One can tell if something is an acid because acids have several distinguishing characteristics, including turning phenolphthalein from pink to colorless and blue litmus paper to red. Acids burn cuts, taste sour and react with other substances to form gases.
Indicators are special chemicals that change colors as the pH of a solution changes. Litmus paper is a common indicator for determining if a substance is an acid or base. This paper is available in blue and red. Phenolphthalein is a liquid indicator, which turns bright pink in bases and becomes clear in acids.
Acids have some unique physical characteristics, including causing wounds to sting and leaving a sour taste in the mouth; however, laboratory safety rules generally recommend against using such tests. The sour taste of vinegar is an example of the taste of an acid. Vinegar, placed on a wound, also causes a characteristic stinging sensation.
Mixing metals with acids results in the formation of hydrogen gas. This flammable gas, when collected in a test tube, explodes when exposed to a flame. Mixing acids with carbonates forms carbon dioxide. Since carbon dioxide does not support combustion, placing a burning wood splint in a test tube of the gas extinguishes the flame.