Acids and bases are everywhere. Any substance that provides hydrogen ions in solution is considered to be an acid and will have a pH lower than seven. Any substance that produces hydroxide ions in solution is considered to be a base and will have a pH higher than seven. Water can produce both hydrogen ions and hydroxide ions in equal quantities, which makes water neutral. When added to water, many substances will cause water to ionize and use the hydrogen or hydroxide in the water to create an acidic or basic environment. The pH of a solution is measured using a digital probe, pH indicator (litmus paper), or it can be calculated by using the hydrogen concentration.