NAS 2 Chemistry Water

Why is water called a polar molecule?

Because oxygen pulls shared electrons closer to itself, giving oxygen a partial negative (δ−) charge and the hydrogens partial positive (δ+) charges.

Which atom in water is more electronegative?

Oxygen.

What does electronegativity mean?

An atom's ability to attract shared electrons in a covalent bond.

What type of bond holds hydrogen and oxygen together within one water molecule?

Polar covalent bonds.

What type of attraction exists between neighboring water molecules?

Hydrogen bonds.

Are hydrogen bonds stronger or weaker than covalent bonds?

Much weaker than covalent bonds.

Why can one water molecule hydrogen-bond with another?

The positive hydrogen of one molecule is attracted to the negative oxygen of another.

How many hydrogen atoms are in one water molecule?

Two.

How many oxygen atoms are in one water molecule?

One.

What is the chemical formula for water?

H₂O.

Why is oxygen slightly negative in water?

It attracts the shared electrons more strongly than hydrogen.

Why are the hydrogens slightly positive in water?

The shared electrons spend more time near oxygen than hydrogen.

Why is water often called the universal solvent?

Because it dissolves more substances than any other common liquid.

Which kinds of substances dissolve best in water?

Polar molecules and ionic compounds.

Why do ionic compounds dissolve in water?

Water molecules surround and separate the positive and negative ions.

What does hydration shell mean?

The layer of water molecules surrounding a dissolved ion.

Why doesn't oil mix with water?

Oil is nonpolar, while water is polar.

What does hydrophilic mean?

Water-loving; readily mixes with or dissolves in water.

What does hydrophobic mean?

Water-fearing; does not mix with water.

Give one example of a hydrophilic substance.

Salt or sugar.

Give one example of a hydrophobic substance.

Oil, wax, or fats.

Why is water essential inside cells?

It provides the medium where most chemical reactions occur.

Why is polarity the key to nearly all of water's unique properties?

Because polarity allows water molecules to form hydrogen bonds with each other and other substances.

Exam Tip: What is the difference between a polar covalent bond and a hydrogen bond?

A polar covalent bond is within a water molecule; a hydrogen bond is between different water molecules.

Memory Trick: What does 'like dissolves like' mean?

Polar substances dissolve in polar solvents like water; nonpolar substances dissolve in nonpolar solvents.

What is cohesion?

The attraction between molecules of the same substance. Water molecules stick to each other because of hydrogen bonds.

What causes cohesion in water?

Hydrogen bonds between neighboring water molecules.

Why is cohesion biologically important?

It helps transport water through plants and contributes to surface tension.

What is adhesion?

The attraction between molecules of different substances.

Give an example of adhesion.

Water sticking to glass or plant cell walls.

What causes adhesion in water?

Hydrogen bonds between water and other polar substances.

How are cohesion and adhesion different?

Cohesion is water-to-water attraction; adhesion is water-to-other-materials attraction.

What is surface tension?

The resistance of a liquid's surface to breaking because of cohesion.

Why does water have high surface tension?

Hydrogen bonds create a strong surface network.

Which insect walks on water because of surface tension?

The water strider.

What is capillary action?

The movement of water through narrow spaces due to cohesion and adhesion.

Why is capillary action important in plants?

It helps pull water upward through xylem.

Which property pulls water molecules together in xylem?

Cohesion.

Which property helps water cling to xylem walls?

Adhesion.

Can capillary action occur without cohesion and adhesion?

No. Both are required.

What is specific heat?

The amount of heat needed to raise the temperature of a substance.

Why does water have a high specific heat?

Hydrogen bonds absorb heat before molecules speed up.

How does high specific heat benefit living organisms?

It keeps temperatures relatively stable.

Why do lakes and oceans warm slowly?

Water has a high specific heat.

What is heat of vaporization?

The energy required to convert liquid water into vapor.

Why does water have a high heat of vaporization?

Hydrogen bonds must be broken before evaporation occurs.

How does sweating cool the body?

Evaporation removes heat from the skin.

What is evaporative cooling?

Cooling caused by evaporation of water.

Why does ice float?

Ice is less dense than liquid water because hydrogen bonds spread molecules farther apart.

Exam Tip: What single property explains most of water's unique characteristics?

Hydrogen bonding.