A combustion reaction involves a substance reacting with oxygen to form one or more oxygen-containing compounds. Combustion reactions are generally highly exothermic, emitting heat. In the presence of sufficient oxygen, carbon within the substance will generally form carbon dioxide, hydrogen will form water vapor, sulfur will form sulfur dioxide, and nitrogen will generally form nitrogen dioxide. For example, in the combustion of octane, carbon dioxide and water vapor are formed. Alkali metals have characteristic reactions with halogen atoms to form salts. For example, solid sodium metal reacts with chlorine gas to produce sodium chloride, an ionic compound. Alkali metals have characteristic reactions with water, which produce hydrogen gas and hydroxide. In this reaction, the two alkali metal atoms are donating one electronic each to the H+ from two water molecules, thus making hydrogen gas. When water loses the H+, the hydroxide ion remains. For example, when solid sodium reacts with water, the sodium atoms donate two electrons and the water molecules donate two H+ ions, resulting in Na+, hydroxide, and hydrogen gas. Metals react with halogens to produce metal halides. In this reaction, the metal donates electrons to the halogen to produce an ionic compound. For example, aluminum metal reacts with chlorine gas to produce aluminum chloride. Hydrogen gas can also react with a halogen to produce hydrogen halides. In this reaction, a covalent bond is formed between the hydrogen and halogen. For example, hydrogen gas reacts with chlorine gas to produce 2 HCl gas.