WATER: PHYSICAL PROPERTIES OF WATER

Prof. Dr. Bülent Mızrak

BAU International-Batumi

2015

Physical properties of water

Physical Properties of Water Property Value

Molar mass 18.015 Molar Volume 55.5 moles/liter Boiling Point (BP) 100°C at 1 atm Freezing point (FP) 0°C at 1 atm Triple point 273.16 K at 4.6 torr Surface Tension 73 dynes/cm at 20°C Vapor pressure 0.0212 atm at 20°C Heat of vaporization 40.63 kJ/mol Heat of Fusion 6.013 kJ/mol Heat Capacity (cp) 4.22 kJ/kg.K Dielectric Constant 78.54 at 25°C Viscosity 1.002 centipoise at 20°C Density 1 g/cc Density maxima 4°C

Physical properties cont.d

Specific heat 4180 J kg 1 K 1 ( T=293…373 K) Heat conductivity 0.60 W m 1 K 1 (T=293 K) Melting heat 3.34 x 105 J/kg Evaporation heat 22.6 x 105 J/kg Critical Temperature 647 K Critical pressure 22.1 x 106 Pa Speed of sound 1480 m/s (T=293 K) Relative permittivity 80 (T=298 K) index of refraction (relative to air) 1.31 (ice; 589 nm; T=273 K; p=p0) 1.34 (water; 430 490 nm; T=293 K; p=p0) 1.33 (water; 590 690 nm; T=293 K; p=p0)

Links between Chemical Structure and Physical Properties

Water (H2O) is a very unusual substance with many strange and unique properties that are so important to Life on Planet Water Life which is based on Water and adapted to its unique and anomalous properties. How does this simple molecule, composed of two hydrogen atoms and one oxygen atom, behave the way it does and how does it support life? Some familiar properties of water are: It's colourless; It's tasteless; It's odourless

It feels wet; It dissolves nearly everything; It exists in three forms: liquid, solid, gas, and is cycled though the water cycle; It can absorb a large amount of heat; It sticks together into beads or drops; It flows and erodes the surface of the earth; it moves sediments to form beaches, river banks and bars;

It shapes lipid and protein molecules and give them their 3dimensional form which is critical to their function It's part of every living organism on the planet.

Many of water's unique properties are largely a result of its chemical structure. The two hydrogen atoms bound to one oxygen atom to form a 'V' shape with the hydrogen atoms at an angle of 105°. When the hydrogen atoms combine with oxygen, they each give away their single electron and form a covalent bond.

Because electrons are more attracted to the positively charged oxygen atom, the two hydrogens become slightly positively charged (they give away their negative charge) and the oxygen atom becomes negatively charged.

This separation between negative and positive charges produces a polar molecule, that is a molecule that has an electrical charge on its surface. The hydrogen lobes have positive charges, and the oxygen atom on the opposite side has two negative charges (associated with two lobes.

The net interaction between the covalent bond and the attracting and repulsion between the positive and negative charges repelling charges produces the 'V' shape of the molecule. The polarity of water allows it to bind with other molecules, including itself. The water molecules form hydrogen bonds, giving shape to water as a liquid.

Each single water molecule can form bonds with four other water molecules in a tetrahedral arrangement. Although these bonds are weak they lead to many other unique properties The Vshape of the water molecule is also important because it allows for other configurations of water to be formed

Ice, for instance, has a very ordered lattice structure. Super cooled water (water below the freezing point) also has water molecules that are structured in a certain way. Snowflakes have yet another shape.

These general rules of molecular interactions with water also apply to macromolecules, such as lipids, carbohydrates, proteins, and nucleic acids, which we will consider in greater detail later.

Most macromolecules contain both hydrophilic and hydrophobic groups.

The hydrophobic groups will tend to be buried in the center of the molecule (or macromolecular complex), minimizing their interactions with water.

Carbohydrates illustrate this rule - they are almost exclusively hydrophilic and tend to adopt an extended structure in solution.

Physical Properties and Life

Life on Planet Water depends on the unique and unusual properties of water. Water is 'mother' and 'matrix' for life. # The large heat capacity and high water content in organisms contribute to thermal regulation and prevent local temperature fluctuations.

# The high latent heat of evaporation gives resistance to dehydration and considerable evaporative cooling. # Water is an excellent solvent due to its polarity, high dielectric constant and small size, particularly for polar and ionic compounds and salts.

# It has unique hydration properties towards biological molecules (particularly lipids, proteins and nucleic acids) that determine their three dimensional structures, and hence their functions, in solution. This hydration forms gels that can reversibly undergo the gel- sol phase transitions that underlie many cellular mechanisms.

# Water ionizes and allows easy proton exchange between molecules, so contributing to the richness of the ionic interactions in biology. # The density maximum at 4°C and low ice density means that all of a body of water (not just its surface) is close to 0°C before any freezing can occur.

Also the freezing of rivers, lakes and oceans is from the top down, so insulating the water from further freezing and allowing rapid thawing, and density driven thermal convection causing seasonal mixing in deeper temperate waters.

# The large heat capacity of the oceans and seas allows them to act as heat reservoirs such that sea temperatures vary only a third as much as land temperatures and so moderate our climate. # The compressibility of water reduces the sea level by about 40 m giving us 5% more land.

