Physical and Chemical Properties of Water

Do Now!

• List the various ways that water is used.
• Using prior knowledge, predict the % of typical water usage for a family of four in North America?

The Water Molecule

• Water is formed when two hydrogen atoms bond to one oxygen atom.
• Not symmetrical
• Electrons spend more time near the (O) and less time near the (H)
• Water molecule is polar

Water molecules are held together by a hydrogen bond

The Water Molecule

• There is a greater concentration of electrons around the nucleus of the oxygen than around the hydrogen.

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• Therefore, the hydrogen end is slightly positive and the oxygen end is slightly negative…...

Dipolar

The Water Molecule

• When water molecules attract to one another, the hydrogen end of one molecule is attracted to the oxygen end of another.

States of Water

• Water is the only substance found on Earth in all three states (phases):

1. Liquid

States of Water

2. Solid (Ice)

States of Water

3. Gas (Steam or Vapor)

Physical States

Vapor

States of Water

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• Water molecules are constantly moving

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• Temperature increase = Increase in movement

Hydrological Cycle

States of Water

• When water molecules move faster, they tend to break their hydrogen bonds.

This is called

Evaporation

States of Water

• When gas or vapor molecules slow down, they clump or join together.

This is called

Condensation

States of Water

• As water becomes cooler, it becomes less dense:

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1 gram per cubic cm at 39.2^o

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0.95865 gram per cubic cm at 212^o

States of Water

• As water changes from a liquid to a solid, molecules form crystals.

In ice crystals, molecules are spaced further apart.

Liquid

Solid (Ice)

States of Water

• Since molecules are spaced further apart, ice is less dense than water (it floats)

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Helpful for aquatic organisms; forms a “blanket”

If ice was more dense than water, lakes would freeze from the bottom upwards.

Thermal Properties of Water

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• Water has a high specific heat, or heat capacity (the amount of heat needed to raise 1g of a substance 1C^o.)

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• This quality of water is important for several reasons:

Thermal Properties of Water

1. Regulates rate at which air temperature changes.

Affects global climate (milder temperatures in coastal regions)

July 9,2005

Thermal Properties of Water

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2. Aquatic organisms face less temperature variation than terrestrial organisms.

Thermal Properties of Water

3. Water is used in cooling systems found in cars and industrial plants

Surface Tension

• Next to mercury, water has the highest surface tension of all liquids.

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This is a result of the tendency of water molecules to attract to one another, or cohere, at the surface of any accumulation of water.

Surface Tension

• Surface tension allows insects to walk on water.
• Interaction between hydrogen bonding and the earth’s gravitational pull

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Surface Tension

• Cohesion- the attraction between water molecules to each other through Hydrogen bonds (H)

Surface Tension

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• Adhesion- the attraction of water molecules to another substance.

Surface Tension

• Capillarity- the movement of water within the spaces of a porous material due to the forces of surface tension, adhesion, and cohesion.

Allows water to climb from

soil into plants

pH: Alkalinity/Acidity

• The measurement of the H+ ions found in that particular substance
• The scale goes from 0 to 14
• 7 is neutral
• Below 7 is acidic
• Above 7 is alkaline (or basic)
• One pH unit represents a ten-fold change in H+ concentration

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The pH Scale

Water as a Solvent

• Since water can dissolve more things than any other natural substance, it is known as the “ Universal Solvent”
• Properties of Solvents include:
• Interacts with other polar compounds
• Is repelled by non-polar compounds
• Small size allows it to saturate areas
• Can convey other substances in solutions

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Water as a Solvent

• Water is especially good at dissolving salts

Salts form from the combination of particles with opposite electrical charges (or ions)

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EX. Na⁺ + Cl^- = NaCl

Water as a Solvent

• When salt is placed in water, the strongly charged salt ions attract to the weaker charged water molecules.

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• Water molecules surround each ion.

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• Salt crystals fall apart, or dissociates, and the salt dissolves.

Water as a Solvent

Salt Water and Salinity

• Solids found in seawater come from 2 main sources:

1. Chemical/Mechanical weathering of rocks on land.

Carried to sea by rivers

2. Come from the Earth’s interior

Released into oceans by hydrothermal vents

Salt Water and Salinity

Chemical/Mechanical weathering

Mississippi

Delta

Salt Water and Salinity

Hydrothermal Vents

Salt Water and Salinity

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• Seawater contains most all elements found on Earth

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• Most of the solutes are made of a small group of common ions

Solute- the substance being dissolved by the solvent

Salt Water and Salinity

• Six (6) ions compose over 98% of solids in seawater.

Na⁺ and Cl^- = 85%

(This is why seawater

tastes salty)

Salt Water and Salinity

• Salinity- total amount of salt dissolved in seawater.

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Expressed as the number of grams of salt left when 1000g of seawater evaporates

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EX. 35 grams left over = salinity of 35 parts per thousand (35 ppt or 35⁰/₀₀)

Salt Water and Salinity

• Rule of Constant Proportions- relative amounts of the various ions in seawater are always the same.

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EX. Chloride is always 55.03% of however much salt/solute is present

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***Indicates the ocean is well mixed***

Salt Water and Salinity

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Salinity varies by the addition or deletion of pure water (not salts)

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Pure water is added by precipitation.

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Pure water is removed by evaporation or freezing.

Salt Water and Salinity

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Average salinity:

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Ocean: 35 ppt

Red Sea: 40 ppt hot,dry climate; evaporation>precipitation

Baltic Sea: 7 ppt excess river runoff; evaporation<precipitation

Useful Constants/Conversions of Water

• Weight: 62.416 pounds per cubic foot at 32°F
• Weight: 8.33 pounds/gallon, 0.036 pounds/cubic inch
• Density: 1 gram per cubic centimeter (cc) at 39.2°F
• 1 gallon = 4 quarts = 8 pints = 128 ounces = 231 cubic inches
• 1 liter = 0.2642 gallons = 1.0568 quart = 61.02 cubic inches
• 1 Acre-Foot = 325,851 gallons = 43,560 cubic feet