Water Molecule and States of Water

The Water Molecule

• Two hydrogen atoms bond to one oxygen atom to form water.
• Water molecules are not symmetrical.
• Electrons spend more time near the oxygen (O) and less time near the hydrogen (H).
• Water molecules are polar.
• Water molecules are held together by a hydrogen bond.
• There is a higher concentration of electrons around the nucleus of the oxygen than around the hydrogen.
• The hydrogen end of a water molecule is slightly positive.
• The oxygen end of a water molecule is slightly negative.
• The hydrogen end of one water molecule is attracted to the oxygen end of another water molecule.

States of Water

• Water is found on Earth in three states: liquid, solid (ice), and gas (steam or vapor).
• Water molecules are constantly moving.
• A temperature increase causes an increase in movement.
• When water molecules move faster, they tend to break their hydrogen bonds, a process called evaporation.
• When gas or vapor molecules slow down, they clump together, which is called condensation.
• As water becomes cooler, it becomes less dense.
• As water changes from a liquid to a solid, molecules form crystals.
• In ice crystals, molecules are spaced further apart compared to liquid water.
• Ice is less dense than water and it floats.
• Ice forming on the surface of bodies of water provides a "blanket" that is helpful for aquatic organisms.
• Lakes would freeze from the bottom upwards if ice was more dense than water.

Hydrological Cycle

• Also known as the Water Cycle
• Describes the continuous movement of water on, above, and below the surface of the Earth.

Thermal Properties of Water

• Water has a high specific heat, or heat capacity, which is the amount of heat needed to raise 1 gram of a substance by 1 degree Celsius.
• The high heat capacity of water regulates the rate at which air temperature changes.
• Water affects global climate, leading to milder temperatures in coastal regions.
• Aquatic organisms face less temperature variation than terrestrial organisms due to the high heat capacity of water.
• Water is used in cooling systems in cars and industrial plants, due to high specific heat capacity.

Surface Tension

• Water has the highest surface tension of all liquids except for mercury.
• The tendency of water molecules to attract to one another, or cohere, results in surface tension.
• Surface tension allows insects to walk on water.
• The attraction between water molecules through hydrogen bonds is called cohesion.
• The attraction of water molecules to another substance is called adhesion.
• Capillarity involves the movement of water within the spaces of a porous material due to surface tension, adhesion, and cohesion.
• Capillarity allows water to climb from soil into plants.

pH: Alkalinity/Acidity

• pH measures the H+ ions in a substance.
• The pH scale ranges from 0 to 14.
• A pH of 7 is neutral.
• A pH below 7 is acidic.
• A pH above 7 is alkaline (or basic).
• Each pH unit represents a ten-fold change in H+ concentration.

Water as a Solvent

• Known as the "Universal Solvent" because it can dissolve more things than any other natural substance.
• Solvent properties include the ability to interact with other polar compounds.
• Water is repelled by non-polar compounds.
• Small size allows it to saturate areas.
• Water can convey other substances in solutions.
• Water is especially effective at dissolving salts.
• Salts form from the combination of particles with opposite electrical charges, known as ions.
• When salt is placed in water, the strongly charged salt ions attract to the weaker charged water molecules.
• Water molecules surround each ion.
• Salt crystals fall apart, or dissociate, and the salt dissolves.

Salt Water and Salinity

• Salinity is the total amount of salt dissolved in seawater.
• Seawater contains most all elements found on Earth.
• Solutes are made of a small group of common ions; a solute is the substance being dissolved by the solvent.
• Six ions make up over 98% of solids in seawater.
• Chloride (Cl-) accounts for 55.03%.
• Sodium (Na+) accounts for 30.59%.
• Sulfate (SO4-2) accounts for 7.68%.
• Magnesium (Mg+2) accounts for 3.68%.
• Calcium (Ca+2) accounts for 1.18%.
• Potassium (K+) accounts for 1.11%.
• Sodium and Chloride make up 85% which contributes to the salty taste.
• Salinity varies by the addition or deletion of pure water, not salts.
• Pure water is added by precipitation.
• Pure water is removed by evaporation or freezing.

Concepts to Remember

• Solvent + solute = solution

Hypotonic

• Solutes in cell are more than outside of cell.
• Outside solvent will flow outside of cell.

Isotonic

• Solutes are equal inside and outside the cell.

Hypertonic

• Solutes are greater outside the cell.
• Fluid will flow out of cell.

Diffusion

• The interpenetration of two substances
• Occurs whenever the solute distributes itself uniformly into the solvent.
• Influenced by the size of molecules, temperature, molecular weight.

Osmosis

• Solutions of unequal concentrations are separated by a semi-permeable membrane
• Fluid flows from the side of low osmotic pressure to the side of high osmotic pressure
• Continues until there is osmotic equilibrium