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. 譯文： 物質都是由被稱為元素的基本物質所組成。化學反應不能將元素分解成更小的單位。元素只含有一種原子。原子是元素最小的結構單位。 一個化合物是一種物質，它可以分解成兩種或兩種以上的元素。水是化合物它可被分解成氫和氧這兩種成分。化合物的分子式提供了組成該化合物分子種原子的種類及其數量的信息。分子式中含有原子符號，下標的數字表示分子中每種原子的數量。比如水的分子式H2O，表示一個水分子含有兩個氫原子和一個氧原子。葡萄糖的分子式C6H12O6，表示含有六個碳原子、十二個氫原子和六個氧原子。 當碳與氧結合形成一種無色、無味、無嗅的氣體叫做二氧化碳，它比空氣重，可以滅火。 二氧化碳在許多方面類似于氮氣，但如果它與石灰水混合，它就使清澈的液體變成乳狀液，而氮氣不能，由此可以鑒別熱氧化碳。 二氧化碳是植物食物的來源。植物有能力從空氣中吸收二氧化碳，使之與水化合轉化成植物的組織，實際上，所有有機碳化合物都是通過這種方式形成的。 無機化合物是由諸如硫、磷、鐵、鉀、鈉和鈣等元素組成。鈣、硫和氧形成硫酸鈣，鈣、磷和氧形成磷酸鈣，鈉、鉀和氧、氮結合生成硝酸鈉和硝酸鉀。