Explain the following properties of water:
- Heat of vaporization
- Surface Tension
- Many of the unique properties of water can be explained by the way its two hydrogen atoms are attached to its oxygen atom. The hydrogen is apart at an angle of 105 degrees, making the water molecules very polar, ie it has a positive end (where the two hydrogens are located) and a negative end (where a pair of oxygen’s electrons are exposed).
- The positive field of the two hydrogen atoms are attracted to the negative field of oxygen’s exposed electrons; this attraction is called hydrogen bonding. The force of hydrogen bonds is generally stronger than covalent bonds/ but not as ionic bonds. Water molecules have a great deal of hydrogen bonding is that and it is this molecular attraction that gives its properties.
- Most substances with molecular weights about that of water [ e.g. ammonia; methane] are gases at room temperature; but water is a liquid because hydrogen bonding gives eater an apparent molecular weight much greater than 18.
- Water requires l calorie per gram to raise its temperature l degree; the highest heat requirement of nearly any substance; this high requirement is due to strong hydrogen bonding which requires substantial energy to separate one molecule from another by heat as occurs in vaporization and boiling.
- This high specific heat also means that substances with high water content require large energy inputs or withdrawals in order to change temperature; thus adding to their thermal stability.
Latent heats of vaporization and fusion:
- Some 586 calories are required to convert l g of water to vapor at 20 degrees more then five times the he heat required to raise the temperature of water from 0 to 100 degree.
- This high heat requirement may create ‘evaporative cooling’ effects in transpiration by removing large amounts of heat .
- An equivalent amount of heat is liberated upon condensation, as in dew These two factors also add temperature stability.
- In order to melt 1 g of ice 80 cal must be supplied. This high rate of fusion is again caused by hydrogen bonding. Also during melting the volume of water decreases (and its density increases) because the packing of water molecules in the liquid state is closer than in the slid. This is the feature that causes ice to float, which is fortunate for life on earth. If the density of ice was greater than liquid water, ice would sink, and because of the high specific heat of water is likely that ice would form at the bottom of lakes and oceans, and not melt.
- Because the oxygen of a water molecule shares hydrogen bonding with two other water molecules, the viscosity of water is less than that predicted on the basis of the strength of its hydrogen bonds. The sharing weakens the effective bonds, and water flows more readily (is less viscous).
Adhesive and cohesive forces; surface tension:
- Adhesion is the attraction of one substance for another, for example, water molecules for cellulose, which both have hydrogen bonding, and thus have strong attraction for each other.
- Cohesion is the attraction of one molecule for another of the same species, and as mentioned before, the attraction of water molecules for each other is very high. On other words, if is the cohesive and adhesive properties of water that allow it to be transported to the tops of the tallest trees.
- Surface tension is associated with water’s cohesive forces, and it is quite high, permitting some thing that has greater density than water to remain on the surface, eg a carefully placed needle, or a water beetle.
Water the universal solvent:
- Because of its polar nature and hydrogen bonding, water is able to “cage” many solutes, preventing them from crystallizing and thence precipitating. Because of this caging effect, water can dissolve a very large number of substances.
Bulk flow and diffusion:
- Many molecules move at a time in bulk flow of liquids, but only one molecule at a time by diffusion. Both methods of transport occur in plants. Bulk flow requires a pressure gradient. Diffusion requires a concentration gradient ie a gradient of chemical activity or potential.
Physical causes of diffusion.
- All molecules greater than 0 degree Kelvin (absolute zero or -273 C) are in motion.
- In the case of solids, this motion is restrained. Less so in liquids and little in gases.
- During their motion molecules in liquids and gases especially are in nearly constant collision with other molecules, colliding off in random vector.
- If a substance is in greater concentration, or has greater potential or activity (eg by having a higher temp) in one area than another, the statistical probability is that these random vectors will travel farther toward the area of less concentration or activity. This tends to equalize the activity or concentration throughout space over time, and this process is called diffusion.
Factors affecting diffusion:
- Diffusion occurs in response to differences in concentration. For example, when C02 is consumed by photosynthesis; a CO2 concentration gradient develops from air to the chloroplast. CO2 diffuses in response to this gradient. The chemical potential or more commonly and accurately in this case, water potential, can also be affected by pressure, temperature, adsorptive surfaces, and by solutes.
- Pressure increases water potential, and thus the tendency for water to diffuse, and so does an increase in temperature.
- Adsorptive surfaces, (e.g. clay, cellulose) decrease water potential by restricting the movement of water molecules, and thus the tendency to diffuse is lowered. Water molecules cage solutes, and the motion of these water molecules becomes restricted, thus they are less free to diffuse. The concentration of water molecules with sufficient energy to diffuse is less in the presence of solutes (even though the total concentration of water molecules per unit volume may remain about the same).
- Although the above factors determine the potential for diffusion, the rate of diffusion is determined by the steepness of the diffusion gradient, i.e., the distance over which the changes in concentration or activity occur. The less the distance the more rapid the rate of diffusion.
- Water vapor creates a partial pressure that is added to the air in proportion to the amount of water vapor present.
- Water at 20 degrees and 100% relative humidity contains 17.31 g m-3 H20 (absolute humidity), and this vapor exerts a partial pressure of 2.34 MPa (MegaPascals). Both the density and vapor pressure increase with pressure and temperature (at 100 degrees the vapor pressure of water equals atmospheric pressure at its boiling point.
- As noted above, vapor pressured increase with temperature and pressure, and decrease with added solutes.
The colligative properties of water:
- The addition of solutes to water causes an increase in boiling point because more energy is required to separate water molecules involved in caging solutes cause a decrease in freezing point for a similar reason in that more energy must be removed from caging water molecules so that they can come to rest with other water molecules in the ice crystal .solutes decrease the vapor pressure of the solution compared to pure diffuse water will tend to diffuse in to areas where solute diffusion is restricted even creating a pressure. This special case of diffusion is called osmosis.
- If any of the colligative values are known, the other can be computed. For example 1 mol of sugar in 1kg of water will lower the freezing point to -1.86 degrees raise the boiling pint to 100.56 degrees lower vapor pressure by 0.041kpa and create an osmotic potential of abut 2.24mpa.
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Naeem Javid Muhammad Hassani is working as Conservator of Forests in Balochistan Forest & Wildlife Department (BFWD). He is the CEO of Tech Urdu (techurdu.net) Forestrypedia (forestrypedia.com), All Pak Notifications (allpaknotifications.com), Essayspedia, etc & their YouTube Channels). He is an Environmentalist, Blogger, YouTuber, Developer & Vlogger.