Investigations One and Two introduced students to qualitative and quantitative properties of matter and the measurement of mass as a method of quantifying matter. Investigation Three introduced volume as a quantitative property and provided students with an opportunity to explore how solids and liquids interact with one another to form mixtures and solutions. In Investigation Four, students had an opportunity to explore how some substances interact to form new substances, a process that is usually signaled by one of the ten common signs of chemical change.

Investigation Five focuses on another property of matter, pH. Matter can be classified as acidic, basic, or neutral. Pure water is neutral. Water molecules can dissociate into H+ (protons) and hydroxide (OH–) ions. It is neutral because its acidic and basic components, hydrogen (H+) ions and hydroxide ions, are in equal amounts. The chemical formula for water is H2O. In pure water, there is a mix of water molecules and equivalent numbers of protons or hydrogen ions (H+) and hydroxide ions (OH-) as shown below in Figure 5.1.

Acids are substances that can increase the number of hydrogen ions in a water solution by supplying a hydrogen ion to a neutral water molecule. When this happens, the number of hydrogen ions exceeds the number of hydroxide ions and the solution becomes acidic. Conversely, bases are substances that can increase the number of hydroxide ions in solution so that the number of hydroxide ions exceeds the number of hydrogen ions and the solution becomes basic.

The acidity, basicity, or neutrality of a substance determines how it will interact with other substances in solution. Scientists compare acidity, basicity and neutrality with a scale called a pH scale. The pH scale ranges from 0 to 14, with 0 being highly acidic, 7 being neutral, and 14 being highly basic. As pH approaches 7, the substance becomes less basic or less acidic.

Scientists can either use specially formulated paper to test pH or an instrument called a pH meter. The pH meter is frequently used in research situations where scientists may need to correct the pH of a solution for use in an experiment. The pH paper, however, is useful when comparing the relative basicity or acidity of substances. The paper reacts to different levels of acidity and basicity by changing color (a chemical change). This colorimetric scale ranges from deep red to represent highly acidic substances to deep blue or violet for highly basic substances. The colors progress along the same spectrum of color as a rainbow (red, orange, yellow, green, blue, indigo, and violet). Green represents the neutral range.

Water traditionally is represented as having a pH of 7. This is true as long as the water is pure. The water standard for neutrality is distilled deionized water. Tap water, distilled water, and spring water are frequently tested as being slightly acidic (pH between 6 and 7) because of lingering ions or other chemicals used to make them safe to drink.

Investigation Five introduces students to pH as a quantitative property of matter. Students will explore the pH scale by testing different acids and bases with pH paper, and discover that the pH of solid substances can be tested by creating a solution of the solid with water. In addition, students will explore the effect of dilution on pH.