Biology CPNotes: The Properties of WaterName:KEY
Chemical Formula:H2O Structural Formula:
Bohr model diagram:
Water is neutral (like other molecules) because it contains 10 protons and 10 electrons (the charges are equal so they balance out)
PolarityPolarity
Draw the subatomic particles of the nucleus of
each atom in the boxes at the right. (each
is a proton)
When comparing the nucleus of an oxygen atom and a hydrogen atom, which one would have a stronger attraction for electrons? Why?Oxygen has 8 protons, each hydrogen has just 1
At any moment, there is a greater likelihood of finding the shared electrons in water close to its oxygen atom than either of the hydrogen atoms. As a result, the oxygen end of the molecule has a slight ____negative______charge and the side of the molecule with the hydrogen atoms has a slight ___positive______charge. This means that water is a polar molecule. Note that thesepartial charges are weaker than the charges on ions such as Na+ or Cl-.
polar:a molecule in which the charges are unevenly distributed. It acts a bit like a magnet with two poles.
Hydrogen Bonding
Polar molecules (such as water) can attract each other because of their partial (+) and (-) charges.
____Hydrogen bond______:the attractive force between the hydrogen in a polar molecule and the oxygen or nitrogen in another polar molecule
Hydrogen bonds are weaker than covalent or ionic bonds.
Water is able to form multiple hydrogen bonds, which account for many of water’s special properties.
How many hydrogen bonds
Are shown in the picture to the right?
Hint: look carefully!4
Water’s special properties
1. Cohesion: An attraction between molecules of the same substance.
This causes water molecules to be drawn together. (“Co-“ together; “haerere”- stick)
Examples: Water beads into droplets on a smooth surface. Surface tension allows some insects to “walk” on water.
2. Adhesion: An attraction between molecules of different substances. (“Ad-“ toward; “haerere”- stick)
Examples: Capillary action (water rises in narrow tube against gravity). A meniscus in a graduated cylinder is curved because of adhesion between the water and the glass.
3. Heat Capacity: The amount of heat energy required to raise the temperature of a substance. Water’s heat capacity is unusually high.
Examples: Large bodies of water can absorb large amounts of heat with small changes in temp – this protects organisms living there. Also water absorbs heat from cell processes, regulating the temp of the cell
4. Density: The amount of mass per unit volume. (D = m / v). Solid water (ice) has a lower density than liquid water.
Examples: Ice floats on top of water instead of sinking. Cold water is denser than warm water and it sinks, causing ocean currents.
5. Water is known as the __universal solvent______: In reality, water can’t dissolve everything…but water is an excellent solvent, and it’s especially common in and around living things.
Solutiona mixture consisting of a solute evenly distributed throughout a solvent
Solventthe substance in which the solute dissolves
Solutethe substance that is dissolved in a solvent
What are some substances that water can easily dissolve?Salts, sugars, minerals, gases, and even other solvents such as alcohol
Water’s polarity gives it the ability to act as an excellent solvent because it can dissolve both ___ionic______compounds AND other __polar______molecules. With polar solutes (like sugar), water molecules surround and separate the polar molecules from each other. With ionic solutes (like salt), water molecules surround and separate the individual ions from each other, a process known as dissociation.
2.2 Properties of Water
The Water Molecule
For Questions 1–4, write True or False on the line provided.
T 1. Water is a polar molecule.
F 2. Hydrogen bonds are an example of adhesion COHESION.
F 3. Covalent bonds give water a low HIGHheat capacity.
F 4. A hydrogen bond is stronger WEAKER than a covalent bond.
Write your answers in the spaces provided.
5. Compared to most other substances, a great deal of heat is needed to raise the temperature of water by a given amount. This is because water…. Has a high heat capacity
6. Explain the distribution of partial charges in a water molecule. The oxygen end is partially negative and the hydrogen end is partially positive
7. Explain the difference between cohesion and adhesion. Cohesion is the attraction between molecules of the same substance; Adhesion is an attraction between molecules of different substances.
8. Which is more dense – ice or water? How do you know? Water because ice (less dense) floats above water (more dense)
9. Explain the difference between how water dissolves a sugar vs. a salt. Water breaks apart a clump of sugar into separate sugar molecules but doesn’t break any covalent bonds. Water dissolves a lump of salt by dissociationwhich means it breaks it apart into its individual ions.
10. Explain the difference between the following terms: Hydrogen Bond, Covalent Bond, Ionic Bond. Hydrogen bond is an attraction between a partially charged hydrogen and a partially charged oxygen or nitrogen. Covalent bond is a sharing of electrons between two atoms. Ionic bond is an attraction between ions as a result of a transfer of electrons.
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