Water and it's Properties

Water and it's properties including hydrophobicity, hydrogen bonding and others

1. Water and its Properties Bhanu Krishan

2. Index S.no Topic Slide Number 1) Introduction to water 3 2) Structure of water 4 3) Properties of water 5 - 9 4) Dissociation of water 10 - 12 5) Water as an Universal Solvent 13 - 14 6) References 15 2

3. Introduction to Water (H₂O) Water is a transparent and nearly colorless chemical substance that is the main constituent of Earth’s, streams, lakes and oceans and the fluids of the most living organisms. It’s chemical formula is H₂O. 3

4. Structure Of Water Water is a covalent structure: H – O – H. Oxygen bonded with two hydrogen molecules. The water molecule itself is bent, with an angle of 104.5° between the hydrogen (compare to 109.5° for sp³ tetrahedron. 4

5. Physical properties of water The physical properties of water differ usually from those of other solvents. For example: Water as a hydride of oxygen has a higher melting point, boiling point, heat of vaporization and surface tension than do the comparable hydrides of sulphur and nitrogen and most other common liquids. 5

6. Expansion on freezing: Most substances decreases in volume (and hence increase in density) as their temperature decreases. But in case of water there is temperature at which its density exceeds that at higher or lower temperature. This temperature is 4 °C. Uniquely high surface tension: Water has the highest surface tension (of 72.8) of any known liquid and it is the reason why water rises to unusually high levels in narrow capillary tubes. This has great significance in physiology. Uniquely high heat capacity : There occurs a smaller temperature rise in water as compared to most substances, when a given amount of heat is applied. Thus water act as a temperature buffer. It maintains its temperature more successfully than most other substances. 6

7. High solvent power : Water is a solvent for a great number of molecules which forms ionized solutions in water. It may thus, be called a universal solvent which facilitates chemical reactions both outside of and within biological systems. Cohesion : Cohesion among water molecules allows water striders to skate across the surface of still waters. 7

8. Chemical properties of Water Hydrogen Bonds: Molecules and ions with water forms hydrogen bonds (such as NaCl) are hydrophilic. On the other hand, ions and molecules that do not form hydrogen bonds with water are hydrophobic. Liquidity at Room Temperature: At room temperature, most compounds with low molecular weights takes gaseous forms. With water, however, hydrogen bonding helps to keep it a liquid at room temperature 8

9. Chemical reaction: Once ionic compounds are dissolve, there anions and cations circulates through the water allowing further reactions to occur. Thus water also sponsors and facilitates chemical reactions. Stable temperature: Water takes more heat to raise its temperature than other common compounds, since much of that heat is required to first break the hydrogen bonds. Freezing point: At 32 °F (or 0°C) and below, water molecules form hydrogen bonds in a crystalline lattice structure. This bonding spaces the molecules a bit, farther apart than usual, causing water to expands when it freezes. This results in ice being less dense than the liquid water, which is why ice floats. 9

10. Dissociation of Water Pure water is not really pure. The purest water contains some hydronium ions and hydroxide ions. These two are formed by the self-ionization of two water molecules. This happens rarely. The process is an equilibrium where the reactants, intact water molecules, dominate the mixture. At equilibrium the molarities for the hydronium ion and hydroxide ion are equal. [H 3 O 1+ ] = [OH 1- ] 10

11. The equation is H 2 O + H 2 O <---> H 3 O 1+  + OH 1- The equilibrium expression is the normal products over reactants. K = [H 3 O 1+ ] [OH 1- ] / [H 2 O] [H 2 O] The morality for the water is a constant at any specific temperature. This means the equation can be rewritten as K[H 2 O] [H 2 O] = [H 3 O 1+ ] [OH 1- ] 11

12. The quantity on the right hand side of the equation " K[H 2 O] [H 2 O] = Kw " is formally defined as Kw. The numerical value for Kw is different at different temperatures. At 25 o C Kw = 1.0 x 10 -14 Kw = K[H 2 O] [H 2 O] Kw = [H 3 O 1+ ] [OH 1- ] = 1.0 x 10 -14 12

13. Water as a Universal Solvent Water is capable of dissolving a variety of different substances, which is why it is such a good solvent and water is called the “universal solvent” because it dissolves more substances than other liquid. This is important to every living thing on earth. It means that wherever water goes, either through the ground or through our bodies, it takes along valuable chemicals ,minerals and nutrients. 13

14. It is water’s chemical composition and physical attributes that make it such an excellent solvent. Water molecules have a polar arrangement of the oxygen and hydrogen atoms, - one side (hydrogen) has a positive electrical charge and the other side, (oxygen) had a negative charge. This allows the water molecule to become attracted to many other different types of molecules. Water can become so heavily attracted to a different molecules like salt(NaCl), that it can disrupt the attractive forces that hold the Sodium and Chloride in the salt molecule together and thus dissolve it. 14

15. References https://water.usgs.gov/edu/qa-solvent.html http://www.800mainstreet.com/acid_base/dissociation_of_water.html https://en.wikipedia.org/wiki/Water 15

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