Properties of Water Read the lesson title aloud to the students.

Learning Objectives Discuss the unique properties of water. Differentiate between solutions and suspensions. Explain what acidic solutions and basic solutions are. Click to reveal each of the learning objectives. Discuss that we often take water for granted, but there would be no life on Earth without it. Explain that about 60 percent of the human body is water. In this lesson, students will learn about the chemical properties that make water such an essential chemical for life.

The Water Molecule (-) (+) (+) Water molecules are polar; the charges are unevenly distributed in the molecule. (-) Point out that water is made of 2 hydrogen atoms and an oxygen atom. Review electron numbers from the previous lesson. Ask: Hydrogen has 1 proton; how many electrons does it have? Answer: 2 Ask: Oxygen has 8 protons; how many electrons does it have? Answer: 8 Ask: How many electrons are there total in a water molecule? Click to reveal the image showing all 10 electrons. Ask: What charge do protons have? Answer: positive Ask: Do protons attract or repel electrons? Answer: Protons attract electrons. (Remind students that “opposites attract.”) Ask: If oxygen has 8 protons and hydrogen only has 1, to which nucleus are the electrons more attracted? Answer: The 8 protons of oxygen have a stronger pull on the electrons. Explain that this means the electrons will spend more time around the oxygen atom than the hydrogen atoms. Ask a volunteer to come to the board and label the regions of the molecule with their relative charges. Click to reveal the charges. Explain that since electrons spend more time around the oxygen end of the molecule, the oxygen “pole” is partially negative, while the hydrogen “poles” are partially positive. (+) (+)

Hydrogen Bonding Hydrogen bond Describe and discuss hydrogen bonding. Click to reveal the hydrogen bond. Explain that it is the hydrogen bonding (resulting from water’s polarity) that accounts for many of water’s special properties. Ask: Is a hydrogen bond more similar to a covalent bond or an ionic bond? Answer: Ionic because they are both the result of charge attraction rather than sharing electrons. Ask: How is a hydrogen bond different from an ionic bond? Answer: Hydrogen bonds are due to attractions between partial charges rather than full charges; therefore, hydrogen bonds are weaker than ionic bonds.

Cohesion Cohesion: attraction between molecules of the same substance Describe and discuss cohesion. Tell students that water molecules can form up to four hydrogen bonds with other water molecules. Explain that the strong attraction between water molecules results in what is called “surface tension,” and that because of surface tension, the surface of water can support very lightweight objects including the spider shown in this photograph. Click to reveal the text. Explain that this attraction is called “cohesion.” Cohesion is also why water molecules form beads on a smooth surface.

Adhesion Adhesion: attraction between molecules of different substances Click to reveal the definition of adhesion. Describe and discuss adhesion. The image shows adhesion between water and glass molecules. Adhesion causes water columns in the glass tubes to rise. Click to point out the height of each water column. Ask: Why do you think water molecules rise higher in narrower tubes? Answer: Water molecules will rise higher in a narrow tube because a larger portion of the surface of the water is adhering to the glass. The adhesion between the water and the glass is stronger than the cohesion between the water molecules and pulls the water upward against gravity. Click to point to the meniscus in the image. Explain why a meniscus forms. Ask: How do adhesion and cohesion compare? Answer: Both are attraction between molecules; cohesion is between molecules of the same substance, adhesion is between molecules of a different substance. Meniscus

Heat Capacity Heat capacity: amount of heat it takes to raise temperature Describe and discuss heat capacity. Tell students that it takes a large amount of heat to raise the temperature of water. Explain the effect this has on organisms that live in water and on coastal areas.

Solutions Water’s polarity gives it the ability to dissolve both ionic compounds and other polar molecules. Point out that solutes like table salt (NaCl) seem to disappear when they dissolve in water. You may wish to demonstrate this by dissolving salt in a beaker of water. Ask: Why does the salt seem to disappear? Answer: Its ions break apart and move throughout the water, so they no longer form visible, solid crystals. Ask: What happens to the mass and volume of water when salt is dissolved in it? Answer: They both disappear. Ask: Besides tasting the water, how could you show that the salt is still there even though you can no longer see it? Answer: You could measure the mass or volume of the water before and after salt is added. Click to highlight the water molecules bound to Cl- and Na+ ions. Ask: Notice that the Cl- binds with the hydrogens of water, while the Na+ binds with the oxygen. Why is this the case? Answer: Hydrogen has a partial positive charge that is attracted to the negative charge of the Cl-, and oxygen has a partial negative charge that is attracted to the positive charge of the Na+.

Acids, Bases, and pH Low pH = acidic; higher concentration of H+ High pH = basic; lower concentration of H+ Human blood Explain that pH is a measure of hydrogen ion concentration. Then explain that acids have a higher hydrogen ion concentration and bases have a lower hydrogen ion concentration than pure water. Ask students to identify the pH that is neutral. Click to reveal the answer: 7, pure water Discuss that pure water is neutral and explain that some water molecules split to form H+ and OH- ions. Ask students to guess the pH of each of the following. Click to reveal each answer. Human blood Bleach Lemon juice Tomato juice Stomach acid Toothpaste Ask: How do the hydrogen ion concentrations of stomach acid and bleach compare with that of pure water? Answer: H+ ion concentration is higher for stomach acid and lower for bleach. Pure water Stomach acid

Buffers Buffers help maintain homeostasis in organisms. Base Neutral Acid Unbuffered base + acid = _________ acidic pH Explain that buffers are weak acids or bases that can react with strong acids or bases to prevent sharp, sudden changes in pH. Remind students that normal blood pH is between 7.35 and 7.45. A lower or higher pH can be a sign of ill health. Have students discuss why regulating blood pH is vital to maintaining homeostasis. Discuss the importance of buffers to health. Ask students what pH would result from adding an acid to an unbuffered base and from adding an acid to a buffered base. Click to reveal the answers. Buffered base + acid = ________ basic pH