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Chemical reactions occur everywhere. The fuel in our cars burns with oxygen .

to provide energy to make the cars move. When we cook our food or bieach our hair, Chemical reactions take place. In our bodies Chemical reactions convert food substances into molecuies to build muscles and into energy to move them. In the leaves oftrees and plants, carbon dioxide and water are converted into carbohydrates.

Some chemica! reactions are simple, whereas others are quite complex. However, they can all be written with the Chemical equations that chemists use to describe Chemical reactions. In every Chemical reaction, the atoms in the reactingsubstances, called reactants, are rearranged to give new substances called products.

' However, the atoms in the reactants are the same atoms as in the products, which means that matter is conserved and no matter is lost during a Chemical change.

In this chapter, we will see how equations are written and how we can determine the amount of reactant or product involved in a Chemical reaction. We do much the same thing at home when we use a recipe to make cookies. An auto mechanic does essentially the same thing when adjustingthe fuel system of an engine to allow for the correct amounts of fuel and oxygen. In the body a certain : amount of 0₂ must reach the tissues for effłcient metabolic reactions. When we knowthe Chemical equation for a reaction, we can determine the amount of . reactant needed or amount of product that can be produced.

7.1 Chemical Reactions

In a physical change, the appearance of a substance is altered, but not its composition. When liąuid water becomes a gas, or freezes to a solid, it is still i water (see Figurę 7.1). If we smash a rock or tear a piece of paper, only the size of the materiał changes. The smaller pieces are still rock or paper because there was no change in the composition of the substances.

In a Chemical change, the reacting substances change into new substances that have different compositions and different properties. New properties may involve a change in color, a change in temperaturę, or the formation of bub-bles or a solid. For instance, when silver tamishes, the bright silver metal (Ag) reacts with sulfur (S) to becóme the duli, black substance we cali tamish i (Ag2S) (see Figurę 7.1). Table 7.1 gives some examples of some typical physical : and Chemical changes.

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