Section Assessment

Section Summary

I Atoms of different elements combine to form compounds.

I Covalent bonds form from shared electrons between atoms.

I Ionic compounds form from the attraction of positive and negative ions.

I There are two types of mixtures— heterogeneous and homogeneous.

I Acids are solutions containing hydrogen ions. Bases are solutions containing hydroxide ions.

Understand Main Ideas

1. imandfffll Explain why molecules do not have electric charges.

2. Differentiate between molecules and compounds.

3. Calculate the number of atoms needed to balance the following equation: CaCO + HCl ^ CO2 + H2O + CaCl

4. Diagram how an acid can be neutralized.

5. Compare and contrast mixtures and solutions by using specific examples of each.

Think Critically

6. Design a procedure to demonstrate whether whole milk, which consists of microscopic fat globules suspended in a solution of nutrients, is a homogeneous or heterogeneous mixture.

7. Predict what kind of chemical bond forms between nitrogen and hydrogen atoms in ammonia (NH3). Sketch this molecule.

CZHEZa^Earth Science

8. Antacids are used to relieve indigestion and upset stomachs. Write an advertisement for a new antacid product. Explain how the product works in terms that people who are not taking a science class will understand.

72 Chapter 3 • Matter and Change

Self-Check Quiz glencoe.com

72 Chapter 3 • Matter and Change

Self-Check Quiz glencoe.com

structure.

N4ldBa All matter on Earth and in the universe occurs in the

Real-World Reading Link When your skin is wet, even on a hot day, it usually feels cool—especially if it is windy. How can warm air feel cold? When the water evaporates, it absorbs heat from your skin. The harder the wind blows, the

Solids are substances with densely packed particles, which can be ions, atoms, or molecules. Most solids are crystalline structures because the particles of a solid are arranged in regular geometric patterns. Examples of crystals are shown in Figure 3.16. Because of their crystalline structures, solids have both a definite shape and volume.

Perfectly formed crystals are rare. When many crystals form in the same space at the same time, crowding prevents the formation of perfect crystals with smooth boundaries. The result is a mass of intergrown crystals called a polycrystalline solid. Most solid substances on Earth, including rocks, are polycrystalline solids. Figure 3.16 shows the polycrystalline nature of the rock granite.

Some solid materials have no regular internal patterns. Glass is a solid that consists of densely packed atoms arranged randomly. Glasses form when molten material is chilled so rapidly that atoms do not have enough time to arrange themselves in a regular pattern. These solids do not form crystals, or their crystals are so small that they cannot be seen. Window glass consists mostly of disordered structure.

Geology FoldablesMelting Granite Into Liquid

Figure 3.17 Each of these containers has the same volume of liquid in it. Explain why the liquids are not all at the same level in the containers.

IFoldables^

Incorporate information from this section into your Foldable.

Liquids

At any temperature above absolute zero ( — 273°C), the atoms in a solid vibrate. Because these vibrations increase with increasing temperature, they are called thermal vibrations. At the melting point of the material, these vibrations become vigorous enough to break the forces holding the solid together. The particles can then slide past each other, and the substance becomes liquid. Liquids take the shape of the container they are placed in, as you can see in Figure 3.17. However, liquids do have definite volume.

^p Reading Check Explain the effect that increasing temperature has on the atoms in solids.

Gases

The particles in liquids vibrate vigorously. As a result, some particles can gain sufficient energy to escape the liquid. This process of change from a liquid to a gas at temperatures below the boiling point is called evaporation. When any liquid reaches its boiling point, it vaporizes quickly as a gas.

In gases, the particles are separated by relatively large distances and they travel at high speeds in one direction until they bump into another gas particle or the walls of a container. Gases, like liquids, have no definite shape. Gases also have no definite volume unless they are restrained by a container or a force such as gravity. For example, Earth's gravity keeps gases in the atmosphere from escaping into space.

Plasma

When matter is heated to a temperature greater than 5000°C, the collisions between particles are so violent that electrons are knocked away from atoms. Such extremely high temperatures exist in stars and, as a result, the gases of stars consist entirely of positive ions and free electrons. These hot, highly ionized, electrically conducting gases are called plasmas. Figure 3.18 shows the plasma that forms the Sun's corona. You have seen matter in the plasma state if you have ever seen lightning or a neon sign. Both lightning and the matter inside a neon tube are in the plasma state.

Figure 3.18 The Sun's temperature is often expressed in kelvins; -273 K is equal to 0°C. The Sun's corona, which is a plasma, has a temperature of about 15,000,000 K.

Compare the temperature of the corona to lightning, which is 30,000 K.

Figure 3.18 The Sun's temperature is often expressed in kelvins; -273 K is equal to 0°C. The Sun's corona, which is a plasma, has a temperature of about 15,000,000 K.

Compare the temperature of the corona to lightning, which is 30,000 K.

Temperature Lightning

74 Chapter 3 • Matter and Change

(t)John Evans, (b)SOHO/NASA

| Changes of State

Cl f Solids melt when they absorb enough thermal energy to cause f their orderly internal crystalline arrangement to break down. This happens at the melting point. When liquids are cooled, they solidify at that same temperature and release thermal energy. The temperature at which liquids solidify is called the freezing point.

When a liquid is heated to the boiling point and absorbs enough thermal energy, vaporization occurs and it becomes a gas. When a gas is cooled to the boiling point it becomes a liquid in a process called condensation, shown in Figure 3.19. Energy that was absorbed during vaporization is released upon condensation.

Evaporation can occur below the boiling point when thermal vibrations enable individual atoms or molecules to escape from a solid. You might have noticed that even on winter days with temperatures below freezing, snow gradually disappears. This slow change of state from a solid (ice crystals) to a gas (water vapor) without an intermediate liquid state is called sublimation.

Conservation of Energy

The identity of matter can be changed through chemical reactions and nuclear processes, and its state can be changed under different thermal conditions. You have learned that a chemical equation must be balanced because matter cannot be created or destroyed. This fundamental fact is called the law of conservation of matter. Like matter, energy cannot be created or destroyed, but it can be changed from one form to another. For example, electric energy might be converted into light energy. This law, called the conservation of energy, is also known as the first law of thermodynamics.

Figure 3.19 As the hot, moist air from the shower encounters the cool glass of the mirror, the water vapor in the air condenses on the glass. Predict What would happen if the glass were the same temperature as the air?

Figure 3.19 As the hot, moist air from the shower encounters the cool glass of the mirror, the water vapor in the air condenses on the glass. Predict What would happen if the glass were the same temperature as the air?

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Responses

  • giuseppe
    When molten material freezes so quickly that the atoms do not have sufficient?
    8 years ago

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