2.6 Chemical Reactions

reactant

A substance present at the start of the reaction

product

A substance produced in the reaction

What four clues serve as a guide when attempting to identify chemical changes.

Possible clues to chemical change include a transfer of energy, a change in color, the production of a gas, or the formation of a precipitate.

chemical property

The ability of a substance to undergo a specific chemical change

What is another word for a chemical change?

chemical reaction

If you see a clue to chemical change, can you be sure that a chemical change has occurred?

could be a physical change (ex. For example, energy is always transferred when matter changes from one state to another. Bubbles form when you boil water or open a carbonated drink.)

Can chemical properties be used to identify substances?

Chemical properties can be used to identify a substance. But chemical properties can be observed only when a substance undergoes a chemical change.

What does every chemical change involve?

Every chemical change involves a transfer of energy. (ex. For example, energy stored in natural gas is used to cook food. When the methane in natural gas chemically combines with oxygen in the air, energy is given off in the form of heat and light. Some of this energy is transferred to and absorbed by food that is cooking over a lit gas burner. The energy causes chemical changes to take place in the food. The food may change color and brown as it cooks, which is another clue that chemical changes are occurring.)

What happens during a chemical change

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

precipitate

a solid that forms and settles out of a liquid mixture (ex. The ring of soap scum that can form in a bathtub is an example of a precipitate. Some bathroom cleaners that you can use to remove soap scum start to bubble when you spray them on the scum. The bubbles are produced because a gas is released during the chemical change that is taking place in the cleaner.)

During any chemical reaction,.

the mass of the products is always equal to the mass of the reactants. (ex. Two of the products of burning wood—carbon dioxide gas and water vapor—are released into the air. When the mass of these gases is considered, the amount of matter is unchanged)

What is the only way to be sure that a chemical change has occurred?

The only way to be sure that a chemical change has occurred is to test the composition of a sample before and after the change.

the law of conservation of mass

The law of conservation of mass states that in any physical change or chemical reaction, mass is conserved. Mass is neither created nor destroyed.

During a chemical change,

the composition of matter always changes