A chemical equation is a representation of a chemical reaction, where reactants under certain chemical conditions react with each other to form products. A chemical equation is written a specific way to show reactants and products, with an arrow separating reactants from products. In reactions with more than one reactant or product, plus signs separate the individual products and reactants from each other. Numbers called coefficients, placed in front of each reactant, specify the ratio required to convert all of the reactants to products. Products also have coefficients that indicate the ratio of each of the products in the reaction. 

The subscripts after a chemical element tell how many atoms of that element are present in the molecule. For example, the chemical equation for the formation of sugar from water and carbon dioxide is as follows:

6CO2 + 6H2O → C6H12O6 + 6O2

The equation obeys the law of conservation of mass and is called a balanced chemical equation. The law states that the mass of the reactants must equal the mass of the products. This can be determined by counting the number of atoms of each element to verify that there is the same number of atoms in the reactants as in the products. 

Ozone (O3) is produced in the upper atmosphere when oxygen gas (O2) absorbs high-energy sunlight. Which equation correctly describes this reaction?


There are six atoms of oxygen on both sides of the reaction; 3 × 2 = 6 atoms as reactants and 2 × 3 = 6 atoms as products. Choice D is correct. 

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