LECTURE № 8 WATER AND ITS PROPERTIES 28.03.2016

water as the cohesion, adhesion, capillary water and surface tension

Chemical

properties of water as solvent

Water Hardness

lakes, oceans and rain, and is the major constituent of

the fluids of living things.

and vasoconstriction)
Water helps to hydrate the body:
Our cells are

composed of roughly 75% water.

Our blood is composed of roughly 90% water.

but it often co-exists on Earth with its solid state,

ice; and gaseous state, steam (water vapor). It also exists as snow, fog, dew and cloud.

snow

fog

dew

clouds

and two hydrogen atoms that are connected by ................ bonds.
Water

is:
Composed of two hydrogen atoms and one oxygen atoms
H2O
Polar Molecule has oppositely charged ends

covalent

electrons:
Oxygen ends slightly negative
Hydrogen end slightly positive
Water molecule is a

polar and a dipole

charge
Charge hydrogen bond between (+) and (-) areas of different

water molecules

slightly negative
charge

a pure state has a neutral pH. As a result, pure

water is neither acidic nor basic. Water changes its pH when substances are dissolved in it.
Water conducts heat more easily than any liquid except mercury.
Water molecules exist in liquid form over an important range of temperature from 0 - 100° Celsius.
Water has a high surface tension.
Water is a universal solvent.

intermolecular forces that hold solids and liquids together
hydrogen bonding locks

molecules together.

ADHESION
Water is attracted to other substances
Water is adhesive to any substance with which it can form hydrogen bonds.

Thus the forces between molecules in a drop of water are cohesive, while the mutual attraction between water and glass represents adhesion.

liquid that allows it to resist an external force, due

to the cohesive nature of the water molecules.
Water molecules want to cling to each other. At the surface, however, there are fewer water molecules to cling to since there is air above (thus, no water molecules). This results in a stronger hydrogen bond between those molecules that actually do come in contact with one another, and a layer of strongly bonded water (see diagram). This surface layer (held together by surface tension) creates a considerable barrier between the atmosphere and the water.

Surface tension allows insects to float and stride on a water surface.

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.

meniscus

one of the few substances that is less dense as

a solid than liquid
Due to hydrogen bonding and resultant pockets

ice floats on water because
it is less dense

°C and freezes at 0 °C.

The boiling temperature of

water decreases at higher elevations (lower atmospheric pressure).

For this reason, an egg will take longer to boil at higher altitudes

water
Substances such as salt are pulled apart by attraction

of opposite charges due to polar structures

a base is called, amphoteric (ampholyte):
acid
base
ampholyte
All the major components in

cells (proteins, DNA and polysaccharides) are also dissolved in water.

significantly with the dissolution of a small amount of ionic

material such as sodium chloride.

can participate in chemical reactions. Example: involvement of water molecules

in dehydration synthesis and hydrolysis, electrolysis.
Metals – Iron, Manganese, Zinc, Copper, Chromium, Lead
Nitrate/ Nitrite
Arsenic, Selenium, Fluoride
Chloride
Total and Free Chlorine
In practice, the chemical properties of water are determined by analytical methods.

1 (IA or alkali) react vigorously with cold water forming hydroxide

and hydrogen gas:


Metals from magnesium to iron in the activity series of metals, react with steam (but not H2O) to form the metal oxide and hydrogen gas:



Noble metals, such as gold and silver, do not react with water at all.

are held together by electrostatic forces of attraction. When an

electrolyte is dissolved in water, these forces are weakened and the electrolyte undergoes dissociation into ions. The ions are solvated.

acid: HCl (g) + H2O (aq) = H+ (aq) + Cl-(aq)

base: NaOH (s) + H2O (aq) = Na+ (aq) + OH-(aq)

salt: NaCl(s) + H2O (aq) = Na+ (aq) + Cl-(aq)

The process of splitting of the molecules into ions of an electrolyte is called dissociation .

into two parts by the addition of a molecule of

water ("chemical decomposition by water," 1880, formed in English from hydro- + Greek lysis "a loosening, a dissolution," from lyein "to loosen, dissolve").

AB + H-OH ⇌ A-H + B-OH

HYDROLYSIS

General hardness (GH) is caused by the presence of a

certain concentration of calcium (Са2+) and magnesium (Mg2+) ions in 1 liter of water:

the metal cations react with soaps, causing them to form

an unsightly precipitate – the familiar "bathtub ring".
More seriously, the calcium and magnesium carbonates tend to precipitate out as adherent solids on the surfaces of pipes and especially on the hot heat exchanger surfaces of boilers. The resulting scale buildup can impede water flow in pipes.

