Chemical and Physical Changes



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Physical and Chemical Changes/Reactions

Chemistry is not just the study of matter, but also of how matter changes. The motor in your car, an oven, and a book of matches are all examples of matter. In some way, each of these is useful as well. The reason they are useful is shown when they undergo changes.

Physical Changes/Reactions

Not all changes produce new material. A change that simply alters the form or appearance of a material but does not make the material into new substance is called a physical change. Physical changes change physical properties, not chemical properties. For example, when you cut a piece of paper, you change the paper’s mass and volume (physical properties), but you don’t change its flammability (chemical property).

Take a minute to think about the water cycle. During the cycle, water takes many various forms, changing between a solid, liquid, and gas. When water is cooled in a freezer or at the cold regions of Earth, liquid water becomes solid ice. You can change ice to liquid water by leaving an ice cube on your kitchen counter. If it is heated on your stove or by energy from the sun, liquid water becomes water vapor. When water vapor becomes liquid again, it returns to the ground as rain. Evaporating, condensing, freezing, and precipitating are all examples of common physical changes. Why? Water is still water, no matter what phase it happens to be in. No new material is formed, so these phase changes are physical changes. Physical properties like volume and density changed, but no chemical properties of water changed. Water is still the same substance in all three forms. It is still made of two parts hydrogen and one part oxygen (H2O). Take a second to consider this example: dry ice in a balloon. Why do you think this is a physical change and not a chemical change?

Here are some common ways to produce a physical change:

• add heat/energy (heat up a substance)

• remove heat/energy (cool down a substance)

• rip, tear, or break a substance

• change a substance’s volume, density, or mass

• add a solute (dissolve something in a substance)

• change the phase of a substance (change its state of matter)

Chemical Changes/Reactions

A change in matter that produces a new substance is called a chemical change. The new substances are made of the same elements as the original substance, but they are now in different combinations. During chemical changes, elements and compounds rearrange to make new materials. Elements may combine to make compounds, compounds may be broken down into elements, or some compounds may change into other compounds. Watch as Mr. Gall shows you how when an acid and base are combined, bonds are broken and compounds change into new compounds.

You have seen the results of many chemical reactions. Rusting turns the strong iron metal of a car into iron oxide, a compound you can easily knock holes through. When wood burns, the compounds that make up the wood combine with oxygen in the air to make carbon dioxide and water and leave behind ash (all new substances).

Observing Chemical Reactions

Imagine you are roasting marshmallows over a campfire. You can use your senses to detect chemical changes. You see the burning logs change from a hard solid to a soft pile of ash. You can hear popping and hissing sounds as gases produced by the reaction cause the wood to expand. You can smell the smoke. You can feel the heat energy released. You can even taste the results of one chemical change. A soft, white marshmallow right out of the bag doesn’t taste at all like one that is roasted by a reaction of sugar with oxygen.

You can detect many chemical reactions by observing changes in the properties of matter. These changes result when new substances form. But how can you tell when a new substance is formed? Sometimes a gas is produced, which you might see as bubbles in a liquid. Other times, a solid, called a precipitate, may appear when two solutions are mixed. Still other times, an unexpected color change may tell you that a new substance has formed. If a pH indicator changes color, it is a sign that a chemical change has taken place making a substance more acidic or basic. Changes in energy also happen during chemical reactions, usually indicated by temperature changes.

All of these kinds of evidence are good indicators of a chemical change. But seeing 1 of these 5 indicators does not GUARANTEE a chemical change. Sometimes physical changes give similar results. For example, think for a moment about the changes to water described earlier. When water boils, gas bubbles form. Since bubbles are present, does this mean gas was produced and a new material was formed? No. Water vapor, ice, and liquid water all are made up of 2 parts hydrogen and 1 part oxygen. The key characteristic of a chemical reaction is the production of new materials that are chemically different from the starting materials. The bubbles seen when liquid water boils to gas is the escaping of water vapor from the liquid water, a phase change and therefore a physical change. There is no new material formed!

