2.4 Chemical Reactions

8/13/2019 2.4 Chemical Reactions

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Section 2.4

Chemical Reactions


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Chemical Reactions

Iron is abundant, easy to shape

when heated, and relatively strong,

especially when mixed with carbon

in steel. Over time, objects made ofiron will rust if they are left exposed

to air. Rusting is a chemical

change. You will learn to

recognize chemical changes and

to distinguish them from physical

changes in this section.

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Chemical Properties and Chemical

Changes

The ability of a substance to undergo a specificchemical 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.

Chemical Change- the composition or identity of

the matter changes. A new substance or new

substances are formed.

This is different from a physical change because

during a physical change the composition of the

matter remains the same.


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Physical Change

A magnet separates iron from sulfur. This is anexample of a physical change


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Chemical ChangesA mixture of iron and sulfur is heated. The iron

and sulfur react and form iron sulfide. This is

an example of a chemical change.

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Chemical Changes

A chemical change is also called a chemical

reaction.

One or more substances change into one ormore new substances during a chemical

reaction.

A substance present at the start of thereaction is a reactant.

A substance produced in the reaction is a

product.


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Chemical Changes

Possible clues to chemical changeinclude:

a transfer of energy

the production of a gas

a change in color the formation of a precipitate:

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A precipitateis a solid that forms and settles out ofa liquid mixture.
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change ALWAYS mean chemical

changes??

Transfer of energy- Fire always means chemical change

Heat, cooling, and light can indicate either type of

change. To be certain of the type of change, the

identity of the substance must be examined.

Production of a gas-

A gas produced when heat is added, such as during

boiling is likely to be a physical change

The production of gas without the addition of heat

indicates a chemical change


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Do these 4 indication of chemical change

ALWAYS mean chemical changes??

Change in color If blue and yellow are mixed and form green, this is

likely a physical change.

If 2 clear solutions are combined and a colored

solution forms, this is a chemical change. In other words, if an unexpected color change occurs, a

chemical change occurred.

Precipitate

If dirt settles out of a mixture of dirty water, this isnot a true precipitate- this is a physical change.

A true precipitate- one that forms from 2 solutions-

always indicates a chemical change.


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Conservation of Mass

The law of conservation of massstatesthat in any physical change or chemical

reaction, mass is conserved.

The conservation of mass is easilyobserved when a change occurs in a

closed container.

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2.4 Chemical Reactions