In boilers, the deposits act as thermal insulation that impedes the flow of heat into the water; this not only reduces heating efficiency, but allows the metal to overheat, which in pressurized systems can lead to catastrophic failure.

CaSO4
MgCl2, MgSO4
GENERAL HARDNESS:
GH = TH + PH

calcium and magnesium. It can be removed by easy means

like boiling. When temporary hard water is boiled, the carbonates decompose with liberation of carbon-dioxide and precipitation of the insoluble Carbonates which are reformed:

MgCO3 is slightly soluble in water but heating will cause its hydrolysis into the much less soluble Mg(OH)2


So simple boiling and filtering of water remove temporary hardness.

TEMPORARY HARDNESS

of calcium and magnesium. These salts do not decompose on

boiling. So permanent hardness can’t be removed easily. It can be removed by soda (Na2CO3), surfactants (Na3PO4) or lime (Ca(OH)2) when MgSO4 is responsible for hardness:

PERMANENT HARDNESS

Surfactant a substance that tends to reduce the surface tension of a liquid in which it is dissolved.

of sodium carbonates
Base exchange process

Addition of Sodium

carbonate
Base exchange process:
Sodium Permutit is used ( Na2Al2 Si2O H2O )
Exchange Na ions for Ca and Mg ions
Ca and Mg Permutit is formed .
Removes 100% hardness
Some raw water is added to protect against corrosive action of 100% soft water

Temporary Hardness

Permanent Hardness

in 1 liter of the water (mmole-Eq/L).
On scale hardness distinguishes:

following 2 analytical methods:
By titration with HCl: In this method

temporary hardness can be measured.
By titration with EDTA: In this method general hardness can be measured.

hardness: 100 ml hard water is taken into a 250

ml conical flask. Then few 3-4 drops of methyl orange is added in it as an indicator. Now titration is carried out by adding 0.1N cold HCl until the yellow color of methyl orange turns red.
Here, each meal 0.1N HCl is equivalent to 0.005 gm of CaCO3. The associated reactions are as follows:

the original water sample. Add 3-4 drops of Eriochrome Black

T indicator. Titrate against 0.05N prepared EDTA solutions in burette until the color charges from wine red (or violet) to pure blue (or turquoise) with no reddish tone; then calculate the general hardness:

ESTIMATION OF GENERAL HARDNESS BY TITRATION WITH EDTA:

changed its color:

Lab # 3 Water Hardness

from total hardness i.e.
Permanent = General hardness – Temporary
hardness

hardness

GH = TH + RH

to 7.5 mmol / L
hardness not more than 7

mmol / L,
the total amount of minerals in which not more than 1 g / l,
harmful chemical impurities do not exceed the maximum allowable concentrations,
and lacking pathogenic bacteria and viruses.

brush, water, and a small amount of detergent. Rinse it

twice with deionized water. Be sure to drain deionized water through the tip.
Rinse the buret again with two 10 mL portions of the titrant (HCl or EDTA solutions)
Fill the buret with titrant and drain a small amount from the buret to dispel any air bubbles that might be in the tip.

a white note card. This will be used to help

locate the bottom of the meniscus when reading the buret.


Place the note card with the black mark behind the buret and just below the meniscus. This will make the bottom of the meniscus much easier to see. Record the volume reading to the nearest 0.01 mL.

of titrant, open the stopcock slightly to allow a small

amount of titrant to accumulate on the tip of the buret.


Rinse the titrant on the tip into the flask using deionized water from your wash bottle.

угля (черные частицы) и катионообменники (гранулы смолы белого цвета).
Уголь адсорбирует

вредные органические вещества и хлориды.
Катионообменники снижают общую жесткость.

an unlike substance.
Cohesion – various intermolecular forces that hold solids

and liquids together.
Buffer is a solution composed of a weak acid and its conjugate base that can be used to stabilize the pH of a solution
Density is a measure of the amount of matter contained by a given volume.
Hydrophobic – lacking an affinity for water; unable to absorb, or be wetted by water
Hydrophilic – having an affinity for water; able to absorb, or be wetted by water
Polarity – The intermolecular forces between the slightly positively-charged end of one molecule to the negative end of another or the same molecule.
Specific heat is the amount of heat, in calories, needed to raise the temperature of 1 gram of water by 1 degree Celsius
Heat capacity – the capability of a substance to absorb heat energy
Surface tension is the property of the surface of a liquid that allows it to resist an external force, due to the cohesive nature of its molecules.