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Chemistry

What is the difference between a chemical and a physical change?

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March 19, 2018 12:43PM

Physical changes are reversible (reversal is often quite easy).*

Chemical changes are not reversible (reversal is often extremely difficult). *

The chemical composition of materials is different after a chemical change but the same after a physical change.

Chemical change:

Bonds have changed; the substances present before the change, are different from those present after the change:

A substance reacts to form a new chemical entity

same atoms are used, but different molecules have been created.

Examples:

The electrolysis of water: the products are two different substances;

Combustion of hydrogen in oxygen; forms water and generates heat;

The body's combustion of glucose produces carbon dioxide and water;

Bread made into toast (partially oxidized bread) is not reversible;

Wood burned in a fireplace is not reversible;

Burning of the World Trade Center.

Physical Change:

State, size, shape, or color of a substance is changed basic character-unchanged; Change of physical properties, leaving basic character-unchanged.

Examples:

Melting of ice: change of state;

Bearing gold into a ring;

Grinding;

Cutting;

Folding;

Collapse of the World Trade Center

* Physical changes aren't always easy to reverse, and chemical changes aren't necessarily impossible to reverse.