Chemistry: Water and pH

The Science of Water

Water Fun Facts Necessary for life Water in our atmosphere helps to keep the planet warm. Our bodies are composed of ~70% dependent on water Although a person can live without food for more than a month, a person can only live without water for approximately one week. Your brain is 75-85% water and plays a vital role in your body's response to dehydration. It controls water intake through altering thirst and varying the water excretion from your kidneys.

Why is water important in Bio? The chemical reactions of all living things take place in an aqueous (water based) environment. Thus, water is one of the most important compounds found in living things!

Forming a Water Molecule Water (chemical formula H2O) has 2 hydrogen atoms and 1 oxygen atom. Bohr model for water: 8P 8N 1P 0N

Water is POLAR covalent Recall that electrons are SHARED in a covalent bond. Polar= electrons are shared unevenly Oxygen’s 8 p+ attract e- more strongly than Hydrogen’s 1 p+ O pulls on e- more, so electrons are more likely to be found near Oxygen

Water is POLAR covalent What does it mean? Water has a partial + pole & a partial – pole (called dipole) Acts like a magnet

So what? Hydrogen Bonds! Opposites Attract! Partial + of 1 water molecule will attract and stick next to partial – of another molecule Called a Hydrogen Bond NOT real bonds, just an attraction  “weak bond” that are easily broken

Who Cares?! Hydrogen bonds give water its special properties!!! https://www.youtube.com/watch?v=3jwAGWky98c

Properties of Water Adhesion & Cohesion High Specific Heat Solid ice is less dense than liquid water Universal Solvent

Cohesion Attraction between molecules of the same substance Responsible for water’s high surface tension A measure of how difficult it is to stretch or break the surface of a liquid Water is very cohesive because one water molecule may form as many as 4 hydrogen bonds at the same time. Jesus Christ Lizard Reason water creates beads Penny experiment!

Adhesion Attraction between molecules of different substances Explains why microscope slides stuck together Measuring volume in graduated cylinder?

Cohesion + Adhesion Capillary action in plants Water in Space

High Specific Heat Specific Heat is the amount of heat energy required to increase the temperature of water. Water’s multiple hydrogen bonds between its molecules gives it a very high specific heat Absorbs large amounts of heat energy with only small changes in temperature Releases heat energy slowly Moderates the Earth’s climate and helps living organisms regulate their body temperature

Ice is Less Dense than Water Density of H2O at different temperatures Temperature (°C) Density (g/cm3) 0 (solid) 0.9150 0 (liquid) 0.9999 4 1.0000 20 0.9982 100 (gas) 0.0006

Ice is Less Dense than Water This is a very rare property! Why is it important that ice floats? Floating ice insulates the water beneath it and prevents it from freezing. Allows aquatic organisms that live in water to survive during cold weather.

Ice Melting Ice (left) has an open lattice structure due to permanent hydrogen bonds that collapse when it melts. In liquid water (right), molecules are connected by short-lived hydrogen bonds because water is fluid. ICE WATER

Water is a Universal Solvent The polarity of water allows nearly any polar (unequal distribution of charges) substance to be dissolved in water Water can dissolve more substances than any other liquid. Solution = liquid consisting of uniform mixture of two or more substances Two parts of a solution: Solvent = liquid (dissolving agent) Solute = substance dissolved Example: Water = solvent Koolaid powder/sugar = solutes Kool-aid = solution

Rule to Making Solutions “Like Dissolves Like” DO NOT CONFUSE WITH OPPOSITES ATTRACT Since water is polar, it will dissolve ions and other polar substances ex: Salt (NaCl)  breaks down into Na+ and Cl- ions Hydrophilic = water loving. Dissolves in water.

Nonpolar molecules Water will not dissolve nonpolar molecules Hydrophobic = afraid of water. Does not dissolve in water. Electrons are shared equally in nonpolar molecules.  Does not form Hydrogen Bonds Oil is an example of a nonpolar molecule.

Solid, Liquid, and Gas Water is the only substance which exists under normal conditions on earth as a solid, a liquid, and a gas.

High Boiling Point Water’s high boiling point, allows water to remain as a liquid over most of Earth’s surface Crucial to life as almost all organisms need liquid water to survive.

On a Scale of 0 to 14 pH Notes

Recall… Solution= Mixture of 1+ substances Solute= substance dissolved in the solution Solvent= substance in which the solute is dissolved

Ions in Water Ions = positively or negatively charged particles Form when certain substances dissolve in a solvent. Example: Salt dissolving in water becomes Na+ and Cl-

Water Ions Water molecules dissociate (break apart) Causes the H2O molecule to ionize, forming two ions: H+ OH- Hydroxide ion

pH When the solution has a higher level of H+ than OH-,, an acid is formed When a solution has a higher level of OH- than H+, a base is formed pH is a measure of how acidic or basic a solution is. ie, how many H+ or OH- a solution has

Acids When H+ > OH- More hydrogen ions than Hydroxide ions

Bases When H+ < OH- Or OH- > H+ More Hydroxide ions than Hydrogen ions

The pH Scale Scientist have developed a Scale to compare the concentration of H+ ions to OH- Ions in Solution 0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 neutral STRONG Acid STRONG BASE

The pH Scale The pH scale is based on a factor of 10. This means that an acid with a pH of 4 is 10 times stronger than one with a pH of 5, while a base with a pH of 13 is 100 times stronger than a pH of 11.

Acid Precipitation! Yikes! Rain, snow, fog pH < 5.6 Recorded as low as 3!!! Sulfur dioxides and nitrogen oxides Burning of fossil fuels (coal, oil, gas) Factories and Cars!!! Impacts: Killing plants, and animals Eroding of buildings/statues/pipes/cables

Buffers A chemical substance that resists changes in pH by accepting hydrogen ions from or donating hydrogen ions to solutions and thus neutralizing acids or bases Maintains homeostasis

pH and breathing