water and liquid properties

7/28/2019 Water and Liquid Properties

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Norolayn K. Said, MST


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Properties of WaterPolar molecule

Cohesion and

adhesionHigh specific heat

Density greatest

at 4oCUniversal solvent

of life


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Polarity of Water In a water molecule two hydrogen atoms

form single polar covalent bonds with anoxygen atom. Gives water more structurethan other liquids Because oxygen is more electronegative, the

region around oxygen has a partial negativecharge.

The region near the two hydrogen atoms has apartial positive charge.

A water molecule is a polar molecule withopposite ends of the molecule withopposite charges.


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Water has a variety of unusual properties because ofattractions between these polar molecules.

The slightly negative regions of one molecule areattracted to the slightly positive regions of nearbymolecules, forming a hydrogen bond.

Each water moleculecan form hydrogen

bonds with up tofour neighbors.

Copyright 2002 Pearson Education, Inc., publishing as Benjamin Cummings

Fig. 3.1


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HYDROGEN BONDS Hold water molecules

together Each water molecule

can form a maximumof 4 hydrogen bonds

The hydrogen bondsjoining watermolecules are weak,about 1/20th as strong

as covalent bonds. They form, break, and

reform with greatfrequency

Extraordinary Properties

that are a result ofhydrogen bonds.

Cohesive behavior

Resists changes intemperature

High heat of vaporization

Expands when it freezes

Versatile solvent


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Changing States of Water

Boiling water- heat speeds up the energy; at 100oC

Evaporation- water escapes into air as water vapor Condensation- cooling releases energy; forms droplets

Freezing- loss of energy; becomes ice; 0oC

Melting- molecules gain energy causing temperatureto rise; ice becomes fluid
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Organisms Depend on Cohesion

Cohesion is responsible for thetransport of the water columnin plants

Cohesion among watermolecules plays a key role inthe transport of water againstgravity in plants

Adhesion, clingingof one substance toanother, contributestoo, as water adheresto the wall of the

vessels.

Hydrogen bonds hold the substancetogether, a phenomenon calledcohesion


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Surface tension, a measure of the force necessary tostretch or break the surface of a liquid, is related tocohesion.

Water has a greater surface tension than most otherliquids because hydrogen bonds among surface watermolecules resist stretching or breaking the surface.

Water behaves as ifcovered by an invisiblefilm.

Some animals can stand,walk, or run on water

without breaking thesurface.


Fig. 3.3


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Moderates Temperatures on Earth

What is kineticenergy?

Heat?

Temperature?

Calorie?

What is the differencein cal and Cal?

What is specific heat?

Celsius Scale at Sea Level

100oC Water boils

37oC Human body

temperature

23oC Room temperature

0oC Water freezes

Water stabilizes air temperatures by absorbing heat from warmer air andreleasing heat to cooler air.Water can absorb or release relatively large amounts of heat with only aslight change in its own temperature.


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Three-fourths of the earth iscovered by water. The waterserves as a large heat sinkresponsible for:

1. Prevention of temperaturefluctuations that are outside

the range suitable for life.

2. Coastal areas having a mildclimate

3. A stable marine environment

Specific Heat is the amount of heat that must be absorbed

or lost for one gram of a substance to change its

temperature by 1oC.


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Evaporative Cooling The cooling of a

surface occurs whenthe liquid evaporates

This is responsiblefor: Moderating earths

climate

Stabilizes

temperature inaquatic ecosystems

Preventingorganisms fromoverheating


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Density of Water Most dense at 4oC

Contracts until 4oC

Expands from 4oCto 0oC

The density of water:

1. Prevents water from freezing from the bottom up.

2. Ice forms on the surface firstthe freezing of the waterreleases heat to the water below creating insulation.

3. Makes transition between season less abrupt.


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When water reaches 0oC, water becomes locked into acrystalline lattice with each molecule bonded to to themaximum of four partners.

As ice starts to melt, some of the hydrogen bonds breakand some water molecules can slip closer together thanthey can while in the ice state.

