Water Water is the most essential and abundant substance on Earth. Cells are made up mostly of water and most cells are surrounded by water. The importance of water is largely due to its many unique characteristics, which all directly relate to one very important property of water.

Water is…….. _______________ Polar!!! Polarity is the unequal sharing of electrons in molecules formed with covalent bonds What is that very important property if water????

Remember… What type of bond forms a water molecule??? Covalent! The chemical formula of water is ___________. The atomic number of hydrogen is ___ and the atomic number of oxygen is ___. Therefore how many protons does the oxygen nucleus have? How many protons does each hydrogen nucleus have? Due to this difference, the oxygen nucleus has a much stronger positive charge than each hydrogen nucleus.

Because the oxygen nucleus has a much stronger positive charge than the hydrogen… the shared negatively-charged electrons are much more attracted to oxygen (because of its greater positive charge). This results in all 10 electrons orbiting around the oxygen end of the molecule just about all the time.

Because of the shape of a water molecule (bent), oxygen is at one end while the hydrogen atoms are at the other. Therefore, the oxygen end of the molecule is slightly negative (due to the presence of all those electrons) and the hydrogen end of the molecule is slightly positive (due to the lack of the electrons). A molecule in which the charges are unevenly distributed is called a polar molecule, because the molecule is like a magnet with poles.

Bonds that form between the _________ charged _____________ in one molecule and a __________ charged _________ in a nearby molecule are called _____________________ positively HYDROGEN BONDS Images from: HYDROGEN atom negativelyatom HYDROGEN BONDS Hydrogen bond animation Is a hydrogen bond a true chemical bond? No- it does not result in the formation of a new compound- it is simply an attraction.

Because of their partial positive and negative charges, polar molecules such as water can attract each other The attraction between the Hydrogen atom on one water molecule and the oxygen atom on the other water molecule is an example of a hydrogen bond.

but it is water’s ability to form ________________________ that is responsible for many of its special properties and characteristics. Hydrogen bonds are not as strong as covalent or ionic bonds… A single water molecule may be involved with as many as ___ hydrogen bonds at the same time. Multiple hydrogen bonds 4

Properties of water Due to water’s polarity, it has unique properties important in maintaining that forever important internal harmony we call… (everyone together!!!) 1. Water is “sticky” –Cohesion –Adhesion –Surface Tension 2. High specific heat 3. High heat of vaporization 4. Expansion on freezing 5. Water as a Solvent

Cohesion “Co” = “together” or “like” “sion” = “stick” The attraction between molecules of the same substance (why droplets of water form) Due to formation of multiple hydrogen bonds Contributes to the upward transport of water in plants- capillary action (roots  shoots)

Adhesion “Ad” = “unlike” The attraction between molecules of different substances Due to formation of multiple hydrogen bonds between polar water molecules and other slightly charged molecules water on a penny; along with cohesion, contributes to capillary action (the ability of water to rise in a narrow tube against the force of gravity)

Surface Tension The measure of how difficult it is to stretch or break the surface of a liquid Results from the H bonding of water molecules to one another at the surface and below the surface (cohesion) Gives still water on a lake or pond the appearance of glass Ex: insects walking on water

High Specific Heat The amount of heat needed to raise the temp of 1 gram of water by 1 degree Celsius Water has a very high specific heat- it takes a lot of energy to raise the temp of water Crucial in temperature stability in living systems so that they can maintain homeostasis Due to H bonding

High Heat of Vaporization A lot of heat energy is required to evaporate water (molecules that move fast enough can overcome their attraction to one another and depart the liquid and enter the air as gas) When water does evaporate from a surface, it draws heat energy out from the layer beneath, creating a cooling effect. This is called evaporative cooling Ex: evaporation of sweat by a human; evaporation of water from leaves of plant (transpiration)

Ice Floats (Expansion on Freezing) Water is one of the few substances that are less dense as a solid than as a liquid- therefore it floats. While other materials contract when they solidify, water expands.

Expansion on Freezing cont’d Water begins to freeze when its molecules are no longer moving fast enough to break their H bonds. As the temp reaches 0 degrees C, the water becomes locked into a crystalline lattice, with each water molecule bonded to the max of 4 other water molecules. Due to H bonding- keeps the molecules at “arms length”- far enough apart to make ice 10% less dense than liquid water. (water’s density= 1.0 g/ml)

Water as a Solvent Water dissolves many substances due to its polarity. Known as a very versatile solvent- other polar substances dissolve very easily in water.

Solution- a completely homogeneous mixture of 2 or more substances Solvent- the dissolving agent of a solution. Ex. Water Solute- the substance that is being dissolved

____________________ means “water loving” _________ groups/molecules try to _________ and touch water or ______________ molecules HYDROPHILIC “Like dissolves like” Water makes a great solvent in living things because so many molecules found in living things are polar or have a charge. be near other polar POLAR

_________________ means “water fearing” ___________ groups/molecules try to _________ other __________ molecules and __________ ________ molecules HYDROPHOBIC “Like dissolves like” polar Oil based paints dissolve in solvents such as turpentine... not water. be near NON-polar away from

Examples of hydrophilic substances- salts, polar compounds (sugar) *Also known as water soluble Examples of hydrophobic substances- oils, fats, lipids, waxes (non-polar compounds) *Also known as water insoluble

“Like dissolves like” ______ works because it has a _____________ that dissolves _______ and a __________ that dissolves in _____ to wash away oily dirt. SOAP NON-polar end Polar end grease water

WATER is important for all living things Image from: Average person ~~ 60-70% water Babies ~~ 78% Human brain ~~ 90%

WHY Water is important to cells: 1. It’s __________ so it can ____________ lots of different substances. 2. It can _________ lots of ______ _______________________________ very much. (That helps with _________________) 3. ____________________ form between water molecules so they stick together. 4.Water is an important ___________________ in many CHEMICAL REACTIONS. POLAR DISSOLVE HOMEOSTASIS HYDROGEN BONDS REACTANT/PRODUCT absorbHEAT WITHOUT changing temperature