Matter: Properties and Changes

Transcription

1 Matter: Properties and Changes Matter and Its Changes What do you think? Read the three statements below and decide whether you agree or disagree with them. Place an A in the Before column if you agree with the statement or a D if you disagree. After you ve read this lesson, reread the statements to see if you have changed your mind. Before Statement After Key Concepts How are physical changes different from chemical changes? How do physical and chemical changes affect mass? 4. Mixing powdered drink mix with water causes a new substance to form. 5. If you combine two substances, bubbling is a sign that a new type of substance might be forming. 6. If you stir salt into water, the total amount of matter decreases. Changes of Matter Matter can change physically and chemically. The identity of a substance does not change in a physical change. A substance does change into a different substance in chemical changes. Chemical changes happen when substances react with one another. In any change, matter can only be rearranged. It cannot be created or destroyed. Matter can change in many ways. You can notice many of these changes in the world around you. Temperature, state of matter, shape, and color are all changes in matter that you have seen and felt many times. These changes can be either physical or chemical. What are physical changes? A change in the size, shape, form, or state of matter that does not change the matter s identity is a physical change. You can change the shape of a ball of clay, and it is still clay. When a physical change occurs, the chemical properties of the matter stay the same. Underline Main Ideas As you read, underline the main ideas under each heading. After you finish reading, review the main ideas that you have underlined. Reading Essentials Matter: Properties and Changes 199

2 Key Sugar crystal Visual Check Crystals of sugar are made up of many sugar particles. The crystals are surrounded by particles of water. 1 Sugar particle 1. Identify Circle the image that shows the sugar completely dissolved in the water. Individual sugar and water particles remain unchanged even after all sugar crystals have dissolved. As the sugar begins to dissolve, the crystals break apart. C12H22O11 1 Water particle H2O Dissolving 2. Explain why dissolving is classified as a physical change. You read in Lesson 1 that solubility is a physical property. Solubility is the ability of one material to dissolve in another. To dissolve means to mix evenly. The picture above shows what happens to sugar molecules when sugar crystals dissolve in water. Dissolving is a physical change because the identities of the substances do not change. Changing State Melting and Boiling When water freezes, it changes into ice. When ice melts, it changes into water. When water boils, it changes into a gas. In each case, the substances that make up water are still the same. Energy and Change in State The energy of particles and Make a half-book to compare information about physical and chemical changes. ical Phys es g Chan 200 Chem i Chan cal ges distances between the particles are different for a solid, a liquid, and a gas. Changes in energy cause changes in the state of matter. Energy must be added to a substance to change it from a solid to a liquid or from a liquid to a gas. Adding energy to a substance can increase its temperature. When the temperature reaches the substance s melting point, the solid changes to a liquid. At the boiling point, the liquid changes to a gas. Matter: Properties and Changes Reading Essentials In Lesson 1 you read about three states of matter solid, liquid, and gas. When matter changes from one state to another, it goes through a physical change.

3 0 20 What are chemical changes? Some changes in matter change the identity of the substance. A chemical change is a change in matter in which the substances that make up that matter change into substances with different chemical and physical properties. Another name for a chemical change is a chemical reaction. In a chemical reaction, the particles that make up two or more substances react, or combine, with each other and form a new substance. Signs of a Chemical Change A chemical change causes changes in physical properties. For many chemical reactions, changes in color, density, or state of matter are signs that a chemical change has taken place. The only sure sign of a chemical reaction is the formation of a new substance. Formation of Gas When a liquid boils, the bubbles show that it is changing into a gas. This is a physical change. When you add a medicine tablet to water, gas bubbles form. The water does not boil, so the gas must be a new substance formed by a chemical reaction. Formation of a Precipitate Some chemical reactions result in a solid forming when two liquids combine. This solid is called a precipitate (prih SIH puh tut). A precipitate is not the result of a change in state from a liquid to a solid. It is a chemical change. A precipitate forms when the particles that were dissolved in the two liquids react to form the particles of the solid, a new substance. Color Change A change in color is another sign of a chemical change. If you painted the walls of your room a different color, this would not be a chemical change. But if a substance changes color, it is a sign that a chemical change has taken place. When you roast marshmallows or leave an apple slice on a plate, the change in color is a sign that there has been a chemical change. 180 ml ml Key Concept Check 3. Describe how chemical changes are different from physical changes. 4. Describe how you can tell whether bubbles are a result of a chemical change or a physical change. Visual Check 5. Explain In each of the images to the left, what new substance can you see that has formed? Reading Essentials Matter: Properties and Changes 201

