Gas Evolution Reactions: Definition & Examples

Instructor: Amanda Robb
In this lesson, we'll look at aqueous reactions that form gas products, called gas evolution reactions. We'll see how this type of reaction happens, how to predict which chemical reactions will commonly form gas products, and how to identify them.

What Are Gas Evolution Reactions?

Have you ever had an upset stomach, or indigestion? One cure for indigestion is Alka-Seltzer, an over the counter medication that helps to settle the stomach. When you drop an Alka-Seltzer tablet into water, it immediately starts to bubble and fizz. Why does this happen? What you're observing when you make an Alka-Seltzer drink is an example of a gas evolution reaction. In this type of chemical reaction one of the products is a gas, which can be seen as bubbles in an aqueous solution.

Gas evolution reactions are one example of double displacement reactions, where two ionic compounds mix. The negatively charged ions, or anions, essentially switch partners, forming new chemical compounds in the product. To picture this, imagine having two Lego structures, each made of two colors. During the reaction, you take one color from each structure and swap it out to the other one. This is how a double displacement reaction works.

Gas evolution reactions are one type of double displacement reaction where a product is a gas
double displacement

If one of the products is a gas, the type of reaction is considered a gas evolution reaction. Today, we're going to look at common chemical reactions that form gases, including the type of reaction that happens when you drop an Alka-Seltzer tablet in water, as well as products that commonly break down into gases.

Chemical Reactions that Form Gases

One trick to recognizing gas evolution reactions is knowing what type of reactions typically produce gases. One of the most common gas evolution reactions is an acid base reaction, where an acid is combined with a base to produce water and carbon dioxide. This is also commonly called a neutralization reaction, since the acid and base neutralize each other to make water.

Our example of Alka-Seltzer in water is a type of acid base reaction. When the Alka-Seltzer tablet starts to dissolve it releases the two components, citric acid and sodium bicarbonate. Citric acid is an acid, and sodium bicarbonate is the base. The atoms of these reactants are rearranged when they come into contact with each other and reassembled to form water, carbon dioxide, the gas product, and sodium citrate. In our Alka-Seltzer reaction, the bubbles are the product carbon dioxide and are what causes the drink to fizz.

Alka-Seltzer tablets create acid base reactions that are one type of gas evolution reaction
Alka Seltzer

Common Types of Gas Products

If you are looking at a reaction that isn't an acid base reaction, how will you know if it forms a gas? There are a few products to look out for that always break down further into gases. First, if you complete the double displacement and you get the product hydrogen sulfide, this product is always a gas.

Other products might be a liquid to start, but quickly break down to release a gas. One example of such a product is carbonic acid. In water, carbonic acid immediately breaks down to carbon dioxide and water. Carbonic acid is a common product of acid base reactions as well, and is one way that carbon dioxide is formed. For example, calcium carbonate mixes with hydrochloric acid in an acid base reaction. A double displacement occurs, giving us carbonic acid and calcium chloride. But, carbonic acid always breaks down into water and carbon dioxide, our gas product.

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