PROPERTIES OF WATER

I. Structure of the Water Molecule Polarity and Hydrogen Bonds

Structure of the Water Molecule H + H + O  H 2 O (chemical formula) HH O Structural formula

Water Molecules are Polar Polar – has a positive end and a negative end. As a result, polar molecules have a slightly negative charge. H+ O-O-

How is a Water Molecule Polar?  The oxygen atom attracts the shared electrons more strongly than the hydrogen atoms do.  As a result, the electrons spend more time near the oxygen atom than they do near the hydrogen atoms. H+ O-O-

How Do Water Molecules Bond With Each Other?  Hydrogen bonds form between neighboring water molecules   The positve H of one molecule “sticks” to the negative O of another molecule.

Hydrogen Bonding  They do not share electrons, so they are weaker than covalent bonds.  They easily break and form again

II. Physical Properties and States of Matter

Physical Properties  Water in its pure state is… Colorless Colorless Odorless Odorless Transparent Transparent  Distilled water is considered “pure”  Rain water is not   Precipitation of any kind (snow, rain, hail) contains dissolved minerals, gasses, acids, dust, pollen grains, viruses and microorganisms!

Water pH   Pure water (distilled) has a pH of 7   Seawater is slightly basic, with a pH of about 8   Most natural water has a pH of between 6-8   Acid rain can have a pH as low as 4

States of Matter

Three States of Water  Water is the only naturally occurring substance that can exist in all three physical states on the planet Gas Water Vapor Widely spread, un-bonded molecules. Liquid Liquid water Density = 1 Small clumps of bonded molecules. Solid Ice Density <1 Hexagonal crystal lattice

States of Matter  Which is ice and which is water?

 After melting, the water molecules are closer together than they were as ice.  As a liquid, the molecules are more randomly arranged and the molecules are able to slide past each other.

Water Phase Changes  Freezing   the substance changes from a liquid to a solid   Water expands when it freezes and becomes less dense  Melting:  the substance changes back from the solid to the liquid  A liquid is a nearly incompressible fluid that conforms to the shape of its container but retains a (nearly) constant volume independent of pressure

Water Phase Changes  Condensation  the substance changes from a gas to a liquid   When water vapor cools it takes up less space and contracts, allowing water molecules to stick to one another forming clouds and precipitation.  Evaporation: the substance changes from a liquid to a gas vapor is always present in the air around us. You cannot see it. When you boil water, the water changes from a liquid to a gas or water vapor. As some of the water vapor cools, we see it as a small cloud called steam. This cloud of steam is a miniversion of the clouds we see in the sky. At sea level, steam is formed at 100° Celsius, 212° Fahrenheit. The water vapor attaches to small bits of dust in the air. It forms raindrops in warm temperatures. In cold temperatures, it freezes and forms snow or hail.

Water Phase Changes  Frost Formation: the substance changes directly from a gas to a solid without going through the liquid phase. the substance changes directly from a gas to a solid without going through the liquid phase.  Sublimation the substance changes directly from a solid to a gas without going through the liquid phase.

III. The Density of Water

 Like most substances, water increases in density as its temperature decreases.  The density of water continues to increase until it reaches 4oC.  At this point, water is unique in that its density begins to decrease as the temperature decreases to 0oC, the freezing point of water.  As the freezing point is approached, hydrogen bonds relax and form a crystal lattice, keeping molecules farther apart than when in liquid form. This is why ice floats and ponds don’t freeze solid (they freeze from the top down, not the bottom up). This is why ice floats and ponds don’t freeze solid (they freeze from the top down, not the bottom up). Temperatures below 0ºC are possible because salt water freezes at colder temperatures than pure water. Temperatures below 0ºC are possible because salt water freezes at colder temperatures than pure water.  The fact that ice floats on liquid water is important because it allows the existence of large areas of polar sea ice.  These affect heat flow between the ocean and the atmosphere, and help stabilize ocean temperatures and Earth’s climate.

