Reading Chapter Two: 2.4

Connecting to Your World Iron is an element with many desirable properties. It is abundant, easy to shape when heated, and relatively strong, especially when mixed with carbon in steel. Iron has one main disadvantage. Over time, objects made of iron will rust if they are left exposed to air. The brittle layer of rust that forms on the surface of the object flakes off, exposing more iron to the air. In this section, you will learn to recognize chemical changes and to distinguish them from physical changes.



Key Concepts

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What always happens during a chemical change?
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What are four possible clues that a chemical change has taken place?
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How are the mass of the reactants and the mass of the products of a chemical reaction related?

Vocabulary

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chemical property
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chemical reaction
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reactant
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product
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precipitate
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law of conservation of mass

Reading Strategy

Predicting Before you read, predict what will happen to the mass of a sample of matter that burns. After you read, check the accuracy of your prediction and correct any misconceptions.

Chemical Changes

The compound formed when iron rusts is iron oxide (Fe2O3). Words such as burn, rot, rust, decompose, ferment, explode, and corrode usually signify a chemical change. The ability of a substance to undergo a specific chemical change is called a chemical property. Iron is able to combine with oxygen to form rust. So the ability to rust is a chemical property of iron. Chemical properties can be used to identify a substance. But chemical properties can be observed only when a substance undergoes a chemical change.

Figure 2.13 compares a physical change and a chemical change that can occur in a mixture of iron and sulfur. When a magnet is used to separate iron from sulfur, the change is a physical change. The substances present before the change are the same substances present after the change, although they are no longer physically blended. Recall that during a physical change, the composition of matter never changes. During a chemical change, the composition of matter always changes. When the mixture of iron and sulfur is heated, a chemical change occurs. The sulfur and iron react and form iron sulfide (FeS).

Figure 2.13 A mixture of iron filings and sulfur can be changed. (a) A magnet separates the iron from the sulfur. (b) Heat combines iron and sulfur in a compound. Classifying Which change is a chemical change? Explain.

A chemical change is also called a chemical reaction. One or more substances change into one or more new substances during a chemical reaction. A substance present at the start of the reaction is a reactant. A substance produced in the reaction is a product. In the reaction of iron and sulfur, iron and sulfur are reactants and iron sulfide is a product.

Recognizing Chemical Changes

How can you tell whether a chemical change has taken place? There are four clues that can serve as a guide. Possible clues to chemical change include a transfer of energy, a change in color, the production of a gas, or the formation of a precipitate.

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Every chemical change involves a transfer of energy. For example, energy stored in natural gas is used to cook food. When the methane in natural gas combines with oxygen in the air, energy is given off in the form of heat and light. Some of this energy is transferred to and absorbed by food that is cooking over a lit gas burner. The energy causes chemical changes to take place in the food. The food may brown as it cooks, which is another clue that chemical changes are occurring.

You can observe two other clues to chemical change while cleaning a bathtub. The ring of soap scum that can form in a bathtub is an example of a precipitate. A precipitate is a solid that forms and settles out of a liquid mixture. Some bathroom cleaners that you can use to remove soap scum start to bubble when you spray them on the scum. The bubbles are a product of a chemical change that is taking place in the cleaner.

If you observe a clue to chemical change, you cannot be certain that a chemical change has taken place. The clue may be the result of a physical change. For example, energy is always transferred when matter changes from one state to another. Bubbles form when you boil water or open a carbonated drink. The only way to be sure that a chemical change has occurred is to test the composition of a sample before and after the change. Figure 2.14 shows examples of practical situations in which different clues to chemical change are visible.

Conservation of Mass

When wood burns, substances in the wood combine with oxygen from the air. As the wood burns, a sizable amount of matter is reduced to a small pile of ashes. The reaction seems to involve a reduction in the amount of matter. But appearances can be deceiving. During any chemical reaction, the mass of the products is always equal to the mass of the reactants. Two of the products of burning wood—carbon dioxide gas and water vapor—are released into the air. When the mass of these gases is considered, the amount of matter is unchanged. Careful measurements show that the total mass of the reactants (wood and the oxygen consumed) equals the total mass of the products (carbon dioxide, water vapor, and ash).

Mass also holds constant during physical changes. For example, when 10 grams of ice melt, 10 grams of liquid water are produced. Similar observations have been recorded for all chemical and physical changes studied. The scientific law that reflects these observations is the law of conservation of mass. The law of conservation of mass states that in any physical change or chemical reaction, mass is conserved. Mass is neither created nor destroyed. The conservation of mass is more easily observed when a change occurs in a closed container, as in Figure 2.15.

Key Concepts

2.1 Properties of Matter

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Properties used to describe matter can be classified as extensive or intensive.Hint
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Every sample of a given substance has identical intensive properties because every sample has the same composition.Hint
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Three states of matter are solid, liquid, and gas.Hint
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Physical changes can be classified as reversible or irreversible.Hint

2.2 Mixtures

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Mixtures can be classified as heterogeneous mixtures or as homogeneous mixtures, based on the distribution of their components.Hint
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Differences in physical properties can be used to separate mixtures.Hint

2.3 Elements and Compounds

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Compounds can be broken down into simpler substances by chemical means, but elements cannot.Hint
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If the composition of a material is fixed, the material is a substance. If the composition may vary, the material is a mixture.Hint
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Chemists use chemical symbols to represent elements, and chemical formulas to represent compounds.Hint

2.4 Chemical Reactions

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During a chemical change, the composition of matter always changes.Hint
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Four possible clues to chemical change include a transfer of energy, a change in color, the production of a gas, or the formation of a precipitate.Hint
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During any chemical reaction, the mass of the products is always equal to the mass of the reactants.Hint

Vocabulary

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Vocabulary Review

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chemical change
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chemical property
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chemical reaction
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chemical symbol
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compound
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distillation
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element
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extensive property
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filtration
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gas
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heterogeneous mixture
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homogeneous mixture
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intensive property
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law of conservation of mass
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liquid
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mass
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mixture
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phase
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physical change
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physical property
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precipitate
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product
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reactant
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solid
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solution
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substance
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vapor
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volume