章节大纲

  • Colorful fireworks exploding in the night sky, creating bright lights and varied colors.

    Communities often use fireworks to celebrate important occasions. Fireworks certainly create awesome sights and sounds! Do you know what causes the brilliant lights and loud booms of a fireworks display? The answer is chemical changes.  
    ::社区经常使用烟花来庆祝重要场合。 烟花当然会创造令人赞叹的景象和声音 。 你知道烟花展示的亮光和响亮声是什么原因吗? 答案是化学变化。

    What Is a Chemical Change?
    ::什么是化学变化?

    A chemical change occurs whenever matter changes into an entirely different substance with different chemical properties. A chemical change is also called a . Many complex chemical changes occur to produce the explosions of fireworks. An example of a simpler chemical change is the burning of methane. Methane is the main component of natural , which is burned in many home furnaces. During burning, methane combines with oxygen in the air to produce entirely different chemical substances, including the gases carbon dioxide and water vapor. 
    ::当物质改变为完全不同的物质,具有不同的化学特性时,就会发生化学变化。化学变化也称为“化学变化 ” 。许多复杂的化学变化会产生烟火爆炸。更简单的化学变化的一个例子是甲烷燃烧。甲烷是天然物质的主要成分,在许多家用炉中燃烧。在燃烧过程中,甲烷与空气中的氧合在一起产生完全不同的化学物质,包括二氧化碳和水蒸气等气体。

    Identifying Chemical Changes
    ::识别化学变化

    Most chemical changes are not as dramatic as exploding fireworks, so how can you tell whether a chemical change has occurred? There are usually clues. You just need to know what to look for. A chemical change has probably occurred if bubbles are released, there is a change of color, or an odor is produced. Other clues include the release of , light, or loud sounds. Examples of chemical changes that produce these clues are shown in the Figure .
    ::大多数化学变化没有爆炸烟花那么戏剧化, 所以您如何判断是否发生了化学变化? 通常有线索。 您只需要知道需要寻找什么。 如果泡沫释放, 颜色变化, 或产生一种气味, 化学变化可能已经发生。 其他线索包括释放, 亮光或响亮的声音。 图中显示了产生这些线索的化学变化的例子 。

    Examples of chemical changes: bubbles, color changes, odors, light, and loud sounds.

    Q: In addition to iron rusting, what is another example of matter changing color? Do you think this color change is a sign that a new chemical substance has been produced?
    ::问题:除了生锈铁外,另一个物质改变颜色的例子又是什么?你认为这种颜色变化是新化学物质已经生产的迹象吗?

    A: Another example of matter changing color is a penny changing from reddish brown to greenish brown as it becomes tarnished. The color change indicates that a new chemical substance has been produced. Copper on the surface of the penny has combined with oxygen in the air to produce a different substance called copper oxide.
    ::A: 物质颜色变化的另一个例子是一便士,从红褐色变为绿褐色,因为它被玷污了。颜色变化表明已经生产出一种新的化学物质。 便士表面的铜与空气中的氧合在一起生产一种不同的物质,叫做氧化铜。

    Q: Besides food spoiling, what is another change that produces an odor? Is this a chemical change?
    ::问题:除了食物变质之外,又有什么变化会产生一种气味?这是化学变化吗?

    A: When wood burns, it produces a smoky odor. Burning is a chemical change.
    ::A:当木材燃烧时,它会产生一种烟雾味。燃烧是一种化学变化。

    Q: Which signs of chemical change do fireworks produce?
    ::问:烟花会产生哪些化学变化的迹象?

    A: Fireworks produce heat, light, and loud sounds. These are all signs of chemical change. 
    ::A:烟花产生热、光和响亮的声音。这些都是化学变化的迹象。

    Can Chemical Changes Be Reversed?
    ::化学变化能否逆转?

    Because chemical changes produce new substances, they often cannot be undone. For example, you can’t change ashes from burning logs back into wood. Some chemical changes can be reversed, but only by other chemical changes. For example, to undo tarnish on copper pennies, you can place them in vinegar. The in the vinegar combines with the copper oxide of the tarnish. This changes the copper oxide back to copper and oxygen, making the pennies reddish brown again. You can try this at home to see how well it works.  
    ::由于化学变化产生新的物质,它们往往无法撤销。例如,将原木烧回木材后,你无法改变灰烬。有些化学变化可以逆转,但只能通过其他化学变化来扭转。例如,为了消除铜小便上的污点,你可以把它们放在醋中。在醋中,铜氧化物与污点的铜氧化物结合在一起。这把铜氧化物改成铜和氧,使硬条再次红褐色。你可以在家里尝试这一点,看看它的效果如何。

    Summary
    ::摘要

    • A chemical change occurs whenever matter changes into an entirely different substance with different chemical properties. Burning is an example of a chemical change.
      ::当物质改变为完全不同的物质,具有不同的化学性质时,就会发生化学变化。燃烧是化学变化的一个实例。
    • Signs of chemical change include the release of bubbles, a change of color, production of an odor, release of heat and light, and production of loud sounds.
      ::化学变化的迹象包括释放气泡、改变颜色、产生一种气味、释放热和光,以及产生响亮的声音。
    • Because chemical changes result in different substances, they often cannot be undone. Some chemical changes can be reversed, but only by other chemical changes.
      ::由于化学变化导致不同物质,因此这些变化往往无法撤销。 一些化学变化可以逆转,但只能通过其他化学变化才能逆转。

    Review
    ::回顾

    1. What happens in any chemical change?
      ::在任何化学变化中会发生什么?
    2. List three signs that a chemical change has occurred.
      ::列出三个化学品变化的迹象。
    3. Give an example of a chemical change. Explain why you think it is a chemical change.
      ::举一个化学变化的例子。 解释为什么你认为这是化学变化。
    4. Why can chemical changes often not be reversed?
      ::为什么化学变化往往不能逆转?