Ice is about 10% less dense than water at 4oC.


Fig. 3.5


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Solvent for Life Solution

Solute

solvent

Aqueous solution

Hydrophilic Ionic compounds

dissolve in water

Polar molecules(generally) are watersoluble

Hydrophobic Nonpolar compounds


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LIQUIDS The molecules of liquids are arranged less tightly

than those of solids but more closely than those ofgases.

Liquids and gases take the shape of theircontainer, unlike solids, which keep their ownshape.

Liquids and solids maintain a definite volume, orsize, while gases will expand to fill a container. Forexample, a liter of liquid will not expand to fill atwo-liter container, but a liter of gas.


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Most substances can exist in the liquid state at the right

temperature and pressure. Water, for example, exists asa liquid at room temperature (20 C/68 F) and normalatmospheric pressure (the pressure of the atmosphereat sea level).

Helium, on the other hand, is a gas under theseconditions. It must be cooled to a very low temperatureor compressed to a very high pressure to become aliquid.

Iron is a solid at room temperature and normal

pressure and must be heated to 1535 C (2795 F) tobecome a liquid. Only three chemical elementsbromine, gallium, and mercuryexist as liquids at roomtemperature and normal pressure.


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All other elements exist as either solids or gasesunder these conditions. Many compounds(combinations of elements) exist as liquids.

Alcohol, gas, oil, and water are examples of

compounds that are liquids at room temperatureand normal pressure.

Many familiar liquids, such as juice, milk, andsoda, are water based, meaning they contain

substances mixed with or dissolved in water.


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Liquids, particularly water, are essential to life. All plantsand animals depend on water to transport nutrients and

wastes. Liquids are also important in everyday activities, such as

cleaning and painting. Liquids clean by dissolving andcarrying away dirt and other solid particles.

Paints contain colored particles in a liquid base. The liquid

enables people to evenly spread color with a brush or rolleron a wall or other surface. After the paint solidifies, it formsa coating on the wall or surface .

Many products, such as metal beams and plastic containers,are made by melting materials into liquid form and thenpouring them into molds. When the material cools, it

solidifies into the desired shape . Various fuels that people burn for heat and energy, such as

oil to heat houses and gasoline to power automobileengines, are also liquids.


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PROPERTIES OF LIQUIDS The BOILING POINT of a liquid is the temperature at

which molecules escape from the liquid and enter thegaseous state.

Heat causes a liquid to boil by adding energy to the liquids

molecules. As the molecules gain energy, they move aboutmore quickly and range farther from each other. When themolecules are far enough apart, intermolecular forces aretoo weak to pull them back together, so the molecules forma vapor.

Boiling starts when bubbles of vapor form within the liquid.These bubbles rise to the top of the liquid and release thegaseous molecules to the atmosphere above the liquidssurface. It takes 2,260 Joules (540 calories) of heat energy toevaporate 1 gram of water at 100 C (212 F) at sea level.


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At the boiling point, the vapor pressure of a liquid mustequal the pressure of the atmosphere above the liquid. For a

liquid boiling in an open container, the atmosphere abovethe liquid is simply Earths atmosphere. The pressure in thebubbles of vapor must equal the pressure of Earthsatmosphere pressing down on the liquid. If this were nottrue, the air pressing down would squeeze and collapse the

bubbles before they could form and rise to the surface. The boiling point of a liquid is lower at higher elevations

because atmospheric pressure decreases as altitudeincreases. For example, the boiling point of water is 100 C(212 F) at sea level, where the air pressure measures one

atmosphere (atm). On top of Mount Everest, which is 8,850m (29,035 ft) above sea level, water boils at only 70 C (158F) because the air pressure at this height is only 1/3 atm.


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Different materials have different boiling points because the

forces of attraction between their molecules differ. For example,water molecules strongly attract each other because of theirstructure.

