6.2 Water & Diffusionp. 152 -

Chapter 6

The Chemistry of Life

Water and Its Importance

• Water is probably the most important substance to life on Earth.

• Water makes up 70 – 95 % of most living organisms.

Properties of Water

• Polar

• Resists temperature change

• Expands when it freezes

Polar Molecules

• When atoms share electrons the force that holds them together is called a covalent bond.

• An unequal sharing is a polar bond.• If molecules have a polar bond we say

they are polar molecules.• Polar molecule = molecule with an

unequal distribution of charge, resulting in the molecule having a positive end and a negative end.

Polar Molecules• Polar molecule =

molecule with an unequal distribution of charge, resulting in the molecule having a positive end and a negative end.

Water Striders use surface tension to walk on water

When the molecules of the liquid are more attracted to the molecules of the tube than they are to each other, the liquid will rise due to surface tension. You can demonstrate the capillary rise in tubes of different internal diameters. The four tubes and reservoir from a single allglass unit, 15cm high, held upright in a metal support.

This Redwood tree may be 300 feet tall yet water can reach the top through capillary action.

If water was not attracted to its self, plants would not be able to get the water they need to the leaves for photosynthesis.

Water dissolves many ionic and molecular compounds because of

its _____.

• A. ionic bonding

• B. polarity

• C. covalent bonding

• D. hydrogen bonding

Water dissolves many ionic and molecular compounds because of

its _____.

• A. ionic bonding

• B. polarity

• C. covalent bonding

• D. hydrogen bonding

What Are the Facts About Water

• Electrons in a water molecule spend more time near the oxygen than near the hydrogens so it is a polar molecule.

• Polar molecules attract ions.• Ions are negatively or positively charged

atoms.• Because it is polar, water can dissolve

many ionic compounds like salt• It can also disolve other polar molecules

like sugar.

What Are the Facts About Water

• Because it is polar, water can dissolve many ionic compounds like salt

• It can also dissolve other polar molecules like sugar.

• Water dissolves so many things, we call it the “Universal Solvent”

What Are the Facts About Water

• Hydrogen bond = weak chemical bond formed by the attraction of positively charged hydrogen atoms to other negatively charged atoms.

Hydrogen bonds

• Water molecules attract other water molecules.

• Positively charged hydrogen atoms of one water are attracted to negatively charged oxygen in another water molecule.

• They form a weak bond called a hydrogen bond.

Hydrogen bonds

• Hydrogen bonds are important to organisms because they hold together many biomolecules such as proteins.

Resists Temperature Change

• Requires more heat to increase temperature than most common liquids

• Loses a lot of heat when it cools

• Water is like an insulator and helps to maintain a steady environment

• Helps to maintain homeostasis

Expands When it Freezes

• Water expands when it freezes

• Ice is less dense so it floats

Diffusion

• Diffusion = net, random movement of particles from an area of higher concentration to an area of lower concentration, eventually resulting in even distribution.

Diffusion

• Random bouncing of particles!

• Move from where there are lots of particles to where there is more room… areas of higher concentration to areas of lower concentration

What Affects the Speed of Diffusion

• Concentration ~ has the greatest effect - more concentrated, more quickly it will diffuse

• Temperature ~ if temperature increases, the particle’s motion increases and diffusion is faster

• Pressure

Results of Diffusion

Diffusion is complete when the particles are evenly distributed… remember…they are still moving around.

Results of Diffusion

This is called:

• Dynamic equilibrium = result of diffusion where there is continuous movement of particles but no overall change in concentration.

Which of the images in Figure 6-4 depicts dynamic equilibrium?

C… just as many are moving in as moving out!

Diffusion in Living Systems

• Cells use diffusion to move things in and out

• If the concentration of a substance it needs is higher outside the cell and lower inside the cell, they will move until concentration levels are equal or dynamic equilibrium has been reached.

Why is the polar property of water important?

• Answers may include: Polarity allows water to dissolve many materials but not react with them chemically in the process.