Physical and Chemical Changes

1. Physical and Chemical Changes Chapter 2 Section 4

2. Energy and Change • Two kinds of changes in matter: • Physical Change • Chemical Change

3. Physical Change • A physical change alters the form of a substance, but does not change it to another substance. • Examples are: tearing paper, when a nail bends, spinning wool into yarn.

4. Chemical Change • When a substance undergoes a chemical change, it is changed into a different substance with different properties. • Examples burning wood, turning sugar into caramel.

5. Thermal and Chemical Energy • Every substance has energy from the movement of its particles, called thermal energy. • The higher the temperature of a substance, the greater its thermal energy. • Another form of energy comes from the chemical bonds within matter. • This form of energy is called chemical energy.

6. Changes in Matter • Any substance can either gain energy or lose energy. • In either case, the substance changes in some way. • Matter changes whenever energy is added or taken away. • When something is heated, it gains thermal energy • When something is cooled, it loses thermal energy.

7. Law of Conservation of Energy • In every physical change and chemical change, the total amount of energy stays the same. • The word conservation comes fromconservewhich means “to protect from loss.” • Energy can change from one form to another but can never be lost. • For example, an unlit match contains chemical energy. When it is lit, the chemical energy changes to light and thermal energy. • The total amount of chemical energy that was in the match before it was lit is equal to the light and thermal energy after it is lit.

8. Changes Between Liquids and Solids • Under certain conditions, a substance can change from any one state of matter to any other. • Each substance changes state at temperatures typical for that substance. • But the overall pattern for the way substances change state is the same.

9. Melting • The change in state from a solid to a liquid is melting. • In most pure substances, melting occurs at a temperature that is specific to the substance, called the melting point. • The melting point of a substance depends on how strongly its particles attract one another.

10. Freezing • Freezing is the change of state from liquid to solid-just the reverse of melting. • When you put liquid water into a freezer, the water loses energy to the cold air. • The water molecules move more slowly. • This means that the temperature of the water is dropping. • When the temperature reaches 0 degrees Celsius, the molecules are moving so slowly that they form regular patterns. These patterns are the ice crystals that form ice.

11. Changes Between Liquid and Gas • The change from liquid water to water vapor is an example of vaporization. • Vaporization occurs when a liquid gains enough energy to become a gas. • There are two main types of vaporization • Evaporation – takes place at the surface of a liquid • Boiling – takes place inside a liquid as well as on the surface.

12. Evaporation • An example of evaporation is when a puddle dries up after it rains. • The water in the puddle gains enough energy from the ground, air, and the sun to change the particles on the surface of the liquid from a liquid to a gas. • The energy enables the molecules on the surface of the puddle to escape into the air.

13. Boiling • Each liquid boils at a specific temperature, called its boiling point. • Like the melting point of a solid, the boiling point of a liquid depends on how strongly the particles of the substance are attracted to one another.

14. Boiling Point and Air Pressure • Boiling point also depends on the pressure of the air above a liquid. • The lower the air pressure above the liquid, the less energy the liquid molecule need to escape into the air. • As you go up in elevation, air pressure decreases. • At the air pressure near sea level, the boiling point is 100 degrees Celsius. • In the mountains, air pressure is lower and so is the boiling point of water.

15. Condensation • The opposite of vaporization is called condensation. • Condensation occurs when a gas loses enough thermal energy to become a liquid. • Examples are clouds form when water vapor in the atmosphere condenses into liquid droplets. • When the droplets get heavy enough, they fall to the ground as rain.

16. Condensation cont. When you observe condensation by breathing onto a mirror, you can see the warm water vapor in your breath condense on the cooler surface of the mirror. The droplets then evaporate in water vapor again. You cannot see water vapor in the air. When you see steam from water boiling it is tiny droplets of liquid water suspended in the air.

17. Changes Between Solid and Gas • Sublimation occurs when the surface particles of a solid gain enough energy to become a gas. • Particles do not pass through the liquid state at all. • An example of sublimation is the change that dry ice undergoes. • Dry ice is the common name for solid carbon dioxide.

18. Why do we use Dry Ice? • As dry ice changes from a solid to a liquid, it absorbs energy, so materials near it stay cold and dry. • For this reason using dry ice is an excellent way to keep temperatures low when a refrigerator is unavailable. • When dry ice becomes a gas, it cools water vapor in the air, which form fog.

19. Chemical Changes • Chemical Reaction is another name for chemical change. • In some chemical reactions, one substance breaks down into two or more other substances. • In other reactions, two or more substances combine, forming one or more new substances. • All chemical reactions have one thing in common. They all produce new substances.

20. Energy and Chemical Reactions • Like all changes in matter, chemical reactions occur when substances gain or lose energy. • All chemical reactions either absorb energy or release energy. • What is the source of energy that a plant uses to grow? • A plant changes the sun’s light energy into the chemical energy that it needs to make it grow.

21. Stored Chemical Energy • Chemical energy that has been absorbed and stored can be later released. • An example is burning wood in a woodstove. • Wood combines with oxygen in the air, producing substances such as carbon dioxide, water, and soot. • Chemical energy is transformed into thermal and electromagnetic energy during this process. • Your body releases chemical energy that you obtain from foods that you eat.

22. Controlling Chemical Energy • How can you make a chemical reaction occur faster or slower? • You just have to add energy or take energy away. • If you are baking a cake, and the kitchen is warm, reactions in the cake batter happen faster. • The hotter the oven the faster the cake bakes. • You discontinue the chemical reaction by taking the cake out of the oven. • Putting the cake in the refrigerator, slows down the chemical reactions that make the cake spoil.