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?

2.4 Chemical Changes The ability of a substance to undergo a specific chemical change is called a chemical property. Chemical properties can be used to identify a substance. But chemical properties can be observed only when a substance undergoes a chemical change.

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.

A chemical change is also called a chemical reaction. 2.4 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.

Recognizing Chemical Changes 2.4 Recognizing Chemical Changes Recognizing Chemical Changes What are four possible clues that a chemical change has taken place?

Recognizing Chemical Changes 2.4 Recognizing Chemical Changes Possible clues to chemical change include: a transfer of energy a change in color the production of a gas the formation of a precipitate.

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.

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?

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

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

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

3. During any chemical change, the mass of the products is a. always equal to the mass of the reactants. b. always greater than the mass of the reactants. c. always less than the mass of the reactants. d. sometimes different than the mass of the reactants.

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.