1. 2.4
Chemical Reactions

2. What always happens during a chemical change?
2.43
Chemical Changes
What always happens during a chemical change?

3. 2.4
The ability of a substance to undergo a specific chemical 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.

4. 2.4
During a chemical change, the composition of matter always changes.
(Recall that during a physical change, the composition of matter never changes.)

5. 2.4
A magnet separates iron from sulfur. This is an example of a physical change.
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.

6. 2.4
A mixture of iron and sulfur is heated. The iron and sulfur react and form iron sulfide. This is an example of a chemical change.
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.

7. 2.4
A chemical change is also called a chemical reaction.
One or more substances change into one or more new substances during a chemical reaction.
A substance present at the start of the reaction is a reactant.
A substance produced in the reaction is a product.

8. Recognizing Chemical Changes
2.4
Recognizing Chemical Changes
What are four possible clues that a chemical change has taken place?

9. 2.4
Possible clues to chemical change include:
a transfer of energy (heat produced or absorbed)
a change in color
the production of a gas
the formation of a precipitate.

10. Clues to chemical changes have practical applications.
2.4
A precipitate is a solid that forms and settles out of a liquid mixture.
Clues to chemical changes have practical applications.
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.

11. 2.4
Conservation of Mass
How are the mass of the reactants and the mass of the products of a chemical reaction related?

12. 2.4
During any chemical reaction, the mass of the products is always equal to the mass of the reactants.

13. 2.4
The law of conservation of mass states that in any physical change or chemical reaction, mass is conserved.
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?

14. 2.4 Section Quiz.
1. Which of the following is a chemical reaction?
melting of lead
dissolving sugar in water
rusting of iron
crushing of stone

15. 2.4 Section Quiz.
2. Which of the following is NOT a possible clue that a chemical change is taking place?
a change of state
a change in color
production of a gas
formation of a precipitate

16. 2.4 Section Quiz.
3. During any chemical change, the mass of the products is
always equal to the mass of the reactants.
always greater than the mass of the reactants.
always less than the mass of the reactants.
sometimes different than the mass of the reactants.

17. END OF SHOW