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Ch. 17: Water and Aqueous Systems • Ch. 17.1 Liquid Water and Its Properties • Ch. 17.2 Water Vapor and Ice • Ch. 17.3 Aqueous Systems • Ch. 17.4 Heterogeneous Aqueous Systems Ch. 17.1: Liquid Water and Its Properties • 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 Ch. 17.1: Liquid Water and Its Properties • Surface Properties – The surface of H2O 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 H2O are spherical Ch. 17.1: Liquid Water and Its Properties • 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 Ch. 17.1: Liquid Water and Its Properties • Specific heat capacity – It takes 4.18J (1 cal) to raise the temperature of 1 gram of water 10C – This is the specific heat capacity of water • The specific heat capacity of water is nearly constant between 00C and 1000C • Because of hydrogen bonding, the specific heat capacity of H2O is very high • Helps moderate daily air temp around large bodies of H2O – Water absorbs heat from warmer surroundings, which lowers the air temperature – At night, heat is transferred from the warmer water to the surrounding air Ch. 17.2 Water Vapor and Ice • 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 Ch. 17.2 Water Vapor and Ice • 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 Ch. 17.2 Water Vapor and Ice • 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 Ch. 17.2 Water Vapor and Ice • 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 Ch. 17.2 Water Vapor and Ice • Ice – As water cools, at first it behaves like a typical liquid • It contacts slightly and it’s density gradually increases (until 40C) • Then the density begins to decrease – Water no longer behaves like a typical liquid – Ice has a 10% lower density than water at 00C – As a result, ice floats (see pg. 481) – Ice is one of only a few solids that floats in it’s own liquid Ch. 17.2 Water Vapor and Ice • 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 Ch. 17.3 Aqueous Solutions • 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 Ch. 17.3 Aqueous Solutions • 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 Ch. 17.3 Aqueous Solutions • 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” Ch. 17.3 Aqueous Solutions • 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 Ch. 17.3 Aqueous Solutions • 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: HgCl2 – See pg. 485 Ch. 17.3 Aqueous Solutions • 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 1000C, hydrates lose their water of hydration Ch. 17.3 Aqueous Solutions • Hydrates – The forces holding the H2O 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 Ch. 17.3 Aqueous Solutions • 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 H2O to dissolve completely and form solutions • Occurs when the soln formed has a lower vapor pressure than that of air Ch. 17.4 Heterogeneous Aqueous Systems • 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 • See pg. 491 for examples Ch. 17.4 Heterogeneous Aqueous Systems • 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 Ch. 17.4 Heterogeneous Aqueous Systems • 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 Ch. 17.4 Heterogeneous Aqueous Systems • 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 Ch. 17.4 Heterogeneous Aqueous Systems • 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
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