Elements and Compounds All matter is composed of basic substances called elements. An element cannot be broken down into simpler units by chemical reactions; it contains only one kind of atom. An atom is the smallest characteristic unit of an element. A compound is a substance that can be split into two or more elements. Water is a compound because it can be split into its components, hydrogen and oxygen. The formula of a compound gives information about the kinds and numbers of atoms that make up each molecule of that compound. A formula contains the symbols of the kinds of atom in the each molecule and subscripts that indicate the number of each kind of atom in the molecule. For example, the formula for water, H2O, indicates that a water molecule contains two hydrogen atoms and one oxygen atom; and a molecule of the glucose, C6H12O6, contains six carbon atoms, twelve hydrogen atoms, and six oxygen atoms. When carbon unites with oxygen, it forms a colorless, odorless, and tasteless gas called carbon dioxide, which is heavier than air and will extinguish a flame. Carbon dioxide is like nitrogen in many ways, but if it is mixed with limewater, it causes the clear liquid to because milky, while nitrogen dose not. This is the test for carbon dioxide. Carbon dioxide is a source of plant food. Plant have the power to take this gas from the air, combine it with water, and make it into their tissues; in fact, it is from this source that all organic carbon comes. Mineral compounds are made of elements such as sulphur, phosphorus, iron, potassium, sodium, and calcium. Calcium unites with sulphur and oxygen to form calcium sulphate, and phosphors and oxygen to form calcium phosphate, sodium and potassium unite with oxygen and nitrogen to form sodium to potassium nitrates. 译文： 物质都是由被称为元素的基本物质所组成。化学反应不能将元素分解成更小的单位。元素只含有一种原子。原子是元素最小的结构单位。 一个化合物是一种物质，它可以分解成两种或两种以上的元素。水是化合物它可被分解成氢和氧这两种成分。化合物的分子式提供了组成该化合物分子种原子的种类及其数量的信息。分子式中含有原子符号，下标的数字表示分子中每种原子的数量。比如水的分子式H2O，表示一个水分子含有两个氢原子和一个氧原子。葡萄糖的分子式C6H12O6，表示含有六个碳原子、十二个氢原子和六个氧原子。 当碳与氧结合形成一种无色、无味、无嗅的气体叫做二氧化碳，它比空气重，可以灭火。 二氧化碳在许多方面类似于氮气，但如果它与石灰水混合，它就使清澈的液体变成乳状液，而氮气不能，由此可以鉴别热氧化碳。 二氧化碳是植物食物的来源。植物有能力从空气中吸收二氧化碳，使之与水化合转化成植物的组织，实际上，所有有机碳化合物都是通过这种方式形成的。 无机化合物是由诸如硫、磷、铁、钾、钠和钙等元素组成。钙、硫和氧形成硫酸钙，钙、磷和氧形成磷酸钙，钠、钾和氧、氮结合生成硝酸钠和硝酸钾。