Hydrophylic ('Water Loving') and

Hydrophobic ('Water Hating') Molecules Hydrophylic Molecules

Substances that dissolve readily in water are termed 'hydrophilic'. They are composed of ions or polar molecules that attract water molecules through electrical charge effects. Water molecules surround each ion or polar molecule on the surface of a solid substance and carry it into solution

SURFACE TENSION

Water is adhesive and elastic, and tends to aggregate in drops rather than spread out over a surface as a thin film. This phenomenon also causes water to stick to the sides of vertical structures despite gravity's downward pull. Water's high surface tension allows for the formation of water droplets and waves, allows plants to move water (and dissolved nutrients) from their roots to their leaves, and the movement of blood through tiny vessels in the bodies of some animals

Ionic substances such as sodium chloride dissolve because water molecules are attracted to the positive (Na+) or negative (Cl charge of each ion. Polar substances such as urea dissolve because their molecules form hydrogen bonds with the surrounding water molecules.

Hydrophobic Molecules Molecules that contain mostly nonpolar bonds are usually insoluble in water and are termed 'hydrophobic'. This is true, especially, of hydrocarbons, which contain many C H bonds. Water molecules are less attracted to such molecules than they are to other water molecules and so have little tendency to surround them and carry them into solution.

But the socalled 'Hydrophobic Effect' does not mean that nonpolar molecules are not attracted to water! When a highly polar substance, such as water, is mixed with a nonpolar or weakly polar substance, such as most oils, the substances will separate into two phases.

This phenomenon is usually rationalized in introductory chemistry text books by saying that oil is hydrophobic. Most people wrongly believe that this means that individual water and oil molecules repel each other, or at least attract each other very weakly. However, this is clearly wrong and misleading!

In face an individual oil molecule is attracted to a water molecule by a force that is much greater than the attraction of two oil molecules to each other. This can be demonstrated when a drop of oil is placed onto a clean surface of water. Originally the oil will be in the shape of a spherical droplet, because the oil molecules are attracted to one another and a spherical shape minimizes the number of oil molecules that are not surrounded by other molecules.

When the oil droplet hits the surface of the water, it spreads out to form a thin layer. This happens because the oil and water bonds formed by the oil forming a layer on the surface of the water are stronger than the oil oil attraction in the oil droplet. If a sufficiently small drop of oil is put on the surface, it will spread to form a single molecular layer of oil.

Given these strong interactions, why doesn't each oil molecule dive into the water solution? and become completely surrounded with water molecules? The reason is that the water water bonds are much stronger! Displacing the water molecules would cost more energy.

Consequently most of the oil molecules stay out of the water, though as many as will fit will hang on to the surface water molecules that do not have a full complement of partners. A similar explanation applies for the meniscus, that is the curved surface of a liquid in a graduated cylinder or any other small diameter glassware. Water adheres to the sides of any container creating a "cup" of surface tension.

The induced structure produced through the interaction with water molecules is very important as it is related to the structure and function of membranes which are very characteristic of life as we know it. Membranes in bacteria are composed of phospholipids and proteins. Phospholipids contain a charged or polar group (often phosphate, hence the name) attached to a 3 carbon glycerol back bone. There are also two fatty acid chains dangling from the other carbons of glycerol.

The phosphate end of the molecule is hydrophilic and is attracted to water. The fatty acids are hydrophobic and are driven away from water. Because phospholipids have hydrophobic and hydrophilic portions, they do remarkable things. When placed in an aqueous environment, the hydrophobic portions stick together, as do the hydrophilic bits. A very stable form of this arrangement is the lipid bilayer.

This way the hydrophobic parts of the molecule form one layer, as do the hydrophilic. Lipid bilayers form spontaneously if phospholipids are placed in an aqueous environment. The cytoplasmic membrane is stabilized by hydrophobic interactions (i.e. water induced) between neighboring lipids and by hydrogen bonds between neighboring lipids. Hydrogen bonds can also form between membrane proteins and lipids. These are known as membrane vesicles and are used to study membrane properties experimentally. There is some evidence that these structures may form abiotically and may occur on particles that rain down on earth from space.

Water as a Solvent Acids & Bases pH Hydration

Water as a Solvent Many substances, such as salt and sugar, dissolve in water. That is, their molecules separate from each other, each becoming surrounded by water molecules. When a substance dissolves in a liquid, the mixture is termed a solution. The dissolved substance (in this case salt or sugar) is the solute, and the liquid that does the dissolving (in this case water) is the solvent. Water is an excellent solvent for many substances because of its polar bonds.

Acids

Substances that release hydrogen ions into solution are called acids. Many of the acids important in the cell are only partially dissociated, and they are therefore weak acids for example, the carboxyl group ( COOH), which dissociates to give a hydrogen ion in solution. This is a reversible reaction.

Bases

Substances that reduce the number of hydrogen ions in solution are called bases. Some bases, such as ammonia, combine directly with hydrogen ions. Other bases, such as sodium hydroxide, reduce the number of H+ ions indirectly, by making OH ions that then combine directly with H+ ions to make H2O. Many bases found in cells are partially dissociated and are termed weak bases. This is true of compounds that contain an amino group ( NH2), which has a weak tendency to reversibly accept an H+ ion from water, increasing the quantity of free OH ions.

Hydrogen Ion exchange

Positively charged hydrogen ions (H+) can spontaneously move from one water molecule to another, thereby creating two ionic species. Since the process is rapidly reversible, hydrogen ions are continually shuttling between water molecules. Pure water contains a steady state concentration of hydrogen ions and hydroxyl ions (both 10 7 M).

pH

The acidity of a solution is defined by the concentration of H+ ions it possesses. For convenience we use the pH scale, where pH = _log10[H+]. For pure water [H+] = 10_7 moles/liter