Density  The density of water: increases as temperature decreases, until 4 o C increases as temperature decreases, until 4 o C Decreases from 4 o C until 0 o C (the freezing point) Decreases from 4 o C until 0 o C (the freezing point) At 0 o C, Hydrogen bonds relax to form crystal lattice (ice) At 0 o C, Hydrogen bonds relax to form crystal lattice (ice)

Density of water  If density of object is = to or < than water it will float. Density of pure water is 1.0 Density of pure water is 1.0 Density of pure ice is Density of pure ice is Density of sea water is 1.03 Density of sea water is 1.03   Most solids are more dense than their liquids. This makes solids sink   Ice is less dense than liquid water due to H- Bonds

Importance of Water’s Density  Ponds don’t freeze from the bottom up (and don’t freeze solid)  Ice floats, and forms an insulating layer for life below polar sea ice polar sea ice  Stabilizes ocean temperatures and climate (causes turnovers…we’ll learn about this later)

IV. Surface Tension

Description  Cohesion water molecules stick together and draw inward water molecules stick together and draw inward This forms a tight layer on the surface. This forms a tight layer on the surface. This layer is resistant to being broken apart, and is called surface tension. This layer is resistant to being broken apart, and is called surface tension.  Surface tension measures the strength of water molecules attracting to one another.  Due to hydrogen bonding.

Importance  Some insects can walk on water  Formation of funnels, water drops, and waves

What Affects Surface Tension?  Gravity Changes the shape of droplets as they fall Changes the shape of droplets as they fall  Warmer temperature Hot water is a better cleaning agent because the lower surface tension makes it a better "wetting agent" to get into pores and fissures rather than bridging them with surface tension. Hot water is a better cleaning agent because the lower surface tension makes it a better "wetting agent" to get into pores and fissures rather than bridging them with surface tension.  Soaps and detergents Breaks the surface tension Breaks the surface tension

Capillary Action and Adhesion  Adhesion is the attraction of water molecules to stick to other substances, and is also a result of its hydrogen bonds.  This property allows water to creep up or flow in tubes, such as a plant’s xylem and phloem, or a human’s blood vessels). This is called capillary action and is important for many life processes.

Cohesion-Adhesion Theory -As water evaporates from leaves, it tugs on the water molecules below -Cohesion and adhesion pull water up and replace missing water molecules -Water enters the roots by osmosis Did you ever wonder: How does water move from roots to leaves when a tree doesn’t have a heart to pump the water?

V. High Specific Heat Capacity

What does it mean?  This means that water can absorb a lot of heat before it begins to get hot.  This is why water is valuable to industries and in your car's radiator as a coolant.  The high specific heat index of water also helps regulate the rate at which air changes temperature, which is why the temperature change between seasons is gradual rather than sudden, especially near the oceans. which is why the temperature change between seasons is gradual rather than sudden, especially near the oceans.

 High Specific Heat Capacity (Resistant to temperature changes)  Because of hydrogen bonding, liquid water has an unusually high heat capacity, compared to all other known liquids.  In fact, in terms of specific heat, water is very high (4.19) when compared to other liquids. (Specific heat capacity is the energy (in joules) needed to raise the temperature of 1 gram of a substance by 1o C. When heat is added to water, most of that heat is used to break the hydrogen bonds that link the water molecules together. Only a small amount of the heat energy is left to increase the vibrations of those water molecules. (The vibrations of the molecules are detected as a rise in temperature.) (Specific heat capacity is the energy (in joules) needed to raise the temperature of 1 gram of a substance by 1o C. When heat is added to water, most of that heat is used to break the hydrogen bonds that link the water molecules together. Only a small amount of the heat energy is left to increase the vibrations of those water molecules. (The vibrations of the molecules are detected as a rise in temperature.)  This means that areas of water (lakes, oceans, etc.) can absorb and release huge amounts of heat energy without much change in the overall temperature of the water. In fact, the sun warms the sea more slowly than it does the land.  It also means that movements of water, such as ocean currents, transfer extremely large amounts of heat energy around the planet (an example of thermal conductivity). This role of ocean currents is vital to Earth’s climate.

Water's High Specific Heat  Heat and Temperature  Kinetic energy = The energy of motion.  Heat = Total kinetic energy due to molecular motion in a body of matter.  Calorie (cal) = Amount of heat it takes to raise the temperature of one gram of water by one degree Celsius.  Kilocalorie (kcal or Cal) = Amount of heat required to raise the temperature of one kilogram of water by one degree Celsius (1000 cal).  Temperature = Measure of heat intensity due to the average kinetic energy of molecules in the body of matter.

