Properties of Water

1. Foundation Science Today’s Lesson: Properties of Water

2. Water is a Polar Molecule Water molecules are polar. P olar = has a positive side and a negative side. The negative part of a water molecule is attracted to the positive part of another.

3. Water Forms Hydrogen Bonds The attraction between water molecules are called hydrogen bonds. Hydrogen bonds between molecules of water is what causes water’s surface tension

4. Water’s Boiling and Freezing Points Water is considered to be so important that the scale we use to measure temperature was set according to water! At sea level (about 1 atmosphere of pressure): Water boils at 100° Celsius Water freezes at 0° Celsius Frozen water (ice) is less dense than liquid water, which is why ice floats. Water is most dense (heaviest) at 4° Celsius.

5. Can Water’s Boiling Point Change? YES! Water’s boiling AND freezing points depend on how much pressure is on the water. When there is less pressure from the atmosphere, water will have a lower boiling point! When there is more pressure from the atmosphere, water will have a higher boiling point! What if there is no pressure on water, like in a vacuum or in outer space? As the pressure is lowered, the boiling point gets lowered. Video…….

6. Can Water’s Boiling Point Change?

7. Air Pressure

8. As you go up, air pressure gets lower, and the boiling point of water goes down

9. The Science of Cooking with Water Can we make water boil hotter than 100° Celsius? YES! Pressure increases the boiling point. So, why do you think people use a pressure cooker? It is so they can cook things in water that is hotter than 100 ° Celsius.

10. Boiling Water and Space How much pressure is there in space? Space has very little, to no pressure. So what is a space suit really for? A human body is about 70% water. With no pressure, all the water in a human would boil! NASA says that a person could live about 1 minute in space without a suit. Note: (don’t need to write in notes) a man accidentally exposed to a vacuum at NASA lost consciousness after about 14 seconds due to lack of oxygen. He said his last memory was “the water beginning to boil on his tongue”. You will NOT explode without a spacesuit in space says NASA.

11. More about Boiling Water How do you know when water is at 100° Celsius when cooking? When the first bubble of air floats all the way to the top of the water. Do you need to make water boil like crazy for it to be 100° Celsius? No ! Just enough for the first bubbles to be rise to the top. After that, it only needs a little heat to stay at 100° Celsius. What are the bubbles in boiling water made of? Steam (=water vapor)

12. Capillary Action Capillary action occurs when water goes up a narrow tube against the forces of gravity. Without hydrogen bonds , capillary action could not occur. Capillary action is how plants and trees transport water from the roots to leaves where photosynthesis occurs. Capillary Action Gravity

13. How Does Capillary Action Occur? 1) Adhesion is the attraction between water and other objects. Example of Water Adhesion Adhesion allows water to stick to the sides of the tube (can be a straw or inside a plant). 2) Cohesion is water being attracted to itself due to its hydrogen bonds Adhesion and cohesion lead to water’s capillary action

14. Water has a Very High Specific Heat Capacity Specific heat is how much energy is required to heat a substance. Water needs a LOT of energy to be heated, so WATER HAS A VERY HIGH SPECIFIC HEAT CAPACITY. To heat things you need to make the molecules move faster. It is hard to make water molecules move fast because its hydrogen bonds keep the molecules stuck together.

15. Think of Heat as Movement of Molecules!! Fast = hot Slow = cold Cold = high density (packed) Hot = low density (a lot of space) Molecules Being Heated…. Low Density High Density

16. Molecules Being Heated Animation

17. Cold (dense) or Hot (less dense)?? Cold and dense Hot, less dense Density is mass/volume!

18. Water Has Surface Tension What is surface tension? Surface tension is the ability of a liquid to “push back” on something. Surface tension allows water striders to walk on water. Surface tension allows this paper clip to “float” on water

19. What Causes Water’s Surface Tension? How does water hold the paper clip up? COHESION causes surface tension. But, surface tension is due to the molecules at the surface having stronger hydrogen bonds Water molecules are surrounded only on one side at the surface, and the H-Bonds are therefore stronger.

20. Is Water an Acid??? Pure water is not an acid (not acidic). BUT, when water is exposed to air, it comes in contact with carbon dioxide (CO 2 ) and forms a weak acid called carbonic acid : CO 2 + H 2 O H 2 CO 3 Carbonic acid can dissolve rock and things like cement. Water can turn slightly acidic, and over time, make holes in rock

21. Is Water an Acid??? Over time, water is responsible for carving out large cave systems like Carslbad Caverns . So , YES , water can be an acid. Water has the power to dissolve limestone Carlsbad Caverns

22. QUESTIONS Tell me 3 things water could NOT do if it was not a polar molecule. Draw a diagram of a hydrogen bond. Label the oxygen and hydrogen atoms and the hydrogen bond. Which is heavier: 5 liters of water at 4° Celsius, or 5 liters of water at 50° Celsius or 5 liters of ice at 0° Celsius? Where will water boil hotter, Bangkok or Chiang Mai? True or False: A pressure cooker can cook faster because the water inside is able to get hotter than 100° Celsius. Tell me how you know when water is 100° Celsius when cooking. What would happen to a human in space without a spacesuit? Draw a diagram of capillary action in a plant. What is adhesion? What is cohesion? Tell me one reason why water’s high specific heat may be important for a human body. You are walking on the street and see some holes in the sidewalk. Explain to me how water may have caused these holes. What does “hydro” mean? What does “phobic” mean? Is water a hydrophobic molecule? Why does water have such a high specific heat capacity?