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Chemistry is the study of the ways various substances are put together and their reactions under different conditions. It is a science that involves all of one's senses: seeing, hearing, tasting, feeling, and smelling. Chemistry is the study of matter. Matter can be found in three basic "states" or forms: liquid, such as water, oil, and saliva; solid, such as wood, bone, and stone; or, gaseous, such as oxygen, helium, and methane. The basic building blocks of matter are the elements. Elements cannot be broken down into simpler matter. Whether in nature or in the laboratory, two or more elements combine chemically to form a compound. The combined form may have different properties from the original elements. For example, when the element oxygen, a gas, combines chemically in one way with the element hydrogen, also a gas, one compound that can be formed is water. As a liquid, it looks and behaves differently from the gaseous forms of hydrogen and oxygen. A chemical change has caused this difference. Two large and important groups of chemical compounds are acids and bases. Water (H2O or H-OH) is both an acid and a base. As its chemical formula indicates, pure water has equal concentrations of H+ (hydrogen ions) and OH- (hydroxide ions). When the concentrations of those two ions are equal, a substance is called neutral. When there are more hydrogen ions than hydroxide ions, that substance is an acid. The opposite condition (more hydroxide ions) makes a material a base. Whether a compound is an acid or a base is indicated by its pH or "power of hydrogen," which represents the amounts of acid or base in a solution. Pure water is neutral, and so registers 7 on the pH scale. The lower the reading below 7, the more acidic a solution is. The higher the reading above 7, the more basic a solution is. The pH of lemon juice is about 2.3 -- acidic; the pH of seawater is about 8.3 -- basic. The cabbage-juice mixture used in the Newton's Apple segment contains compounds that change color as the pH changes. Therefore, it can be used as an indicator to show different levels of the pH scale. Litmus paper, which turns red for acidic solutions and blue for basic solutions, or the pH kits used to test aquariums, can also be used as indicators.