A water molecule consists of one oxygen atom and two hydrogenatoms. The oxygen atom attracts the electrons it shares with thehydrogen atoms more strongly than the two hydrogen atoms do.

Electrons have a negative electric charge and thus make theoxygen end of the water molecule more negatively charged, whilethe hydrogen end of the molecule has a positive charge. Thisseparation of charge makes the water molecule strongly polar.

The negative charge on the oxygen atom attracts positivehydrogen atoms from other water molecules, causing the watermolecules to bond tightly to each other. Breaking this bondrequires considerable heat, which is why the boiling point forwater, 100 C (212 F), is relatively high .

Without this bonding, water would boil near -80 C (-112 F).Ethyl alcohol is also a polar liquid, and its boiling point is 78.5 C(173.3 F).


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Nonpolar liquids have lower boiling points than polar

liquids because electric charge is evenly distributedaround their molecules. This even distribution makesthe molecule-to-molecule attractions in nonpolar liquidsrelatively weak. Examples of nonpolar liquids are thehydrocarbons, substances that consist entirely ofhydrogen and carbon molecules.

Many common fuels, such as gasoline and methane, arehydrocarbons. In the molecules of these substances, thecarbon and hydrogen atoms share their electrons moreequally than do the hydrogen and oxygen atoms of water.

As a result, the bonds between the molecules arerelatively weak, and the liquids boil at lowertemperatures. The hydrocarbon propane boils at 42.1 C(-43.8 F), and butane boils at 0.5 C (31.1 F). Thesesubstances exist as gases at room temperature.


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Sometimes a liquid can be superheatedthat is,

heated above its usual boiling point withoutchanging into vapor. Superheating occurs when

vapor bubbles inside a liquid dont have anappropriate surface on which to form.

For example, when water in a smooth-walledcontainer is heated in a microwave oven, it canreach a higher temperature than its boiling pointand remain a liquid. If a rough surface enters the

liquid, such as a teabag, vapor bubbles can form andthe liquid will begin to boil rapidly.


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Freezing Point The freezing point of a substance is the

temperature at which the liquid form of thesubstance becomes a solid. The molecules of aliquid arrange into a more ordered structure as theliquid freezes. The freezing point of a substance isessentially the same as its melting pointthat is,the point at which a solid becomes a liquid.


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When a liquid freezes to become a solid, its volume usually

shrinks by approximately 10 percent as its molecules movecloser together. In solid aluminum, for example, each atomhas 12 neighboring atoms, each at a distance of 2.86 x 10-8 cm.In liquid aluminum, each atom has 10 or 11 neighboring atomsat a distance of 2.96 x 10-8 cm.

Thus, the atoms are less tightly packed in the liquid, and theliquid must contract as it freezes. The exceptions to this ruleare water and the liquid forms of gallium and bismuth. Thesesubstances expand upon freezing.

The structure of their solid state is less dense than that of theirliquid state near the freezing point. In ice, each water

molecule is solidly packed into a lattice, surrounded by fourmolecules equally distant from each other. This structure isactually less dense than the molecular patterns that can occurin the liquid form of water, which is why ice floats on water.


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Viscosity The viscosity of a liquid is a measure of how much the liquid

resists flow. Flow allows a liquid to take the shape of the containerthat holds it. A liquids viscosity depends on the structure of theliquids molecules and on the attractive forces between the liquidsmolecules.

Highly viscous liquids often contain molecules that havecomplicated structures. These molecules can become entangled

with one another, impairing their ability to flow past one another.The viscosity of water is lower than that of heavy oils, for example,because oils contain large, convoluted molecules that catch on oneanother.

The polarity of the molecules in water, however, causes them toattract one another, making water more viscous than a nonpolarliquid, such as propane. Viscosity decreases as temperatureincreases because additional heat energy enables molecules toovercome attractions to one another and move more freely.


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Surface Tension


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Liquids behave as though they have a delicate

skin on their surface. This property is calledsurface tension. In rain droplets, surface tensionacts like a thin balloon, holding the watermolecules together in each droplet. Water-strider

bugs take advantage of surface tension by flittingacross the surfaces of ponds without fallingthrough the surface.


