Chapter 2-2

Properties of Water

Why is Earth called the blue planet

• Covered by water

Water is unique because:

• 1. Liquid at temperature found on most of the Earth

• 2. Expands as it freezes, unlike most substances– Because ice is less dense than water, it floats.

Water Facts

• Water exists mainly as a liquid over much of Earth’s surface

Water Facts• As water freezes it expands; this makes

ice less dense than water causing it to float

Water molecules as a liquid

As water freezes the molecules arrange themselves in a very specific pattern (ice is classified as a crystalline solid)

A Molecule of WaterOne Atom of Oxygen; O

Two Atoms of Hydrogen; H

This gives us

Water is a neutral molecule. It has 10 proton and 10 electrons

H2

2 p+

2 e- 08 p+

8 e-

Although water is an electrically neutral molecule it is does exhibit polarity.

Polarity refers to the unequal sharing of electrons.

POLARITYThe shared electrons between oxygen and

hydrogen tend to spend more time orbiting the oxygen atom giving it a unequal charge distribution

e

In a water molecule, are the electrons more likely to be near the oxygen nucleus or the hydrogen nucleus? _______________Oxygen nucleus

The oxygen end gets a partial negative charge

Hydrogen end gets a partial positive charge

Negative charges are attracted to positive charges.

In a water molecule the negative oxygen end is attracted to the positive hydrogen end of another molecule

This creates a weak Hydrogen Bond between water molecules

Hydrogen bonding

H

H

HH

H

H

HH

H HH H

O

O

O

O

OO

(-)

(-)

(-)

(-)

(-)

(-)

(+)(+)

(+)

(+)

(+)

(+)

(+)

(+)

(+)(+)(+)

(+)

Why are the charges in parentheses? they are partial charges

Water can form multiple hydrogen bonds between molecules

Cohesion • Cohesion is an attraction between

molecules of the same substance.

• Water is very cohesive because of hydrogen bonding.

Cohesion creates surface tensionThe weight of the paper clip isn’t enough to break the weak hydrogen bonds between the water molecules

Water striders rely on cohesion between water molecules

Adhesion• Adhesion is an attraction between

molecules of different substances.

•Water also sticks to, or adheres, to other surfaces well. This is why a meniscus forms when you are measuring liquid.

Capillary action

• Cohesion and adhesion combined allows water to move up certain materials.

This is how plants get water from the ground up through their stems

Polarity also affects Solubility

Solubility refers to the ability of one substance (solute) to dissolve in another (solvent)

Solubility

• Polar substances can dissolve other polar substances.

• Non-polar substances dissolve other non-polar substances.

• Polar substances and non-polar substances do not mix.

Urea dissolves in water because both are polar

H H

O

(-)

(+)(+)(-)

H H

O(+)(+)

(-)

(-)(+)

(+)

Weak Hydrogen bonds

H H

O(+)(+)

(-)C

CC

C

C

C

C

CC

C

C

C

CC

C

C

C

C

CC

CC

CC

C

C

C

C

CC

C

Molecules such as Naphthalene (White Tar) will not dissolve in water because Naphthalene is a non-polar molecule

H H

O(+)(+)

(-)

• Water is a polar molecule

• Oil is a non-polar molecule

• Oil and water do not mix for this reason

Mixtures

• A mixture is two or more elements or compounds that are physically mixed together but not chemically combined.

• Example: Salt and pepper mixed together

• Mixtures made with water include solutions and suspensions.

Mixtures made with Water

• Solutions: a mixture of two or more substances in which the substances are evenly distributed.

• A solution has both a solvent and a solute.

Kool-aidOH OH

YEAHYEAH!!

• A solvent is a substance in which a solute is dissolved to form a solution. – Ex: water

• WATER is the UNIVERSAL SOLVENT because of its polarity

• A solute is a substance that is dissolved in a solvent to make a solution. – Ex: kool-aid, salt, sugar

Mixtures made with Water

• Suspensions: a mixture of water and a non-dissolved material.

• Mud is a suspension, the dirt is not fully dissolved in the water so the particles are “suspended”

Other suspensions:

Blood, which is mostly water contains many dissolved and undissolved particles

So is blood a solution or suspension?

• Both!

• Blood is mostly water, & many substances are dissolved in it. Here blood is a solution.

• Cells in the blood remain in suspension.

Water can form ions, but remains neutral.

H2O H+ + OH-

Water Hydrogen + Hydroxide

Acid or Base

• The pH scale represents how many H+ and are in a solution. The pOH scale represents how many OH – ions

pH scale ranges from 0 (strong acid) to 14 (strong base)

Lets draw our scale…

• Acids have a low [OH-]; high [H+]

• Bases have high [OH-]; low [H+]

• What are examples of each?

Acids

Have extra H+ ions

Have a pH less than 7

Hydrochloric Acid and Vinegar

Bases

• Have more OH- ions

• Have a pH greater than 7

• Also called Alkalines

• Soap and Ammonia

Neutral

Neutral solutions have exactly the same number of H+ as OH- ions

Pure Water….. Has one H+ for every OH-

Tap water usually has a pH just above 7

Myth: All acids are harmful and will burn your skin.

Busted: Not all acids cause burns…some examples are lemon juice and vinegar

Myth: Bases are safe to handle

Busted: Lye or Sodium Hydroxide is very dangerous

• Ph scale is logarithmic- each number increase is 10X more.

• pH of 1 is 10x more acidic than a pH of 2 and 100x more acidic than a pH of 3.

Buffers

• Resist changes in pH

• A buffer is a weak acid or base that reacts with strong acids or bases to prevent sudden changes in pH.

Buffers

• Why are buffers important in the body? – to help maintain homeostasis.– The pH in the body needs to be between 6.5-

7.5 for chemical reactions to occur properly.