Lecture 2.4- Chemical Changes

BELLWORKElement, compound, or mixture?

Classify the following materials as an element compound or mixture.

Give reasons for your answers.

a. Table salt (NaCl)

b. Salt water

c. Apple juice

d. Water (H2O)

e. Helium gas (He)

Lecture 2.4- Chemical changes

A magnet separates iron from sulfur.

During a physical change, the composition of matter never changes.

Physical Changes

Chemical Changes

A mixture of iron and sulfur is heated.

The iron and sulfur react and form iron sulfide.

During a chemical change, the composition of matter ALWAYS changes.

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)

Recognizing Chemical Changes

It is usually a chemical change if there is :•a transfer of energy (ex. heat or light)

•a change in color

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

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.

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

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.

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.

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 FeSiron sulfur iron sulfide

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 FeSiron sulfur iron sulfide Reactants Product

Chemical Changes

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

Chemical vs. Physical PropertiesQuickTime™ and a

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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.

Conservation of Mass

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