Physical and Chemical Changes

1. Physical and Chemical Changes of Matter

2. What is “Change”? • It is the act of altering a substance. • An event, NOT a trait. Before condition  After condition. • Can be PHYSICAL or CHEMICAL.

3. Physical Change • Does not alter the chemical composition or identity of the substance, only the form. • Melting ice (change in state or phase) • Freezing Kool-aid • Tearing paper • Boiling water (change in state or phase) • Stretching silly putty • Making a mixture (ex. Sugar water) • Unmixing a mixture (ex. sorting)

4. Chemical Changes • Does alter the chemical composition or identity of a substance and makes new substances. • Burning paper • Digesting food • Rotting • Iron reacting with oxygen gas • A chemical change is also called a chemical reaction.

5. Is it Physical or Chemical?

6. All Changes of Matter Involve Energy being Tranfered and Transformed • Energy always moves between the chemical system and the surroundings during all changes of matter, either physical or chemical. System = the chemicals of interest undergoing a change (either physical or chemical) Surroundings = everything else (including the beaker and the thermometer) • During any change in matter, energy cannot be created or destroyed. It can only be transferred from place to place and transformed from type to type (Law of Conservation of Energy).

7. Exothermic change - Heat moves out of the chemical system and into the surroundings. The surroundings get hotter and has a positive temperature change. (exo = out; therm = heat). Surroundings System System System System Tf – Ti = +T Tf = final temperature Ti = initial temperature T = change in temperature of surroundings

8. Endothermic change – Heat moves into the chemical system from the surroundings, so the surroundings gets colder and has a negative temperature change! (endo = in; therm = heat) Surroundings System Tf – Ti = -T Tf = final temperature Ti = initial temperature T = change in temperature of surroundings

9. Changes of Matter Demos • Thermite Chemical Reaction • (used in underwater welding of ship hulls) • Exothermic Chemical Change • Mixing Ammonium Nitrate with water to make a solution • (used in icy packs for muscle sprains) • Endothermic Physical Change

10. Parts of a Chemical Reaction Reactants  Products Reactants: Starting substances that are broken down by the chemical change. Products: Ending substances that are created by the chemical change.  Means “Yields”

11. Law of Conservation of Mass (1789) • Applies to all chemical reactions. • Matter is never created or destroyed in chemical reactions. • Mass of reactants = Mass of products • Why??? Because atoms are simply rearranged in new ways during chemical reactions. No new atoms are created, no atoms are destroyed, and none are transformed into other atoms of different elements. Chemical bonds between atoms break and new chemical bonds form. However, during the process, molecules may break down and new molecules of new substances may form. **As an analogy, think of playing with Legos when you were younger….You put them together and took them apart over and over again! You did not create or destroy the legos.

12. A Lego chemical reaction….. CH4 (g) + O2 (g)  CO2 (g) + H2O (g) Methane gas + oxygen gas yields carbon dioxide gas + water vapor Yellow legos = hydrogen atoms, Red legos = oxygen atoms, White legos = carbon atoms

13. Chemical Reactions Always Produce New Substance with New Physical and Chemical Properties Ex. Iron Plus Oxygen Yields Rust 4 Fe (s) + 3O2 (g)  2 Fe2O3 (s) Note that the sum of the masses of the reactants equals the mass of the product!

14. Evidence for Chemical Reaction 1) Evolution of light.

15. Evidence for Chemical Reaction 2) Temperature Change.

16. Evidence for Chemical Reaction 3) Formation of a new Gas/Bubbles.

17. Evidence for Chemical Reaction 4) Color Changes.

18. Evidence for Chemical Reaction 5) Formation of a solid precipitate.

19. Evidence for Chemical Reaction 5) Formation of a solid precipitate. A precipitate is a solid chemical substance formed during a chemical reactions between two aqueous (water-based) solutions. A precipitate is definitely a new substance!