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  3. "Physical and Chemical Properties and Changes" → Sugar, water

"Physical and Chemical Properties and Changes" → Sugar, water

游客2024-01-04  61
问题                "Physical and Chemical Properties and Changes"
   → Sugar, water, and aluminum are different substances. Each substance has specific properties that do not depend on the quantity of the substance. Properties that can be used to identify or characterize a substance—and distinguish that substance from other substances--are called characteristic properties. They are subdivided into two categories: physical properties and chemical properties.
   The characteristic physical properties of a substance are those that identify the substance without causing a change in the composition of the substance. They do not depend on the quantity of substance.A Color, odor, density, melting point, boiling point, hardness, metallic luster or shininess, ductility, malleability, and viscosity are all characteristic physical properties. For exam pie, aluminum is a metal that is both ductile and malleable.B Another example of a physical property is water. Whether a small pan of water is raised to its boiling point or a very large kettle of water is raised to its boiling point, the tem- perature at which the water boils is the same value, 100 degrees C or 212 degrees F.C Similarly, the freezing point of water is 0 degrees C or 32 degrees F. These values are independent of quantity. D
   Characteristic properties that relate to changes in the composition of a substance or to how it reacts with other substances are called chemical properties. The following questions pertain to the chemical properties of a substance.
   1. Does it burn in air?
   2. Does it decompose (break up into smaller substances) when heated?
   3. What happens when it is placed in an acid?
   4. What other chemicals will it react with, and what substances are obtained from the reaction?
   Characteristic physical and chemical properties—also called intensive properties—are used to identify a substance. In addition to the characteristic physical properties already mentioned, some intensive physical properties include the tendency to dissolve in water, electrical conductivity, and density, which is the ratio of mass to volume.
   Additional intensive chemical properties include the tendency of a sub stance to react with another substance, to tarnish, to corrode, to explode, or to act as a poison or carcinogen (cancer-causing agent).
   Extensive properties of substances are those that depend on the quantity of the sample, including measurements of mass, volume, and length. Whereas intensive:properties help identify or characterize a particular kind of matter, extensive properties relate to the amount present.
   If a lump of candle wax is cut or broken into smaller pieces, or if it is melted (a change of state), the sample remaining is still candle wax. When cooled, the molten wax returns to a solid. In these examples, only a physical change has taken place; that is, the composition of the substance was not affected.
   → When a candle is burned, there are both physical and chemical changes. After the candle is lighted, the solid wax near the burning wick melts. This is a physical change; the composition of the wax does not change as it goes from solid to liquid. Some of the wax is drawn into the burning wick where a chemical change occurs. Here, wax in the candle flame reacts chemically with oxygen in the air to form carbon dioxide gas and water vapor. In any chemical change, one or more substances are used up while one or more new substances are formed. The new substances produced have their own unique physical and chemical properties.
   The apparent disappearance of something, like the candle wax, however, is not necessarily a sign that we are observing a chemical change. For example, when water evaporates from a glass and disappears, it has changed from a liquid to a gas (called water vapor), but in both forms it is water. This is a phase change (liquid to gas), which is a physical change. When attempting to determine whether a change is physical or chemical, one should ask the Critical question: Has the fundamental composition of the substance changed? In a chemical change (a reaction), it has, but in a physical change, it has not.
   Glossary
   ductility: can be drawn into wire
   malleability: can be shaped
   viscosity: thick, resistant to flow [br] In paragraph 8, the author contrasts the concepts of physical and chemical changes by
选项 A、listing several types for each concept
B、providing clear definitions for them
C、identifying the common characteristics
D、using a wax candle as an example
答案 D
解析 "When a candle is burned, there are both physical and chemical changes." Choice A is not correct because only the example of the candle is mentioned. Choice B is not correct because the meaning is explained by example, not by definition. Choice C is not correct because the common characteristics were mentioned in previous paragraphs.
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