Chapter 2

Section 2Water and Solutions

Grade 10 Biology Fall 2010

Bell Ringer

• http://www.youtube.com/watch?v=orW7CEwcAW8

1.List some common acids and bases2.Why do you think water is such an essential

compound?

Objectives

• Analyze the properties of water• Describe how water dissolves substances• Distinguish between acids and bases

Water in Living Things

• 70 percent of your body is made of water• About two-thirds of the molecules in your

body are water molecules• Water is the medium in which most cellular

events take place

Storage of Energy

• Water absorbs heat more slowly and retains this energy longer than many other substances do

• Many organisms release excess heat through water evaporation – Ex. Sweating

• In organisms, the ability to control temp. enables cells to maintain a constant internal temp. when the external temp. changes– Homeostasis

Cohesion and Adhesion • Hydrogen bonds between

molecules cause the cohesion of liquid water

• Cohesion: an attraction between substances of the same kind

• Molecules at the surface of water are linked together by hydrogen bonds

• This attraction between water molecules causes a condition known as surface tension

Cohesion and Adhesion

• Water molecules are attracted to similarly polar substances

• Adhesion: an attraction between different substances

• Adhesion powers process called capillary action

Cohesion and Adhesion

Aqueous Solutions

• Solution: a mixture in which one or more substances are evenly distributed in another substance

• Many important substances in the body have been dissolved in blood or other aqueous fluids

• Because these substances can dissolve in water, they can more easily move within and between cells

Polarity

• The polarity of water enables many substances to dissolve in water

• Water is a polar substance

Polarity• Ionic compounds and polar molecules dissolve

best in water – Why?

• When dissolved in water ions become surrounded by polar water molecules

• Ions are attracted to the ends of water molecules with the opposite charge, solution is mixture of water molecules and ions

• Similar attraction results with polar molecules• Ions or molecules become evenly distributed in

water

Polarity

• Non-polar molecules do not dissolve well in water – Why?

• When non-polar substances are placed in water, the water molecules are more attracted to themselves than the non-polar molecules

• Result is the non-polar molecules are shoved together

Acids

• Acids: compounds that form hydrogen ions when dissolved in water

• When an acid is added to water, the concentration of hydrogen ions in the solution is increased above that of pure water

• Hydrogen ion = H+ • Acidic compounds have a pH value less than 7

Bases

• Base: compounds that reduce the concentration of hydrogen ions in a solution

• Many bases form hydroxide ions when dissolved in water

• Hydroxide ion = OH-

• Basic compounds have a pH value greater than 7

pH• pH scale: based on the

concentration of hydrogen ions in solution

• All solutions have a pH value between 0 and 14

• Each whole number represents a factor of 10

• A solution with a pH value of 5, has 10 times as many hydrogen ions as one with a pH value of 6

Review

1. T/F Non-polar molecules dissolve very easily in water.

2. T/F Cohesion and adhesion of water molecules help a plant move water from its roots to its leaves.

3. T/F When a base is added to water, the concentration of hydrogen ions increases.

4. T/F A substance has a pH value of 11 therefore it is basic.

Answer

1. F: Non-polar molecules dissolve very easily in water.

2. T: Cohesion and adhesion of water molecules help a plant move water from its roots to its leaves.

3. F: When a base is added to water, the concentration of hydrogen ions increases.

4. T: A substance has a pH value of 11 therefore it is basic.