Water

The Single most abundant compound in most living things

Properties of Water

• Water:– Expands upon freezing– Has adhesion properties (capillary action)– Has cohesion properties due to surface tension– Has a high specific heat– Has a high heat of vaporization– Has a high heat of fusion– Is the greatest solvent on Earth

Expansion upon Freezing• Water is a liquid across most of the earth• Water expands as it freezes (ice is less dense

than liquid water = ice floats on water)• Fish and aquatic life can survive under ice

Polarity

• Oxygen has 8 protons in its nucleus. Hydrogen only has 1 proton in its nucleus. Oxygen has much stronger attraction for electrons than hydrogen

• Greater probability of finding the shared electrons near the oxygen atom than the hydrogen atom (covalent bonding)

PolarityThus, the oxygen end of the molecule has a partial negative charge while the hydrogen end of the molecule has a partial positive charge.

Hydrogen Bonds

• Because of the partial charges, polar molecules such as water can attract each other

• Hydrogen bonds form between hydrogen atoms of one water molecule and oxygen atoms of another water molecule

Cohesion / Surface Tension

• Cohesion is an attraction between molecules of the same substance.

• Water is extremely cohesive due to H bonding.

Adhesion / Capillary Action

• Adhesion is an attraction between molecules of different substances

• Adhesion is why water is drawn out of the roots of plants and up into its stems and leaves

Meniscus of water occurs because the adhesion between water molecules and glass molecules is stronger than the cohesion between water molecules – called capillary action

High Specific Heat

• Specific heat means the ability of a substance to raise 1 gram of body mass by 1°C.

• Water has high specific heat because a lot of energy is required to break H bonds

• Important because water buffers organisms' internal temperatures

High Heat of Vaporization

• Water evaporates only at a high heat.

• It resists evaporating because H bonds must be broken to change states from liquid to gas

• Allows you to cool from sweating

High Heat of Fusion

• Amount of energy needed to change state from solid to liquid

• Also due to breaking H bonds

• Resists melting. Keeps environment cool

Solutions

• A mixture is composed of two or more elements or compounds that are physically mixed but not chemically combined (salt and pepper)

• Two types of mixtures can be made with water:

• Solutions• Suspensions

Water as a Solvent

• All of the components of a solution are evenly distributed throughout.

• In a salt-water solution, salt is the solute – the substance that is dissolved

• Water is a solvent – the substance in which the solute dissolves.

• Water’s polarity gives it the ability to dissolve ionic compounds and polar molecules.

• Water is the greatest solvent on earth.

Solutions and Suspensions

– When a crystal of table salt is placed in warm water, sodium and chloride ions are attracted to the polar water molecules.

Na+

Na+

Cl -

Cl -

Water Water

Solutions and Suspensions

– Ions break away from the crystal and are surrounded by water molecules.

Cl -

Solutions and Suspensions

– The ions gradually become dispersed in the water, forming a solution.

Solutions and Suspensions

• Suspensions are mixtures of water and nondissolved materials.

• Blood that circulates through your body is composed of suspended cells in a liquid plasma matrix

Acids, Bases, and pH

• A water molecule can react to form hydrogen and hydroxide ions.

• Water is neutral because the number of positive hydrogen ions (H+) produced is equal to the number of negative hydroxide ions (OH-) produced.

• The pH scale indicates the concentration of H+ ions in solution. The pH scale ranges from 0 to 14.

• At a pH of 7, the concentration of H+

ions and OH- ions is equal.

• Pure water has a pH of 7.

The pH Scale

Human blood

Milk

Sea water

Normal rainfall

Pure water

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

The lower the pH, the greater the acidity.

Acid rainTomato juice

Lemon juice

Stomach acids

The pH Scale

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

The higher the pH, the more basic the solution.

The pH Scale

Soap

Bleach

Oven cleaner

Ammonia solution

Acids

• An acid is any compound which forms H+ ions in solution.

• Acidic pH is below 7, therefore has more H+ ions that pure water

• Examples include: acetic acid (vinegar), gastric juice (stomach acid), sulfuric acid (in car batteries)

Bases

• A base is a compound that produces OH- ions in solution.

• Basic, or alkaline, solutions contain fewer H+ ions that pure water, and have a pH above 7

• Basic examples include: baking soda (sodium bicarbonate), ammonia (NH3), lye (NaOH), milk of magnesia

Buffers

• The pH of fluids within most cells of the human body is kept between 6.5 and 7.5

• If pH is lower or higher if will affect chemical reactions. Therefore, pH balance is an important part of homeostasis.

• Buffers are weak acids of bases that react with strong acids or bases to prevent sudden sharp changes in pH.