Chemical Reactions 2.4 Iron is abundant, easy to shape when heated, and relatively strong, especially when mixed with carbon in steel. Over time, objects made of iron will rust if they are left exposed to air. You will learn to recognize chemical changes and to distinguish them from physical changes.

Chemical Changes 2.43 What always happens during a chemical change?

EX: “reacts with oxygen” or “does not react with sodium” 2.4 Chemical Changes The ability of a substance to undergo a specific chemical change is called a chemical property. Chemical properties (a type of intensive property) can be used to identify a substance. But chemical properties can be observed only when a substance undergoes a chemical change. EX: “reacts with oxygen” or “does not react with sodium”

During a chemical change, the composition of matter always changes. 2.4 Chemical Changes During a chemical change, the composition of matter always changes. Recall that during a physical change, the composition of matter never changes.

2.4 Chemical Changes A magnet separates iron from sulfur. This is an example of a physical change. 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.

2.4 Chemical Changes A mixture of iron and sulfur is heated. The iron and sulfur react and form iron sulfide. This is an example of a chemical change. 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.

2.4 A chemical change is also called a chemical reaction. Chemical Changes 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. The product(s) has different intensive properties than the reactant(s) because there is a change in composition (arrangement of atoms)

Recognizing Chemical Changes 2.4 Recognizing Chemical Changes Recognizing Chemical Changes What are four possible clues that a chemical change has taken place? What is the only true method to verify that chemical change has taken place?

Recognizing Chemical Changes 2.4 Recognizing Chemical Changes Possible clues to chemical change include: a transfer of energy (such as light or a change in temp) a change in color the production of a gas* the formation of a precipitate* *if these phases were not present in the reactants

Clues to chemical changes have practical applications. 2.4 Chemical Changes A precipitate is a solid that forms and settles out of a liquid mixture. Clues to chemical changes have practical applications. Clues to chemical change often have practical applications. a) Bubbles of carbon dioxide gas form when a geologist puts acid on a rock that contains compounds called carbonates. b) When a test strip is dipped in urine, the color change is used to estimate the level of the sugar glucose in urine. c) One step in the production of cheese is a reaction that causes milk to separate into solid curds and liquid whey.

Recognizing Chemical Changes 2.4 Recognizing Chemical Changes Recognizing Chemical Changes What is the only true method to verify that chemical change has taken place? Verify a change in intensive properties Ex: Sulfur is odorous and Iron is magnetic, but the compound iron sulfide is not magnetic and does not have an odor

2.4 Conservation of Mass Conservation of Mass How are the mass of the reactants and the mass of the products of a chemical reaction related?

2.4 Conservation of Mass During any chemical reaction, the mass of the products is always equal to the mass of the reactants.

2.4 Conservation of Mass The law of conservation of mass states that in any physical change or chemical reaction, mass is conserved. The conservation of mass is easily observed when a change occurs in a closed container. When the liquids in photograph A are mixed, they react. None of the products are gases. Analyzing Data How do you know that a reaction took place and that mass was conserved during the reaction?

2.4 Conservation of Mass Example MME question: 7 g of iron (Fe) reacts completely with 32 g of phosphorus (P) to form iron phosphide (Fe3P). Write the chemical equation: 3Fe + P  Fe3P b) Circle the reactant, underline the product C) What is the mass of the product? Show the reasoning: Mr = Mp  7g + 32 g = Mp  Mp = 39 g

2.4 Section Quiz. 2.4.

2.4 Section Quiz. 1. Which of the following is a chemical reaction? melting of lead dissolving sugar in water rusting of iron crushing of stone

2.4 Section Quiz. 2. Which of the following is NOT a possible clue that a chemical change is taking place? a change of state a change in color production of a gas formation of a precipitate

2.4 Section Quiz. 3. During any chemical change, the mass of the products is always equal to the mass of the reactants. always greater than the mass of the reactants. always less than the mass of the reactants. sometimes different than the mass of the reactants.

Concept Map 2 Concept Map 2 Solve the Concept Map with the help of an interactive guided tutorial.

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