-Cola is the world’s best-selling pop. It was invented in 1886 by a pharmacist who originally wanted to sell it as a medicine. Back then, Coca-Cola cost five cents a glass!

Why is pop fizzy?

Many people love the feeling of bubbles bursting in their mouths when they drink pop. These bubbles happen because of carbonation. Carbonation occurs when carbon dioxide (CO₂) dissolves in water (H₂O) or an aqueous (watery) solution.

Carbon dioxide doesn’t easily dissolve in water under everyday conditions. To make this happen, manufacturers need to increase the pressure in the can (or bottle) and keep it at a low temperature. Under these conditions, water molecules can trap lots of CO₂ molecules.

The can is then sealed so that it is airtight. This way, the inside of the can maintains enough pressure to prevent the extra CO₂ molecules from escaping. Inside the can, CO₂ exists in two forms. Some CO₂ dissolves in water. Some CO₂ sits in gas form between the top of the bottle or can and the liquid.

What chemical reactions are going on inside a pop can?

When CO₂ dissolves in H₂O, water and gaseous carbon dioxide react to form a dilute solution of carbonic acid (H₂CO₃).

The chemical reaction for this process is: H₂O + CO₂ ⇋ H₂CO₃  

Note that the reactants, H₂O and CO₂ , are on the left. The product,  H₂CO₃ , is on the right.

Notice the symbol ⇋ in the chemical equation. This symbol shows that the reaction is reversible. That means it can go forwards and backwards. The carbonic acid can be converted back to water and carbon dioxide. When a can of pop is sealed, high pressure inside the can forces the chemical reaction to the right (the forward reaction).

In other words, carbonic acid is produced. The forward reaction continues until concentration of the reactants and products no longer changes. In other words, when the two chemical reactions are in equilibrium.

But once you open that can, the pressure is released. This shifts the chemical reaction to the left (the backward reaction).

Remember that gaseous CO₂ at the top of the can? Once you open the can, it escapes. And now that the can is no longer under pressure, the dissolved CO₂ starts coming out of the solution. This forms bubbles, which release the CO₂ into the air.

The escaping CO₂ lowers the concentration of CO₂ in the pop. Because of this lower concentration, the carbonic acid turns back to CO₂ and H₂0. The result is a new equilibrium.

Why does fizz taste so good?

Have you ever noticed that fizzy pop tastes better than flat pop? That has nothing to do with the bubbles. It’s actually because in a fizzy drink, the dilute carbonic acid creates a slight burning sensation on your tongue.

This doesn’t happen when pop goes flat. When pop is left out in the open, CO₂ continually escapes. Let’s look at that chemical equation again.

H₂O + CO₂ ⇋ H₂CO₃ 

As CO₂ bubbles away from the liquid, the reactants and products move again towards equilibrium. This causes the backward reaction to take over. As that happens, the concentration of carbonic acid in the drink gets lower and lower. As the amount of H₂CO₃ in the beverage goes down, so does the soda’s ability to bring about that tingling sensation on your tongue.

Did you Know?

The carbonation process was invented by a famous English chemist named Joseph Priestley in 1767.  A few years later, a Swiss scientist named Jacob Schweppe created the world’s first soft drink: Schweppe’s Tonic Water! 

Why do people like drinking pop?

Why do some people enjoy the tingling sensation of carbonated drinks in the first place? Scientists are not entirely sure. In fact, studies have found that many other mammals actually avoid drinking carbonated liquids. Some scientists believe that humans like fizzy drinks because we like a bit of risk in our lives. It’s like why some people really enjoy spicy food. And let’s not forget that other reason why people might like fizzy drinks: they contain a lot of sugar!

What have you learned about pop?

Let’s recap.

• Pop contains carbonic acid, which forms when CO₂ is dissolved in water.
• In every can (or bottle) of pop, carbonic acid and gaseous CO₂ exist in chemical equilibrium.
• When you open a can of pop, gaseous CO₂ escapes. You see bubbles and hear the sound of fizz. The chemical equilibrium between the carbonic acid and the CO₂ shifts. 
• When pop goes flat, that’s the concentration of carbonic acid in the drink getting lower.
• Many animals really dislike pop.
• And finally: if you love pop, you may be a bit of a risk-taker!