Element, compound, or mixture? Classify the following materials as an element compound or mixture. Give reasons for your answers. Table salt (NaCl) Salt water Apple juice Water (H2O) Helium gas (He)

Lecture 2.4- Chemical changes

Physical Changes During a physical change, the composition of matter never changes. A magnet separates iron from sulfur. 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. 3

Chemical Changes During a chemical change, the composition of matter ALWAYS changes. A mixture of iron and sulfur is heated. The iron and sulfur react and form iron sulfide. 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. 4

Recognizing Chemical Changes It is usually a chemical change if there is :

Recognizing Chemical Changes It is usually a chemical change if there is : a transfer of energy (ex. heat or light) 6

Recognizing Chemical Changes It is usually a chemical change if there is : a transfer of energy (ex. heat or light) a change in color 7

Recognizing Chemical Changes It is usually a chemical change if there is : a transfer of energy (ex. heat or light) a change in color production of a gas 8

Recognizing Chemical Changes It is usually a chemical change if there is : a transfer of energy (ex. heat or light) a change in color production of a gas formation of a precipitate. 9

A precipitate is a solid that forms and settles out of a liquid mixture. 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.

A chemical change is also called a chemical reaction.

A chemical change is also called a chemical reaction. A substance present at the start of the reaction is a reactant. 12

A chemical change is also called a chemical reaction. A substance present at the start of the reaction is a reactant. A new substance created by the reaction is a product. 13

Fe + S  FeS A chemical change is also called a chemical reaction. A substance present at the start of the reaction is a reactant. A new substance created by the reaction is a product. Fe + S  FeS iron sulfur iron sulfide 14

Fe + S  FeS A chemical change is also called a chemical reaction. A substance present at the start of the reaction is a reactant. A new substance created by the reaction is a product. Fe + S  FeS iron sulfur iron sulfide Reactants Product 15

Chemical Changes The ability of a substance to undergo a specific chemical change is called a chemical property.

Chemical vs. Physical Properties

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

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?

Conservation of Mass