Chemical Structure of Water Water is composed of two atoms of Hydrogen (H) and one atom of Oxygen (O). The chemical structure of water looks a lot like a famous Mouse- M-I-C…K-E-Y…M-O-U-S-E. The head being the O and the ears being the H.
Water has three states. Below freezing water is a solid ice or snowflakes Between freezing and boiling water is a liquid Above its boiling point water is a gas.
Why Does Ice Float? Most liquids contract (get smaller) when they get colder. Water is different. Water contracts until it reaches 4 C then it expands until it is solid. Solid water is less dense than liquid water because of this. Therefore, it Floats! If water worked like other liquids, then there would be no such thing as an ice berg, the ice in your soft drink would sink to the bottom of the glass, and ponds would freeze from the bottom up!
Density Demo Inside the Balloons you’ll find- Hot, cold, salt, and freshwater. If we place the balloon filled with hot water and the balloon filled with cold water in a tank of water what do you think will happen to each? Sink or Float? Now lets try the same thing with the salt water balloon and the freshwater balloon.
Water Is Sticky Water is attracted to other water. This is called cohesion. Water can also be attracted to other materials. This is called adhesion. (Remember Adhesive tape picks up things)
Sticky Water Prevents the Storm: Adhesion/ Cohesion Demo. If we pour the water from one pitcher to the other pitcher down the string, will the rain coat clad volunteer get wet? Why do you think what you saw happening happens? Here’s why …..
Polarity at Work The oxygen end of water has a negative charge and the hydrogen end has a positive charge. This means Waters net charge is spread out unevenly, we call this polarity. The O end of one water molecule sticks to the H end of the other water molecule because the + and – ends are attracted
Hydrogen Bonding The hydrogens of one water molecule are attracted to the oxygen from other water molecules. This is due to waters Polarity and we call it Hydrogen Bonding. This attractive force is what gives water its cohesive and adhesive properties.
Adhesion and Solubility Demo Lets see what happens to the ink on this paper towel if we stick just the bottom of the paper towel in the water. Predictions…..
Surface Tension Surface tension is the name we give to the cohesion of water molecules at the surface of a body of water. Water Strider
Surface Tension Demo Lets place a drop of water onto a piece of wax paper. Look closely at the drop. What shape is it? Why do you think it is this shape?
What is happening? Water is not attracted to wax paper (there is no adhesion between the drop and the wax paper). Each molecule in the water drop is attracted to the other water molecules in the drop. This causes the water to pull itself into a shape with the smallest amount of surface area, a bead (sphere). All the water molecules on the surface of the bead are 'holding' each other together or creating surface tension.
Can You Float A Paper Clip? Surface tension allows water striders to 'skate' across the top of a pond. A metal pin or paper clip is heavier than water, but because of the surface tension the water is able to hold up the metal.
Breaking The Surface Tension Add a drop of detergent. What happens to the paper clip? Why? The detergent has phosphate in it. The phosphate attracts to the water molecules and breaks the surface tension.
Capillary Action Capillary action is related to the adhesive properties of water. Capillary action is when water moves up a cylinder.
Capillary Action Demo You can see capillary action 'in action' by placing a straw into a glass of water. The water 'climbs' up the straw. What is happening is that the water molecules are attracted to the straw molecules. When one water molecule moves closer to the straw molecules the other water molecules (which are cohesively attracted to that water molecule) also move up into the straw.
More Capillary Action Capillary action is limited by gravity and the size of the straw. The thinner the straw or tube the higher up capillary action will pull the water. Let’s do an experiment to see if this is true.
Plants and Capillary Action Plants take advantage of capillary action to pull water from the soil into themselves. From the roots water is drawn through the plant by another force, transpiration.
Specific Heat Water has a high heat capacity. Specific heat a measure of heat capacity, is the heat required to raise the temperature of 1 gram of water 1°C. Specific heat Water, with its high heat capacity, therefore, changes temperature more slowly than other compounds that gain or lose energy. http://waterontheweb.org/resources/glossary.html
Water’s resistance to sudden changes in temperature makes it an excellent habitat because organisms adapted to narrow temperature ranges may die if the temperature fluctuates widely. The heat requiring and heat retaining properties of water provide a much more stable environment than is found in terrestrial situations. Fluctuations in water temperature occur very gradually, and seasonal extremes are small in comparison to terrestrial (Land) environments. Water As a Habitat.
Think Local: The Lake Effect. Have you heard of the Lake effect? In general in the summer it is cooler by the lake, in winter it is warmer by the lake. This is because of the specific heat of the water in Lake Michigan- it takes longer for it to get warm in the summer and cold in the winter. If you follow the lake temperature you will notice that at the changing of the seasons (Sept, Oct) the temperature of the lake is usually warmer than the air. It stays this way for awhile until it has been cold enough for long enough for the lake temp to change. The reverse happens in the spring.
Water is the Universal Solvent. Because of water’s polarity and cohesive properties (it’s sticky remember). Water will attract other molecules and these molecules will dissolve in water.
Human Water Molecules 6 Student volunteers = H atoms. 3 Student volunteers = O atoms. H2O joins One H from each H2O puts his/her hand on the shoulder of the O molecule to represent H bonding.
Lets Review: Properties of Water. Polar- along with this comes H bonding, adhesion and cohesion. High specific heat. Universal Solvent. High Surface Tension. Has capillary action.
Quick Quiz Now it’s your turn. Put away your notes, take out a piece of paper and a pencil/ pen. List 5 things from the power point which make water special or unique.
What is pH. To put it simply pH is a measure of how acidic, neutral or basic a substance is. It specifically measures the amount of H+ ions and OH- ions in a liquid. A substance with more H+ ions is Acidic. A substance with more OH- ions is basic. A substance with an = # of each is neutral. The pH scale goes from 1 to 14. 1 to just less than 7 is acidic, just over 7 to 14 is basic and 7 is neutral.