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water properties in civil engineering.pdf
- 1. DEPARTMENT OF CIVIL ENGINEERING CIV2103: HYDRAULICS & HYDRAULIC INSTRUCTOR: BYAMUKAMA DAN [email protected] 0789541607/0705418906
- 2. INTRODUCTION TO PROPERTIES OF WATER Water (H20) is the “universal solvent” and the most abundant surface on Earth. It is also the only common substance to exist as a solid, liquid, and gas naturally. Water molecules form hydrogen bonds and are extremely polar. The five main properties of water are its high polarity, high specific heat, high heat of vaporization, low density as a solid, and attraction to other polar molecules.
- 3. Polarity and Structure One oxygen atom and two hydrogen atoms make a water molecule. It has a bent molecular geometry with the oxygen having two lone pairs of electrons. The difference in electronegativity between the oxygen and hydrogens causes the oxygen to have a partial negative charge and the hydrogen to have a partial positive charge. This difference in charge causes polarity. The partial positive charge of the hydrogen of one water molecule attracts the partial negative charge of the oxygen of another water molecule. This attraction is called hydrogen bonding. Hydrogen bonding is weaker than the covalent bonds between the oxygen and hydrogens of the same molecule but causes many of water’s unique properties. For example, more energy is required to break hydrogen bonds, so water has a higher melting and boiling point.
- 4. Universal Solvent Water is the solvent of life. Hydrophilic substances are those that dissolve in water, while hydrophobic substances do not mix well with water. Substances can dissolve in water if they can match or overcome the hydrogen bonding between water molecules. If they cannot, the substance forms a precipitate. Acids, alcohols, and salts are rather soluble in water while fats and oils are hydrophobic. An ionic solute dissolved in water becomes separated. For example, NaCl separates into Na+ cations and Cl- anions surrounded by water molecules. Water is amphoteric, meaning it can act as either an acid or a base depending on the solution. It can produce both H+ and OH- ions.
- 5. Specific Heat Capacity Water has a high specific heat of 4184J/(kg x K) at 20 ºC and high heat of vaporization because of hydrogen bonding. This allows bodies of water to have minimal fluctuations in temperature to regulate climate. Water’s high heat of vaporization allows humans to use sweat to cool off. Sweat is mostly made of water. It absorbs excess body heat as it evaporates. This process is known as evaporative cooling.
- 6. Density Water’s density is 1 gram per cubic cm. This is used to define the gram. Instead of undergoing thermal expansion, the density rises with temperature to a peak of 3.98 ºC and then decreases. Negative thermal expansion is the increase in density between 32 and 39.16 F. As a result, ice is less dense than water, which has a decrease in density by about 10%. This is why bodies of water may have a layer of ice on the surface but contain liquid underneath. This allows fish and marine life to survive under the ice. High specific heat keeps the temperature of the water relatively stable through the winter so that marine life can survive. Salt content lowers the freezing point of the ocean by almost 2 ºC. Ice still floats on the ocean, but the ice is near salt-free with a similar density to ice on bodies of freshwater. The salt adds to the salinity and density of the remaining water which sinks by convection. This process is called brine rejection.
- 7. Compressibility Compressibility is a result of pressure and temperature. The compressibility of water is so low that is often assumed to be incompressible. Low compressibility allows water in deep oceans with high pressure to only decrease by 1.8% in volume. Electrical Conductivity Pure water is a good insulator, but deionized water is never completely free of ions. Water undergoes a process called autoionization as a liquid. This means that two water molecules can form one hydroxide anion (OH-) and one hydronium cation (H30+).
- 8. Cohesion and Adhesion Hydrogen bonds between water molecules are constantly breaking and reforming with other water molecules. Cohesion is the ability of water molecules to stick together. Water’s polarity also gives water high adhesion: the ability to stick to other surfaces. The adhesive forces are stronger than cohesive forces. Strong cohesion and adhesion cause water to exhibit capillary action. Capillary action is the process of liquid flowing through a narrow space without and often against gravity. Water adheres to the walls of plants’ roots and rises up into the plant. Porous materials such as water also exhibit capillary action. Trees can transport water through capillary action over 100 meters.
- 9. Surface Tension The hydrogen bonding also causes water to have a high surface tension of 71.99mN/m at 25. The surface tension is high enough for insects to walk on water. Surface tension is a result of water’s cohesive properties. Water droplets and water rising above the rim of a glass show water’s high surface tension.
- 10. Capillary Action Even if you've never heard of capillary action, it is still important in your life. Capillary action is important for moving water (and all of the things that are dissolved in it) around. It is defined as the movement of water within the spaces of a porous material due to the forces of adhesion, cohesion, and surface tension. Capillary action occurs because water is sticky, thanks to the forces of cohesion (water molecules like to stay close together) and adhesion (water molecules are attracted and stick to other substances). Adhesion of water to the walls of a vessel will cause an upward force on the liquid at the edges and result in a meniscus which turns upward. The surface tension acts to hold the surface intact. Capillary action occurs when the adhesion to the walls is stronger than the cohesive forces between the liquid molecules.
- 11. Hydraulics, branch of science concerned with the practical applications of fluids, primarily liquids, in motion. It is related to fluid mechanics, which in large part provides its theoretical foundation. Hydraulics deals with such matters as the flow of liquids in pipes, rivers, and channels and their confinement by dams and tanks.