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Surface tension results from the intermolecularforces of attraction in a liquid. A water moleculedeep inside a droplet experiences attractive forces inall directions from other molecules in the drop. Thesum of these forces is zero, leaving no net force onthe molecule.

A molecule that is close to the surface, however, hasmore neighboring molecules inside the drop than it

has near the surface. The forces pulling themolecule toward the center of the drop are strongerthan those at the surface, so the molecule sticks tothe drop instead of falling away.


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Intermolecular forces of attraction make liquids pull together

and minimize their surface area. Liquids do this because, likeall matter, they seek to minimize the amount of energy theyrequire to maintain their molecular structure.

A liquid requires the least amount of energy when it has thesmallest possible surface area. For small amounts of liquid in

air, such as raindrops, the sphere is the shape with the smallestsurface area.

Gravity, another force acting on raindrops, stretches thedroplets so that they are not exactly round. To overcome theattraction between molecules in a liquid and increase theliquids surface area takes energy. For instance, a moving carcan transfer energy to a droplet of rain by hitting it, making itbreak apart or deform against the car into a shape that hasmore surface area.


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Capillary Action


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Water will climb up a paper towel if the edge of the

towel touches a puddle, and it will climb up a thinglass tube if the tube is dipped in water. Waterbehaves this way because of an effect calledcapillary action.

Capillary action occurs when the attraction of aliquids molecules for themselves differs from theirattraction for a solid that the liquid contacts. The

water in the paper towel example climbs the towel

because the water molecules are more attracted tothe paper than they are to each other.


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Chemistry students demonstrate capillary action usinga glass tube called a capillary tube and a beaker of

water. Water climbs the glass tube when it is dipped inthe beaker because the water is more attracted to theglass than it is to itself.

Several forces are acting on the water: the attraction ofthe water molecules to the glass tube, the weight of

gravity pressing down on the water in the tube, andthe attraction of the water molecules for each other.The water rises in the tube until all these forcesbalance.

For some liquids, such as mercury, the attraction

between the molecules of the liquid is stronger thantheir attraction to the glass tube. When a glass tube isdipped in a beaker of mercury, capillary action makesthe level of mercury in the tube drop below the level ofmercury in the beaker.


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Miscibility Miscibility is a measure of how easily different liquids

will dissolve when mixed together. Miscibility dependson the polarity of a liquids molecules. For example,

water will mix with alcohol because they are both polarliquids, so their molecules attract one another.

But water will not mix well with oil, which is a nonpolarliquid. Oil floats on top of water because the polar watermolecules are much more strongly attracted to eachother than to the oil molecules.

The rule for determining miscibility is that like dissolveslike. Polar liquids are miscible with other polar liquids,

while nonpolar liquids are miscible with other nonpolarliquids.


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When a substance dissolves in a liquid, the resulting mixture iscalled a solution. Osmosis occurs when molecules of the initial

liquid pass through a membrane, but molecules of the dissolvedsubstance do not. The molecules of the initial liquid can passthrough the membrane because they are relatively small.

Osmosis tends to equalize the concentration of the solutions onboth sides of a membrane. The membrane in this case is calledsemipermeable, because it allows one part of the mixture to pass

through but not another. Cells in living organisms consist mostly ofwater, and they are surrounded by a watery environment. If the concentration of a dissolved substance, such as sugar or salt,

differs inside and outside a cell, osmosis causes water to passthrough the cells membrane from the area of lower concentrationto the area of higher concentration, until the concentration on eachside of the membrane is equal.

Osmosis makes sugar and salt good food preservatives. Whenharmful bacteria encounter sugary or salty foods, water flows fromthe area of lower concentrationthe cells of the bacteriato thearea of higher concentrationthe food. The f low of water out fromthe bacterias cells dehydrates the bacteria, which kills it.

water and liquid properties

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