4 ACADEMIC VOCABULARY expose (verb) to uncover; to make visible 6. Identify one physical change that can be reversed and one that cannot. Key Concept Check 7. Explain how physical and chemical changes affect mass. Energy and Chemical Change Chemical changes involve a change in energy. Think about a fireworks display. The release of thermal energy, light, and sound are signs of chemical changes. All chemical reactions involve energy changes. Chemical reactions can also take in energy. Thermal energy is often needed for a chemical change to take place. To bake bread, for example, you have to put dough in a hot oven. Some chemical reactions need energy in the form of light. Plants and some unicellular organisms use the Sun s energy for photosynthesis. Photosynthesis is the chemical reaction by which these organisms make sugar and oxygen. This process occurs only if the organisms are exposed to light. Can changes be reversed? Think again about a fireworks display. After the fireworks explode, they cannot go back to the chemicals they were before. Chemical changes cannot be reversed. Some physical changes cannot be reversed, either. If you cut an apple, it cannot be put back together. Other physical changes can be reversed. For example, if you dissolve salt in a pan of water and then boil the mixture, the water will change to a gas and the salt will be left behind. Conservation of Mass Physical changes do not affect the masses of substances. For example, when an ice cube melts, the mass of liquid water will be the same as the mass of the ice cube. If you cut a sheet of paper into pieces, the total mass of the pieces will be the same as the mass of the paper you started with. Mass is conserved, or unchanged, during a physical change. Mass is also conserved during chemical changes. Antoine Lavoisier, a French chemist, discovered this in the 1700s. The masses of two substances that will chemically react can be measured and added together. After the two substances react to form new substances, the total mass after the reaction can be measured. You will find that the total mass before and the total mass after the reaction are the same. The law of conservation of mass states that the total mass before a chemical reaction is the same as the total mass after the chemical reaction. This is always true because particles are only rearranged. They cannot be created or destroyed, so the total mass cannot increase or decrease. 202 Matter: Properties and Changes Reading Essentials

5 Comparing Physical and Chemical Changes The difference between a physical change and a chemical change is that the identity of matter changes during a chemical change but does not change during a physical change. You might not be able to tell just by looking at a substance whether its identity has changed. The particles that make up the matter can look the same. You might have to look for more than one clue to tell whether a change is a physical change or a chemical change. The table below gives a summary of physical changes and chemical changes. Comparing Physical and Chemical Changes Type of Change Examples Characteristics 8. Identify some clues you can use to determine whether a change is a physical change or a chemical change. Physical change melting boiling changing shape mixing dissolving The substance is the same before and after the change. Only physical properties change. changing temperature Chemical change changing color burning rusting formation of gas formation of precipitate spoiling food tarnishing silver digesting food The substance changes into something else. Both physical and chemical properties change. Visual Check 9. State What type of change occurs when ice melts? When a log burns? Reading Essentials Matter: Properties and Changes 203

6 Mini Glossary chemical change (KEM ih kul CHANJ): a change in matter in which the substances that make up the matter change into other substances with different chemical and physical properties physical change (FIH zih kul CHANJ): a change in the size, shape, form, or state of matter that does not change the matter s identity law of conservation (kun SURV ay shun) of mass: states that the total mass before a chemical reaction is the same as the total mass after the chemical reaction 1. Review the terms and their definitions in the Mini Glossary. Write a sentence that describes some ways in which physical changes and chemical changes are similar and some ways in which they are different. 2. A chemical change is shown in the picture below. Name three signs that you could use to tell that it is a chemical change. Rust 3. How did underlining the main ideas help you learn the material in this lesson? What do you think Reread the statements at the beginning of the lesson. Fill in the After column with an A if you agree with the statement or a D if you disagree. Did you change your mind? ConnectED Log on to ConnectED.mcgraw-hill.com and access your textbook to find this lesson s resources. END OF LESSON 204 Matter: Properties and Changes Reading Essentials