Reactant in Chemistry: Definition & Examples

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  • 0:00 Reactants Defined
  • 1:55 Reactant Scenarios
  • 2:57 Lesson Summary
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Lesson Transcript
Instructor: Nathan Crawford

Nathan, a PhD chemist, has taught chemistry and physical science courses.

This lesson defines reactants within chemical reactions and how reactants are written in chemical reactions. The role of chemical reactions in reactions is explained, and several examples are provided to provide important context.

Reactants Defined

Imagine a loaf of bread fresh from the oven. Now, imagine the ingredients that were used to make the bread: dough, flour, eggs, sugar, etc. You might be surprised to know that you have been visualizing the necessary components of a chemical reaction!

A chemical reaction describes how atoms, the fundamental building blocks of matter, rearrange themselves into new combinations. The raw materials that react with each other are known as reactants. Under the proper conditions, such as temperature, time, or pressure, the chemical bonds of the reactants are broken, and the atoms form new bonds that give different combinations. The substances that result from this recombination of atoms are called the products of the reaction. When chemical reactions are written down, the reactants are written on the left side, in a similar manner to a math equation; one reactant plus another. An arrow is written to the right of the reactants to indicate that a reaction occurs, and the products are then written on the right side of the arrow. Here is an example where ammonia and hydrochloric acid are written on the left side of the arrow, and the product, ammonium chloride, is written on the right side of the arrow.

Example of written reaction with reactants and products labeled

An easier way to understand reactions is to once again consider cooking, but this time, think about making a pizza. The foundation of pizza is, of course, the crust, and pizza dough is the raw ingredient or reactant. The toppings include tomato sauce, cheese, veggies, and other tasty ingredients, which make up the remaining reactants. Obviously, the assembled uncooked pizza does not just cook on its own. It requires the heated interior of a hot oven. Once the pizza has been placed in the hot oven for an appropriate amount of time, the cooked pizza emerges as the product of this culinary reaction.

Reactant Scenarios

The minimum number of reactant substances that can be involved in a chemical reaction is one type. One type of reaction that involves the splitting of a single reactant substance into two or more products is known as a decomposition reaction. A good example of this is the decomposition reaction that occurs when an electric current is passed through water (H20). The energy from the electric current breaks the bonds of water molecules to form hydrogen gas (H2) and oxygen gas (O2) as products.

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