Water Review

Structure of Water

- polar - partially negative end - partially positive end - "bent" in shape (contributes to the polar property)

Properties of water

adhesive, cohesive, thermal and solvent properties

How does hydrogen bonding and polarity contribute to the thermal properties of water?

- Hydrogen bonds break with heat. - Hydrogen bonds form with cooling.

Use of water as a coolant in sweat

- When a molecule evaporates, other molecules separate into liquid (vapor molecule) - Heat is thereby incorporated (latent heat of vaporization) - To cool down, evaporation takes place (hydrogen bonds are broken)

2. Why is it that methane can evaporate at -180°C on Titan whereas water is frozen solid at this temperature?

In a gas, the bonds holding the molecules together are weaker, so it takes less heat energy to evaporate them.

2. Where do hydrogen bonds form? a) Between the slight negative charge of hydrogen and slight positive charge of oxygen within a water molecule b) Between the slight positive charge of hydrogen and slight negative charge of oxygen within a water molecule c) Between the slight positive charge of hydrogen and slight negative charge of oxygen in different water molecules d) Between the slight negative charge of hydrogen and slight positive charge of oxygen in different water molecules

c

4. What property of water makes it suitable as a coolant? a) It takes a lot of energy to increase the temperature of water. b) It takes a lot of energy for water to evaporate. c) Water molecules are cohesive and stick to the skin. d) Water is a good solvent so it can transport heat from the body.

b

Hydrogen bonds are ______

forces of attraction that exist between the positive and negative ends of adjacent water molecules

1. What state is methane in the earth's atmosphere?

Gas

Thermal properties of water

High specific heat capacity, High latent heat of vaporization, High boiling and melting point Example: Sweating - lots of energy is needed, energy is taken from surroundings, the body is cooled down

Solvent properties of water

the polar nature of water means that it forms shells around other polar and charged molecules, preventing them from clumping together and keeping them in solution

3. What is the "special force" which is found between water molecules that is not found in methane?

hydrogen bonds

How does hydrogen bonding and polarity contribute to solvent properties of water?

- Water forms hydrogen bonds with polar molecules - Its partially negative oxygen pole is attracted to the positively charged ions and the partially positive hydrogen pole is attracted to the negatively charged ions - Both positively and negatively charged ions dissolve in water Example: When salt is mixed with water, the salt dissolves because the covalent bonds of water are stronger than the ionic bonds in the salt molecules.

Explaining the hydrogen bonding and polarity of water

- two hydrogens and one oxygen - unequal sharing of electrons (polar covalent bond) - hydrogen atoms are slightly positive, oxygen is slightly negative - molecule is bent, atoms have opposite poles - positive ions and negative ions attract each other (ionic bond) - the attraction between the water molecules are hydrogen bonds

8. Another difference between Titan's lakes and the oceans of the earth will be the substances dissolved in each. Water is a polar molecule and an excellent solvent of ionic substances like salts and other polar molecules like amino acids and sugars. Methane is non-polar and thus will dissolve lipids and small molecules better. Suggest why cytoplasm or blood made with water would be better for living things than cytoplasm or blood made from methane.

A cytoplasm of water can dissolve lots of proteins and salts which are essential to life. The reactions of metabolism happen in the solution, and they cannot happen unless they dissolve. Blood transports many substances including oxygen, nutrients, hormones, and antibodies. Many of these molecules would not dissolve in methane.

6. Sweating cools the skin because when the water in sweat evaporates, it carries heat away from the skin. If humans made sweat with methane instead of water, how would it change the effectiveness of sweating to cool the skin?

Methane would not take as much heat away as water when it evaporated because it has a low specific heat capacity.

Adhesive property of water

The dipolarity of water molecules makes them adhere to surfaces that are polar (therefore hydrophilic) In other words, the adhesive property of water allows water to be attracted and adhere to OTHER substances Helpful: adhesive forces between water and cellulose in cell walls in the leaf cause water to be drawn out of xylem vessels, keeping the call walls moist and able to act as a gas exchange surface

4. What effect does this force have on the melting and boiling point of water?

The strong bonds in the hydrogen bonds require more energy to break, causing a higher melting and boiling point.

7. If extra-terrestrial life on Titan does exist, and if it uses methane in sweat to cool down, explain why Titan's organisms would be happier to be sweating with methane and not water.

Titan's organisms would not be able to sweat water, as the water would be frozen at -160°C

Cohesive property of Water

Water molecules cohere (stick to each other) because of the hydrogen bonds that form between them Helpful: strong pulling forces can be exerted to suck columns of water up to the tops of the tallest trees in tubes call xylem vessels. These columns of water rarely break despite the suction forces

5. Suggest how this "special force" might change the cohesion of water molecules compared to methane molecules? (Cohesion is the ability of the same molecules to stick to each other)

Water molecules will have greater cohesion between molecules. This is useful in blood and xylem tubes where water flows.

Drops on a penny lab

Which molecule would demonstrate greater surface tension? Why? Water. This is because water can form more hydrogen bonds with other molecules compared to ethanol. Ethanol is less polar than water, due to the many non-polar bonds in ethanol. Water only has covalent bonds. Number of drops (water) that fit on a penny: 43 Number of drops (ethanol) that fit on a penny: 23 Explanation of results: Water had a greater number of drops compared to ethanol. Water is more polar than ethanol. There are "greater intermolecular forces" (hydrogen bonds), which allows for more water to sit on top of the penny before the tension breaks and the water bubble "pops".

3. Which can be explained by the solvent properties of water? a) Sodium chloride is transported as Na+ and Cl- in blood. b) Movement of water occurs under tension in the xylem. c) Water is the coolant in sweat. d) Ice floats on liquid water.

a

5. What is a consequence of the specific heat capacity for liquid water, ice, and water vapor? a) Less energy is needed to warm water vapor than liquid water. b) Salt dissolves more rapidly in liquid water than in ice. c) Small insects can walk on liquid water. d) Ice floats on liquid water.

a

1. What is a property of water? a) Water has a low specific heat capacity so large increases in heat energy cause a small temperature change. b) Water is an excellent solvent for non-polar substances. c) Covalent bonds between adjacent water molecules are responsible for its unique properties. d) Water molecules are highly cohesive which is important for transport of water in plants.

d