Chemical reactions result in new material being formed and a change in both physical and chemical properties

Reactants: 1. the substances present at the beginning of a chemical reaction,

2. the substances mixed together to make the reaction happen, and

3. found to the left of the arrow in an equation

6 C02 + 6 H2O C6H12O6 + 6 O2

Products: 1. the substances formed by a chemical reaction, and

2. found to the right of the arrow in an equation

( Two things must happen for a chemical change to occur: Bonds must be broken or formed AND the chemical & physical properties of the reactants change

Color Change: A color change is often a sign that

a chemical reaction has occurred. The brilliant

colors of fall leaves result when green chlorophyll

in leaves is broken down. Then colors of other

substances in the leaves become visible.

Chemical and Physical Changes

Define physical change:

List three examples of physical changes that take place during the water cycle.

1.

2.

3.

A balloon filled with water is placed in a microwave until it bursts. Name two different physical changes that occur during this reaction.

1.

2.

Define chemical change:

Give three examples of chemical changes you have seen in your life.

What are the five major pieces of evidence that indicate chemical change?

1.

2.

3.

4.

5.

A bag of citric acid, baking soda, and red cabbage juice (a pH indicator) are placed in a zip lock bag. When the contents of the bag are mixed, the bag begins to expand and it becomes very cold. Soon, the red cabbage juice begins to turn blue and purple, creating a neutral (not an acid or a base) substance. Give 3 pieces of evidence that show this is a chemical change.

True or False? For any statements you believe are false, change the wording in the statement to make it true. You must be able to interact with these notes to make sense of them.

1. Physical reactions can change the physical properties of a substance.

2. In a physical reaction, the chemical properties of the substances change.

3. If the bonds in the reactants are broken, a chemical reaction has occurred.

4. Reactants can be found to the right of the arrow in a chemical equation.

5. The products are the things that are mixed together to start a reaction.

6. In Element X bonds with Chemical Y, the product will have a combination of the properties of the original elements.

7. The products of a reaction can be found to the right of the arrow in a chemical equation.

8. Tearing a piece of paper in half is a physical change.

9. Burning a match is a chemical change.

10. Gas production is a sign of a chemical change.

11. Boiling is a sign of a chemical change.

12. All phase changes are physical changes.

13. Food coloring is dissolved in water. This is a chemical change.

14. Dry ice sublimates from a solid to a gas. This is a physical change.

15. A precipitate is formed when two solutions mix to form a solid.

16. An acid and a base mix to form something neutral. The pH change indicates a chemical reaction occurred.

17. The leaves change color in the fall. This is a sign of a chemical change.

18. A student makes a volcano for a science fair project. She mixes vinegar and baking soda together and carbon dioxide gas is formed. The vinegar and baking soda are the products.

19. A student makes a volcano for a science fair project. She mixes vinegar and baking soda together and carbon dioxide gas is formed. The gas production is a sign of a chemical change.

20. After a chemical reaction, you would expect to find the same molecules that you started with.

21. If the chemical properties of the reactants change, a physical change has occurred.

22. Photosynthesis is a chemical reaction.

23. Adding energy, adding a solute, and tearing matter are ways to cause a physical change to occur.

24. Dissolving is a physical change.

25. Water is boiling in a pot and bubbles are rising to the top of the water. The gas production indicates a chemical change.

26. Color change, temperature change, and pH change are all signs of a chemical change.

27. Anytime something is burned by fire, there is a chemical reaction.

28. Adding salt to water increases the boiling point. This is an example of a chemical change.

29. Physical and chemical reactions change the properties of the substances involved.

30. Chemical changes cause changes to both physical and chemical properties.

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Photosynthesis is a chemical reaction.

Chemical Reactions produce new substances. The signs of a reaction vary. Many reactions include one or more of the following types of evidence. Here are some examples…

pH Change:

A pH indicator changes color and a substance becomes more or less acidic or basic.

Gas Production:

Pouring hydrogen peroxide on skin sometimes produces bubbles. This is a chemical reaction.

Precipitation:

Two clear solutions react when mixed, forming a precipitate. The presence of the precipitate tells you a chemical change has taken place.

Changes in Temperature:

The burning of a campfire causes a great increase in temperature. This is a chemical change.

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