Properties of WaterChapter 2 Section 2

Objectives Discuss the unique properties of

water Differentiate between solutions

and suspensions Explain what acidic solutions and

basic solutions are

The Water Molecule Its properties and functions stem

from its chemical structure It is one of the few compounds

found in a liquid state over the Earth’s surface

It is a neutral molecule Is essential for all biological

processes

Structure of Water Made of 2

hydrogen atoms covalently bonded to 1 oxygen atom

Polarity The electrons are not shared

equally between the oxygen atom and the hydrogen atoms

This unequal sharing causes partial charges to occur at opposite ends of the molecule

A molecule in which the charges are unevenly distributed is called polar

The polarity of water molecules also gives it a bent shape

Hydrogen Bonding Because of polarity, water molecules are

attracted to each other They form hydrogen bonds between the

partial positive charge of the hydrogen atoms of one molecule to the partial negative charge of oxygen on another molecule

Hydrogen bonds are the force of attractions between hydrogen molecules and other atoms such as oxygen, nitrogen & fluorine

These bonds form, break , and reform with great frequency

Polarity of water and hydrogen bonding are responsible for the other properties of water

Properties of Water Cohesion Adhesion Heat Capacity Solvent nature

Cohesion

Cohesion is an attraction between molecules of the same substance

Water molecules can be involved in as many as 4 hydrogen bonds at the same time, making it extremely cohesive

Cohesion causes water molecules to be drawn together, which is why drops of water forms beads on a smooth surface

It is also responsible for surface tension which makes water acts as if it has a thin “skin” on its surface Surface tension also enables small creatures

such as spiders and water-striders to run on water without breaking the surface

Adhesion Adhesion is the attractive force between

molecules of different substances For example: water molecules and glass

molecules The surface of water becomes curved

because the attraction between the water and glass molecules is greater than the cohesion between water molecules

Adhesion between water and glass molecules also cause water to rise in a narrow tube against the force of gravity

This affect is called capillary action Plants use capillary action to draw water

from its roots to its leaves

Cohesion and Adhesion in Action Trip to ISS Mini Lab! How many drops of water can

you get on a penny before it spills over?

Heat Capacity Due to multiple hydrogen bonds between water

molecule, it takes a large amount of heat energy to speed those molecules up and create a rise in temperature

Water’s heat capacity, the amount of heat energy required to increase temperature, is relatively high

This allows: Bodies of water to absorb large amounts of

heat with only small changes in temperature Cells to regulate temperature Stabilizing of global temperatures Evaporative cooling

Water Mixtures Water is not always pure It is often part of a mixture A mixture is a material composed of two or more

elements that are physically combined and not chemically combined

For example: Salt & pepper Sugar & sand Air

Living things are mixtures involving water Two types of mixtures with water:

Solutions Suspensions

Solutions A solution is a mixture in which 1 or more

substances are uniformly distributed in another substance

They can be mixtures of liquids, solids, or gases

For example: Blood Plasma

They are made up of two components: the solute & the solvent

Water’s polar nature allows it to dissolve polar substances

Examples: sugar, ionic compounds, some proteins

Solute vs. Solvent Is a substance

dissolved in the solvent

Can be ions, atoms, or molecules that are dispersed

Is the substance that does the dissolving

Example: water (universal)

In a solution of sugar water, sugar is the ___________ and water is the ____________.

Solutions Continued Solutions can have varying

concentrations which is the amount of solute in

a certain amount of solvent Example: 5% saltwater solution is 5g

of salt dissolved into 100 mL of water Saturated solution: is one in which

no more solute can dissolve Aqueous solution: is one in which

water is the only solvent

Suspensions Some materials do not dissolve when

placed in water, but separate into pieces so small that they do not settle out

The movement of the water molecules keeps the small particles suspend

Mixtures of water and undissolved material are called suspensions

Some of the most important biological fluids are both solutions and suspensions

For example: blood

Blood as a Solution & Suspension

Ionization of Water Occurs when water molecules are pulled

apart or dissociate into ions

When this happens it leaves hydrogen to bond with another water molecule forming the hydronium ion

Hydroxide ion

Hydrogen ion

pH Chemists devised a

measuring system called the pH scale to indicate the concentration of H+ ions in solution

The pH scale ranges from 0 to 14

Each step on the pH scale represents a factor of 10

pH Determinations The pH of a solution is determined by the

amount of hydrogen ions or hydroxide ions present

At a pH of 7, the concentration of the H+ ions and OH- ions is equal so its called neutral

Solutions with a pH below 7 are called acidic because they have more H+ ions than OH- ions

Solutions with a pH above 7 are called basic because they have more OH- ions than H+ ions

Acids & BasesAcids

Is any compound that forms H+ ions in solution

Strong acid pH values range from 1 to 3

Tend to have a sour taste Can be corrosive For example HCl

H2SO4

Bases Is any compound that

forms OH- ions in solution

Strong base pH values range from 11 to 14

Tend to have a bitter taste

Tend to feel slippery

For example:

NaOH

NaHCO3

Buffers

The pH of the fluids within most cells in the human body must generally be kept between 6.5 and 7.5

If the pH is higher or lower it affects the chemical reactions within the cells

Controlling pH is a matter of homeostasis and is done using buffers

Buffers are weak acids or bases that can react with strong acids or bases to prevent sudden changes in pH

For example: bicarbonate and phosphate ions