I.The Celsius Thermometer: (Figure 3.6) The Celsius scale at sea level: I.100 degrees C (212 degrees F) = water boils II.37 degrees C (98.6 degrees F) = human body temperature III.23 degrees C (72 degrees F) = room temperature IV.0 degrees C (32 degrees F) = water freezes II.Specific heat = Amount of heat that must be absorbed or lost for one gram of a substance to change its temperature by one degree Celsius. I. Specific heat of water = One calorie per gram per degree Celsius (1 cal/g/degree C).

VI. High Heat of Vaporization

Description  Takes a lot of energy to make water evaporate.  Most of the initial energy is used to break the hydrogen bonds.  This causes water to be resistant to temperature changes

Importance  Slows evaporation of water bodies  Helps our bodies and our planet maintain a certain stable temperature range

 Heat of vaporization is the amount of heat required for 1 gram of a substance to be converted from a liquid to a gas. is the amount of heat required for 1 gram of a substance to be converted from a liquid to a gas. Hydrogen bonds make it difficult for water molecules to escape the liquid state and are responsible for water’s high heat of vaporization. Hydrogen bonds make it difficult for water molecules to escape the liquid state and are responsible for water’s high heat of vaporization. The hydrogen bonds must be broken before water can evaporate and this requires considerable energy. The hydrogen bonds must be broken before water can evaporate and this requires considerable energy. Water has a relatively high heat of vaporization (540 cal/g).Water has a relatively high heat of vaporization (540 cal/g).  This property of water helps our bodies (through sweating) and our planet (surface water evaporation) to maintain a temperature within a specific range.

I.Water's high heat of vaporization: I. Moderates the earth's climate. I.Solar heat absorbed by tropical seas dissipates when surface water evaporates (evaporative cooling). II.As most tropical air moves poleward, water vapor releases heat as it condenses into rain. II. Stabilizes temperature in aquatic ecosystems (evaporative cooling). III. Helps organisms from overheating by evaporative cooling Evaporative cooling = Cooling of a liquid's surface when a liquid evaporates. The surface molecules with the highest kinetic energy are most likely to escape into gaseous form; the average kinetic energy of the remaining surface molecules is thus lower.

True or False?  Water boils and evaporates at higher altitudes faster than at sea level.

True At sea level water boils at 100o C. At 2,400 m, water boils at 92o C. This is because as the altitude gets higher, the lower air pressure makes it easier for the water molecules to break their bonds and attraction to each other and, thus, it boils more easily.

True or False  It takes more energy to heat cold water to boiling (212 o F or 100 o C) than it does to change boiling water to steam.

False   It takes 5x as much energy to turn boiling water into steam then it takes raise cool water to boiling 212o F.

VI. Water is the Universal Solvent

What Does This Mean?  Water has the ability to dissolve both bases and acids, so it is called a universal solvent.  Everything else dissolves in water as well…except lipids  The movement of water and its hydrogen bonds cause this

Water As An Inert Solvent  Water is not altered by any substance it dissolves, and in turn, those substances being dissolved are not altered by the water.  This makes water an inert solvent.

Hydrophilic vs. Hydrophobic Substances  Hydrophilic “Water loving” “Water loving” Substances that are easily dissolved in water Substances that are easily dissolved in water Ex. Nutrients, minerals, etc. Ex. Nutrients, minerals, etc.  Hydrophobic “Water hating” Water won’t form solutions with these substances Ex. Lipids (fats, oils waxes)

True or False  More things can be dissolved in a strong acid than in water.

False   The movement of liquid water along with its ability to form hydrogen bonds, allows water to pull substances apart and dissolve them better but not always faster then an acids!

A Recap on the Importance of Water

Remember Homeostasis?  Helps maintain a constant physiological condition of cells, and organisms’ global ecosystems because… a. Makes a good insulator a. Makes a good insulator b. Resists temperature change b. Resists temperature change c. Universal solvent c. Universal solvent d. Acts as a coolant d. Acts as a coolant e. Controls climate e. Controls climate