When a liquid changes into a gas, the density changes tremendously. Some elements are very reactive and dangerous, while others are not. Some elements are found alone in nature, while others are found only compounds. Some pairs of elements, such as Ca and Mg, or Na and K, are often found together in nature. Some elements are very reactive and dangerous, but compounds containing those elements may be stable and unreactive.
Every compound has one certain formula and no other.
All substances can be frozen if the temperature is low enough. All ionic substances are solids at room temperature, but many covalent polar substances are liquids at room temperature. Many nonpolar substances are gases at room temperature, but some are liquids or even solids. 13 Polar substances dissolve in other polar substances. Nonpolar substances dissolve in other nonpolar substances.