The Chemistry of Life 2.2 Properties of water

Bellwork question Why is it important that biologists have a good understanding of chemistry? Like all matter, living things are made up of elements and chemical compounds Survival of living things depends on chemical reactions Take place both within and outside their body To understand living things, biologists need to have a good understanding of chemistry

Water is essential to life on Earth It makes up 60% of the human body Is known as the universal solvent Presence of liquid water means that life might be present somewhere Why is water so special?

The Water molecule Found in a liquid state over most of the Earth’s surface Neutral charge – 10 Protons and 10 electrons Polarity of water molecule A polar molecule is one where the charges are unevenly distributed

Hydrogen bonding Polar molecules can attract to each other Attraction between partial +ve charge of H atom and partial –ve charge of another molecule is a hydrogen bond Oxygen, nitrogen and fluorine are common partial negative charged particles Hydrogen bonds are not strong But they give water a number of unique characteristics Explain why water expands upon freezing Explains why water can dissolve so many substances

Cohesion Cohesion is an attraction between molecules of the same substance Water is very cohesive One water molecule can have 4 hydrogen bonds at the same time Results in water being molecules being drawn together Explains why drops of water form beads on a smooth surface Provides surface tension – allowing for some insects to walk on the a ponds surface

Adhesion The attraction between molecules of different substances Water will stick to the side of glass tubes, or graduated cylinders Results in a meniscus Adhesion between water and glass is stronger than cohesion between water molecules Water can also rise in narrow tubes against the force of gravity – capillary action. One of the forces that draws water out of roots and into stems and leaves in plants But, cohesion holds the column of water itself together as it rises

Heat Capacity Heat Capacity – the amount of heat energy required to rise the temperature The multiple hydrogen bonds means that it takes a large amount of heat energy to cause the molecules to move faster, raising the temperature of the water Water has a relatively high heat capacity Large bodies of water can absorb a lot of heat with only small changes in temperature Protects organisms in the water Water can also aid in the regulation of cell temperature

What is a mixture? Mixture – material composed of 2 or more elements or compounds Physically mixed together Not chemically combined Water can form 2 types of mixtures Solutions and Suspensions

How does water’s polarity influence it’s properties of a solvent? In an ionically bonded compound, ions are attracted to polar water molecule Ions break away Become dispersed Form a solution Solutions are evenly distributed throughout the solution In Salt water solution Table salt – solute Water - solvent

The universal solvent Water is a very versatile solvent It can break dissolve: Salts Sugars Minerals Gasses Other solvents… All because of it’s polarity When no more solute can be dissolved the solution is saturated.

Besides tasting the water, how could you test to show that the salt is there, even though you can’t see it? Weigh it Measure changes in volume….

Suspension When something in water separates into pieces so small that they do not settle out Movement of water molecules keeps them suspended Mixture known as a suspension Blood is a great example of a solution and suspension Blood – mostly water Many dissolved compounds Cells can’t dissolve, and are held in suspension

Complete the table Substance Definition Examples Physical combination of to or more substances Cinnamon sugar Solute Water in saltwater Mixture of water and non dissolved substance blood Solution

Complete the table Substance Definition Examples Mixture Physical combination of to or more substances Cinnamon sugar Solute Substance that is dissolved Salt in saltwater Solvent Substance in which Solute is dissolved Water in saltwater Suspension Mixture of water and non dissolved substance blood Solution Mixture in which all substances are evenly distributed Saltwater

Acids, bases and pH Water molecules can split into H+ ions and OH- ions In pure water only 1 in 550 million molecules split Number of Positive and Negative ions are equal pH is neutral - 7

The pH Scale Indicates the concentration of H+ ions in solution Ranges from 0 to 14 Below 7 is acidic Above 7 is basic The lower pH the greater the acidity pH scale is logarithmic Each step on the pH scale represents a factor of 10 increase from the previous step

Acids and bases Both can be very corrosive

Acid Any compound that forms H+ ions in solution Have higher H+ concentrations than pure water Therefore have a pH of less than 7 1 – 3 is a strong acid Example is hydrochloric acid in your stomach

Base A base is a compound that produces OH- ions (hydroxide) Lower concentrations of H+ ions than pure water pH > 7 Strong bases such as NaOH (used in soap making) have pH’s ranging from 11 – 14

Buffer A weak acid and/or a base that can react with a strong acid or base to prevent a sharp, sudden change Important in the human body Where pH must be kept between 6.5 and 7.5 If pH is lower or higher than can affect chemical reactions that take place within cells Controlling pH is one form of homeostasis Blood must be kept at pH 7.4 Changes are controlled by bicarbonate and phosphate ions

Buffers cont… Buffers do not keep a solution neutral There are many different buffers in the world – each is designed for a fixed pH range One may work from 2 – 6 Another from 10 – 12 All contain a weak acid and a weak base component Acid component will release H+ ions if base is added Base component will absorb H+ ions if acid is added The solution does not become as acidic or basic as it would have without the buffer

Writing exercise Suppose you are a writer for a natural history magazine for children. This month’s issue will feature insects. Write a paragraph explaining why some bugs, such as the water strider, can walk on water