PA Assessment Anchor and Eligible Content: S11.A.3.3.2: Compare stationary physical patterns to the objects properties.KEY CONCEPT

All living things are based on atoms and their interactions.

Water’s Unique NatureWater

•Water covers more than 75% of the Earth's surface.

•It is the most abundant compound in nearly all living organisms.

•The most important property of the water molecule is that, due to an uneven distribution of electrons, it is slightly charged on each end.

Water•Such molecules are called “polar” because they have distinctive regions, or “poles” with opposite electric charge.

•Due to the polar covalent bonds that hold a water molecule together, hydrogen bonds form where the negative oxygens and the positive hydrogens are located.

•This polarity results in the water molecule being like a little magnet which gives it unique physical and chemical properties.

Water

5 Unique Properties of Water

1. High Specific Heat•Water is unusual in being able to absorb a lot of heat energy without having its temperature increase by very much.

•This property of water helps to reduce temperature fluctuations in animals and plants.

•It allows oceans to act as a tremendous heat sink to moderate the earth's temperature.

5 Unique Properties of Water

2. Heat of Vaporization•This is the amount of heat energy required to evaporate a given weight of a liquid.

•Water has a very high heat of vaporization. This means that it takes a lot of heat to evaporate just a little water.

•During the summer this prevents a lot of water loss from rivers, lakes, ponds, etc.

5 Unique Properties of Water

3. Heat of Fusion•This is the heat energy that must be removed from a given weight of water in order to freeze it.

• Water’s relatively high heat of fusion means that it takes longer for lakes and streams to freeze in the winter.

•This gives living things more time to adjust to the change.

5 Unique Properties of Water

4. Freezing and Expansion of Water

• When water is cooled below 3.8o Celsius, the water molecules arrange themselves into a crystal structure.

•What is most unusual is that this crystal structure called ice, is less dense than water.

•If ice didn’t float, lakes would freeze from the bottom up killing much of the life that lives there.

5 Unique Properties of Water

5.Water is a major solvent in nature.

•This is due to many other molecules being ionic or polar and their electrical charge making them attracted to the water molecules.

•When water and another substance is mixed the resulting solution is called an aqueous solution.

5 Unique Properties of Water

•Any solution contains the following parts:

Solute (what's being dissolved) + Solvent (what is doing the dissolving) = Solution

•Thus water dissolves many kinds of salts, sugars, and proteins.

Unique Properties of Water

Dissociation of Water

•When ionic compounds dissolve in water, their ions separate from one another in a process called dissociation.

•Water and many other covalent compounds can also dissociate into ions.

Dissociation of Water • Water molecules sometimes split apart to form

hydrogen ions and hydroxide ions.

• This reaction can be summarized by a chemical equation in which double arrows are used to show that the reaction can occur in either direction. (Draw on back of PowerPoint)

Dissociation of Water

•The hydroxide ion retains the electron left behind by the departed hydrogen.

•It therefore has one more electron than it has protons.

•This means that it has a full unit of negative charge and is symbolized as OH-- .

Acids

•Certain molecules dissociate in such a way that they release a hydrogen ion without releasing a hydroxide ion.

•These substances are called acids.

•The chemist’s definition of an acid is a “proton donor”.

Acids

•If many hydrogen ions (protons) are donated the effect can be very profound, for example, burning your skin.

•Hydrochloric acid is an example of a strong acid.

Bases•The opposite of an acid is a base, also known as an alkali.

•A base is defined as a “proton acceptor”. It is the hydroxide ion that accepts the proton.

•A strong base can give your skin a worse burn than an acid. An example of a strong base is sodium hydroxide.

Neutralization

•When a strong base & strong acid are mixed, the hydroxide ions from the base combine with the hydrogen ions from the acid to form water.

•This process is called neutralization.

The pH Scale•The pH scale ranges from 0 to 14.

•Each change in one pH unit indicates a tenfold increase or decrease in hydrogen ion concentration and the opposite tenfold change in hydroxide ion concentration.

•Pure water has a pH of 7, which means it has equal concentrations of hydrogen ions and hydroxide ions.

The pH Scale•Acids have pH values of less than 7.

•A solution with a pH of 6 has ten times the hydrogen ion concentration of a pH 7 solution and only one-tenth the hydroxide ions concentration. It is therefore slightly acidic.

•Strong acids, such as those that help digest food in the stomach, have pH values of 1 to 3.

The pH Scale•Bases have pH values greater than 7.

•Strong bases, like ammonia have high pH values, ranging from 11 to 14.

•In order to maintain homeostasis in the human body, the pH values within most cells range from 6.5 to 7.5.

The pH Scale•These near-neutral values within cells are maintained by dissolved compounds called

buffers, which are weak acids or bases that can react with strong acids or bases to prevent sharp, sudden changes in pH.

pH scale

CHEMICAL COMPOUNDS IN LIVING THINGS

•Of the 94 naturally occurring chemical elements 11 are common in living organisms.

•Another 20 are found in trace amounts.

•The following 4 elements make up 96.3% of the total weight of the human body: carbon, hydrogen, oxygen, and nitrogen (CHON).

•These elements make up practically all the chemical compounds in living things.

Inorganic Compounds•Inorganic compounds are primarily those compounds that do not contain carbon.

•An exceptions to this definition is carbon dioxide.

•Living things contain many inorganic compounds.

Inorganic CompoundsSome examples of inorganic compounds in living things:

•water

•carbon dioxide

•calcium phosphate (used in bone formation) •salts (which help balance pH)

Organic Compounds•Organic compounds are carbon containing compounds.

•Carbon is a unique element because of its remarkable ability to form covalent bonds that are strong and stable.

•Because carbon has 4 electrons in its outermost energy level, it can form 4 single covalent bonds.

Organic Compounds

Carbon Atom

Organic Compounds•Carbon can form chains of almost unlimited length by bonding to other carbon atoms.

•The bonds between carbon atoms in these chains can be single, double, or triple covalent bonds or combinations of these bonds.

•These chains can close on themselves to form rings.

Organic CompoundsThese ring structures may have single or double bonds, or a mixture of both. No other element can equal carbon in this respect.

Polymerization•Many carbon-based compounds are formed by a chemical process called polymerization.

•Polymerization is the process where large compounds or polymers are constructed by smaller compounds or monomers bonding together.

•A large polymer is called a macromolecule (meaning giant molecule).

Polymerization

PolymerizationThe chemical diversity that polymerization allows living things is similar to the diversity that our alphabet allows us. Although there are only 26 letters in the alphabet, our ability to join them together (polymerize them) to form words gives us an almost infinite variety of possible words (molecules).