Quimica

| Section 2: Properties of Matter |
Key Ideas | Why are color, volume, and density classified as physical properties? |
| Why are flammability and reactivity classified as chemical properties? |
Why It MattersProperties determine uses. For instance, the properties of a substance called aerogel enable it to trap fast-moving comet particles. |
When playing sports, you choose a ball thathas the shape and mass suitable for your game. It would be hard to play soccer with a football or to play softball with a bowling ball. The properties of the balls make the balls useful for different activities.Physical PropertiesShape and mass are examples of physical properties. Some other physical properties are color, volume, and texture. The balls in Figure 1 have different physicalproperties.  Physical properties are characteristics that can be observed without changing the identity of the substance. For example, you can determine the color, mass, and shape of a ball without changing the substance that makes up the ball.
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Figure 1 The physical properties of these balls make the balls useful in differentsports.-------------------------------------------------
Physical properties are often very easy to observe. For instance, you can easily observe that a tennis ball is yellow, round, and fuzzy. Matter can also be described in terms of physical properties that are not as obvious. For example, a physical property of air is that it is colorless.Physical properties can help identify substances.Because many physical properties remainconstant, you can use your observations or measurements of these properties to identify substances. For example, you recognize your friends by their physical properties, such as height and hair color. At room temperature and under atmospheric pressure, all samples of pure water are colorless and liquid. Pure water is never a powdery green solid. The physical properties of water help you identifywater. | |
Physical properties can be observed or measured.You can use your senses to observe some of the basic physical properties of a substance: shape, color, odor, and texture. Another physical property that you can observe is state—the physical form of a substance. Solid, liquid, and gas are three common states of matter. For example, water can be in the form of solid ice, liquid water, orgaseous steam.
Other physical properties, such as melting point and boiling point, can be measured. The temperature at which a substance changes from a solid to a liquid is the melting point. The temperature at which a liquid changes to a gas is the boiling point. A characteristic of any pure substance is that its boiling point and its melting point are constant if the pressure remains the same.At sea level, water boils at 100 ºC and freezes at 0 ºC. At constant pressure, pure water always has the same boiling point and the same melting point. Regardless of the mass or volume of water, the physical properties of the water are the same. This principle is true for all pure substances.
Other physical properties that can be measured are strength, hardness, and magnetism. The ability toconduct electricity or heat is also a physical property. For instance, copper conducts electricity well, while plastic does not.Physical properties help determine uses.Every day, you use physical properties to recognize substances. Physical properties help you decide whether your socks are clean (odor), whether your books will fit in your backpack (volume), or whether your clothes match (color).Physical properties are often used to select substances that may be useful. Copper is used in power lines, telephone lines, and electric motors because it conducts electricity well. As Figure 2 shows, aluminum is used in foil because it is lightweight yet durable and flexible. Car frames are made of steel, which is a strong solid that provides structure. Tires are made of a flexible solid that...