 Liquid Water and Its Properties  Water Vapor and Ice  Aqueous Systems  Heterogeneous Aqueous Systems

 Water is a unique compound Covers 75% of Earth’s surface A simple triatomic molecule Highly polar with a bent shape Water molecules are attracted to one another by intermolecular attractions, mainly hydrogen bonding, which causes:  High surface tension  High specific heat capacity  High heat of vaporization  High boiling point

 Surface Properties The surface of H 2 O acts like a skin Surface tension is a result of hydrogen bonding Water is cohesive, especially at the surface  Water cannot form bonds with the air  Instead, molecules are pulled inward  Explains why drops of H 2 O are spherical

 Surface Properties All liquids have a surface tension, but water’s is higher than most It is possible to lower the surface tension of water by adding a surfactant  A wetting agent such as soap or detergent  The detergent molecules interfere with the attraction between the water molecules  Hydrogen bonding also explains water’s unusually low vapor pressure Limits water’s ability to vaporize or evaporate

 Specific heat capacity It takes 4.18J (1 cal) to raise the temperature of 1 gram of water 1 0 C This is the specific heat capacity of water  The specific heat capacity of water is nearly constant between 0 0 C and C  Because of hydrogen bonding, the specific heat capacity of H 2 O is very high  Helps moderate daily air temp around large bodies of H 2 O  Water absorbs heat from warmer surroundings, which lowers the air temperature  At night, heat is transferred from the warmer water to the surrounding air

 Specific Heat Capacity (abbreviated “C”) - the amount of heat it takes to raise the temperature of 1 gram of the substance by 1 o C often called simply “Specific Heat” Note Table 17.1, page 508 (next slide)  Water has a HUGE value, when it is compared to other chemicals 6

Note the tremend ous differenc e in Specific Heat. Water’s value is VERY HIGH.

 To calculate, use the formula: q = mass (in grams) x  T x C  heat is abbreviated as “q”   T = change in temperature  C = Specific Heat Units are either: J/(g o C) or cal/(g o C) 8

 How much energy is required to raise the temperature of 65 mL of water from 20 degrees C to 88 degrees C?  (remember C = specific heat, and on the chart on the previous slide, the C for water is 4.18 J/gC

 Evaporation and Condensation Water absorbs a large amount of heat as it evaporates/vaporizes  Heat of vaporization is the energy needed to convert 1g of substance from a liquid to a gas at the boiling point  Hydrogen bonds must be broken before the liquid changes to the gaseous state

 Evaporation and Condensation The reverse of vaporization is condensation  The heat of condensation is equal to the heat of vaporization of water  Heat is released during condensation, gained during evaporation Evaporation and condensation are important to regional temperatures on Earth

 Boiling point Water has a very high boiling point  Due to hydrogen bonding  Molecular compounds of low molar mass are usually gases or liquids and have low boiling points at normal atmospheric pressure  Water is an exception  It takes a great deal of heat to to disrupt the bonding between the molecules in water  If this were not true, water would be a gas at the usual temperatures found on Earth

 Ice Liquids usually contract as they cool  Density increases while mass stays constant  Eventually the liquid will solidify  Because the density of the solid is greater than the liquid, the solid will sink

 Ice As water cools, at first it behaves like a typical liquid  It contacts slightly and it’s density gradually increases (until 4 0 C)  Then the density begins to decrease  Water no longer behaves like a typical liquid  Ice has a 10% lower density than water at 0 0 C  As a result, ice floats  Ice is one of only a few solids that floats in it’s own liquid

 Ice The fact that ice floats has important consequences for living organisms  Acts as an insulator in bodies of water Water molecules require a considerable amount of kinetic energy to return to the liquid state  Known as heat of fusion  Very high in water, compared to other low molar mass molecules

 Solvents and solutes Water samples containing dissolved substances are called aqueous solutions  The dissolving medium is the solvent  The dissolved particles are the solute  Solutes and solvents may be solids, liquids or gases Solutions are homogeneous mixtures  They are stable mixtures

 Solvents and solutes Substances that dissolve most readily in water include ionic cmpds and polar covalent molecules  Non-polar molecules like grease do no dissolve in water  Non-polar molecules will dissolve in other non-polar molecules

 The Solution Process Solvation is the process that occurs when a solute dissolves  The negatively and positively charged particles are surrounded by solvent molecules  In some ionic cmpds, internal attractions are stronger than external attractions – these cmpds cannot be solvated and are said to be insoluble The rule is “like dissolves like”

 Electrolytes and nonelectrolytes Cmpds that conduct an electric current in aqueous solution or the molten state are called electrolytes  All ionic cmpds are electrolytes  Some are insoluble in water Cmpds that do not conduct an electric current are called nonelectrolytes  They are not composed of ions  Most carbon cmpds are nonelectrolytes Some very polar molecular cmpds are nonelectrolytes in the pure state, but become electrolytes when they dissolve

 Electrolytes and nonelectrolytes Not all electrolytes conduct an electric current to the same degree  Some electrolytes are strong  When dissolved, almost all of the solute exists as separate ions  Ex: NaCl  Some electrolytes are weak  When dissolved, only a fraction of the solute exists as separate ions  Ex: HgCl 2

 Water of hydration The water in a crystal is called the water of hydration or water of crystallization  A cmpd that contains water is called a hydrate  When writing the formula, a dot is used to connect the formula of the cmpd and the number of water molecules per formula unit  Hydrates appear dry and are unchanged in normally moist air  When heated above C, hydrates lose their water of hydration

 Hydrates The forces holding the H 2 O in hydrates is not very strong  Held by weak forces  Results in a higher that normal vapor pressure  If the vapor pressure is higher than the vapor pressure in the air, the hydrate will effloresce by losing the water of hydration

 Hygroscopic substances Some hydrated salts that have a low vapor pressure remove water from air to form higher hydrates Salts and other substances that remove water from air are hygroscopic  Many are used as dessicants  Some cmpds are so hygroscopic that they become wet when exposed to air – these are called deliquescent cmpds  Remove enough H 2 O to dissolve completely and form solutions  Occurs when the soln formed has a lower vapor pressure than that of air

 Suspensions Mixtures from which particles settle out upon standing  Colloids Mixtures containing particles that are intermediate in size between suspensions and true solutions  The particles are in the dispersed phase  They are spread through the dispersion medium, which can be a solid, liquid or gas

 Colloids Properties differ from suspensions and solutions  May be cloudy when concentrated, clear when dilute  Intermediated sized particles cannot be filtered and do not settle out  Exhibit the Tyndall effect – scattering of visible light in all directions  Colloids scintillate (flash light) when studied under a microscope  Due to the erratic movement of the particles that reflect light  This chaotic movement is known as Brownian motion

 Colloids Properties differ from suspensions and solutions  Colloids scintillate (flash light) when studied under a microscope  Due to the erratic movement of the particles that reflect light  This chaotic movement is known as Brownian motion  Caused by collisions of molecules, which prevent the colloidal properties from settling

 Colloids Colloids may also absorb ions onto their surface  All the particles in a particular system will have the same charge  Repulsion of like charges keep the colloids from forming aggregates  Adding an opposite charge will cause separation of the colloid

 Emulsions Colloidal dispersions of liquids in liquids Requires an emulsifying agent  Ex: soap and detergents  Allow formation of colloidal dispersions between liquids that do not normally mix by forming bonds with the water molecules