Chemistry: Getting Our Ingredients

Getting Our Ingredients

As in cooking, when we perform chemical changes, a list of ingredients is required. However, the form used to express these ingredients is a bit different. Let's take a look at the list of ingredients needed to make water:

  • H2 + O2 ⇔ H2O

This statement tells us a great deal of information. Everything written before the arrow is required to make the chemical reaction take place and is called a reactant. The formulas written after the arrow are the chemicals that are formed and are called the "products" of the reaction. Together, the whole statement is the beginning of a chemical equation.

Molecular Meanings

The starting materials for a chemical reaction are referred to as reactants or reagents. The chemicals that are made in the reaction are referred to as products. Together, the entire statement that includes both the products and reactants is called a chemical equation.

All chemical changes are expressed using equations in this general form. The numbers of reactants and products may change from one equationto another, but the general format is always the same.


Most chemistry textbooks write the arrows for chemical reactions like this: →. However, you'll notice that in this section and beyond, I'll write the arrows like this: ⇔. The reason I do such an unusual thing is that all chemical reactions are reversible, meaning that in addition to reactants forming products, the products can also react in reverse to form reactants. Though in many cases the backwards reaction doesn't occur at a significant rate, it does occur, making it important to write the ⇔ arrow instead of the → arrow. We'll discuss this in greater detail in Solution Chemistry/emical Equilibria.

Excerpted from The Complete Idiot's Guide to Chemistry © 2003 by Ian Guch. All rights reserved including the right of reproduction in whole or in part in any form. Used by arrangement with Alpha Books, a member of Penguin Group (USA) Inc.

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