The Properties of WATER

1. The Properties of WATER

2. What is so special about water? • It is the most abundant compound in most living things • Water is the only compound that exists in all 3 phases (solid, liquid and gas)on Earth • Water expands as it freezes; thus, ice is less dense than liquid water which allows it to float on the surfaces of lakes and rivers

3. Thinking cap time….. • With your partner, discuss the following: why is the fact that ice floats on water (remember, ice is less dense than water) essential to much of life on Earth? • Time: One minute

4. The Chemical Makeup of H20 • Water is bonded covalently • The Hydrogen atoms are not bonded together; they are each bonded to the Oxygen atom • In a water molecule, the electrons are shared unequally. They tend to hover around the Oxygen atom

5. Polarity • Oxygen atoms have a much stronger attraction for electrons than do Hydrogen atoms; thus there is a greater probability of finding the shared electrons near the oxygen atom • This results in the Oxygen end of the water molecule having a slightly negative charge and the Hydrogen end having a slightly positive charge • Polarity refers to this uneven distribution of charge(electrons) within a molecule

6. Hydrogen Bonds • Because water molecules are polar (slightly negative and positive ends)they can attract each other • A hydrogen atom in one water molecule is attracted to the oxygen atom of another water molecule • This is referred to as Hydrogen Bonding • Hydrogen bonds are not as strong as covalent or ionic bonds • This type of bonding accounts for the many unique properties of water

7. A Hydrogen Bond

8. Think-Pair-Share • THINK: On your own, write down why you think Hydrogen bonds would not be as strong as ionic or covalent bonds • PAIR: discuss and compare answers with your partner • SHARE: be prepared to discuss as a class • TIME: 2 minutes

9. Cohesion • Cohesion is an attraction between molecules of the same substance • Because of hydrogen bonding, water is very cohesive • Waters cohesion causes molecules on the surface of water to be drawn inward, which is why drops of water form beads on a smooth surface

10. Cohesion and Surface Tension • The cohesive property of water explains why some insects and spiders can walk on the surface of water • Surface Tension: the strong attraction between molecules of water creates a force which acts like a film and can support very light objects

11. Adhesion • Adhesion is the attraction between molecules of different substances (review: what was cohesion?) • Have you ever been asked to read the volume in a graduated cylinder at eye level? The surface of the water dips slightly in the center because the adhesion between the water molecules and the glass molecules is stronger than the cohesion between water molecules

12. Check for Understanding • With your partner, summarize what cohesion and adhesion are • Come up with a clever way to not get the two confused • TIME: 2 minutes

13. Cohesion, Adhesion and Capillary Action • Capillary Action is the force that draws water out of the roots of plants and up into its stems and leaves • Adhesion between the water molecules and the root/stem walls causes water to pull up the sides • Cohesion between water molecules holds the column of water together as it rises • Cohesion and adhesion work together to allow for capillary action

14. Water as a Mixture • Water is not always pure; it is often found as part of a mixture • A mixture is a material composed of two or more elements or compounds that are physically mixed together but not chemically combined • Two different mixtures that can be made with water: • Solution: all of the components are evenly distributed (ie. Salt in water- the water serves as the solvent and salt is the solute) • Suspension: when materials do not dissolve in water but form small pieces (clumps) to do not settle (ie. Sand in water)