Aqueous Systems and Solutions

Solutions An aqueous solution is water that contains dissolved substances. Solvents and Solutes In a solution, the dissolving medium is the solvent. The dissolved particles in a solution are the solute.

Solutions A solvent dissolves the solute. The solute becomes dispersed in the solvent. Solvents and solutes may be gases, liquids, or solids. Solutions are homogeneous mixtures. Solute particles can be atoms, ions, or molecules. If you filter a solution through filter paper, both the solute and solvent pass through the filter.

Solutions Substances that dissolve most readily in water include ionic compounds and polar covalent compounds. Nonpolar covalent compounds do not dissolve in water. Examples of nonpolar covalent compounds include methane (CH 4 ), and compounds found in oil, grease, and gasoline. However, oil and grease will dissolve in gasoline. What types of substances dissolve most readily in water?

Solutions The Solution Process A water molecule is polar, with a partial negative charge on the oxygen atom and partial positive charges on the hydrogen atoms. As individual solute ions break away from the crystal, the negatively and positively charged ions become surrounded by solvent molecules and the ionic crystal dissolves.

Solutions The process by which the positive and negative ions of an ionic solid become surrounded by solvent molecules is called solvation. Solvated ions Surface of ionic solid The Solution Process

Solutions The Solution Process Polar solvents such as water dissolve ionic compounds and polar compounds. Nonpolar solvents such as gasoline dissolve nonpolar compounds. This relationship can be summed up in the expression “like dissolves like.”

Which of these compounds should not dissolve in water? A.HCl B.C 4 H 10 C.KI D.NH 3

Which of these compounds should not dissolve in water? A.HCl B.C 4 H 10 C.KI D.NH 3

Electrolytes and Nonelectrolytes electrolyte - a compound that conducts an electric current when it dissolves in water or is melted. All ionic compounds are electrolytes because they dissociate into ions. nonelectrolyte - a compound that does not conduct an electric current ever. nonelectrolytes do not dissociate in water

Electrolytes and Nonelectrolytes In order for the bulb to light, an electric current must flow between the two electrodes that are immersed in the solution. Sodium chloride, a strong electrolyte, is nearly 100% dissociated into ions in water. To (+) electrode To (–) electrode

Electrolytes and Nonelectrolytes Mercury(II) chloride, a weak electrolyte, is only partially dissociated in water. To (+) electrode To (–) electrode In order for the bulb to light, an electric current must flow between the two electrodes that are immersed in the solution.

Electrolytes and Nonelectrolytes Glucose, a nonelectrolyte, does not dissociate in water. A nonelectrolyte is a compound that does not conduct an electric current in either an aqueous solution or the molten state. To (+) electrode To (–) electrode In order for the bulb to light, an electric current must flow between the two electrodes that are immersed in the solution.

Electrolytes and Nonelectrolytes Some polar molecular compounds are nonelectrolytes in the pure state but become electrolytes when they dissolve in water. (ex. Ammonia NH 3 ) For example, ammonia (NH 3 (g)) is not an electrolyte in the pure state. Yet an aqueous solution of ammonia conducts an electric current because ammonium ions (NH 4 + ) and hydroxide ions (OH – ) form when ammonia dissolves in water. NH 3 (g) + H 2 O(l)  NH 4 + (aq) + OH – (aq)

Electrolytes and Nonelectrolytes Not all electrolytes conduct electric current to the same degree. In a solution that contains a strong electrolyte, all or nearly all of the solute exists as ions. A weak electrolyte conducts an electric current poorly because only a fraction of the solute in the solution exists as ions.

Electrolytes and Nonelectrolytes Your cells use electrolytes, such as sodium and potassium ions, to carry electrical impulses across themselves and to other cells. An electrolyte imbalance can occur if you become dehydrated. When you exercise, you can lose water and electrolytes from your body through perspiration.

Explain why you must be extremely careful when using electricity near a swimming pool. The chlorinated water in a swimming pool is a solution that can conduct an electric current. If a current is introduced into the water, any swimmers could be shocked.

The water contained in a crystal is called the water of hydration or water of crystallization. Hydrates hydrate – a compound that contains water of hydration anhydrous - A substance that does not contain water.

Why do hydrates easily lose and regain water? The forces holding the water molecules in hydrates are not very strong, so the water is easily lost and regained. Hydrates

Heating of a sample of blue CuSO 4  5H 2 O begins. After a time, much of the blue hydrate has been converted to white anhydrous CuSO 4. CuSO 4  5H 2 O(s) CuSO 4 (s) + 5H 2 O(g) – heat + heat

Hydrates A piece of filter paper that has been dipped in an aqueous solution of cobalt(II) chloride and then dried is blue in color (anhydrous CoCl 2 ). When the paper is exposed to moist air, it turns pink because of the formation of the hydrate cobalt(II) chloride hexahydrate (CoCl 2  6H 2 O).

Each hydrate contains a fixed quantity of water and has a definite composition. Hydrates Some Common Hydrates FormulaChemical nameCommon name MgSO 4  7H 2 OMagnesium sulfate heptahydrateEpsom salt Ba(OH) 2  8H 2 OBarium hydroxide octahydrate CaCl 2  2H 2 OCalcium chloride dihydrate CuSO 4  5H 2 OCopper(II) sulfate pentahydrateBlue vitriol Na 2 SO 4  10H 2 OSodium sulfate decahydrateGlauber’s salt KAl(SO 4 ) 2  12H 2 O Potassium aluminum sulfate dodecahydrate Alum Na 2 B 4 O 7  10H 2 OSodium tetraborate decahydrateBorax FeSO 4  7H 2 OIron(II) sulfate heptahydrateGreen vitriol H 2 SO 4  H 2 OSulfuric acid hydrate (mp 8.6 o C)

Hydrates Efflorescent Hydrates If a hydrate has a vapor pressure higher than the pressure of water vapor in the air, the hydrate will lose its water of hydration, or effloresce. The water molecules in hydrates are held by weak forces, so hydrates often have an appreciable vapor pressure.

Hydrates Hygroscopic Hydrates Hydrates and other compounds that remove moisture from air are hygroscopic. Hydrated ionic compounds that have low vapor pressure remove water from moist air to form higher hydrates.

Hydrates Hygroscopic Hydrates Calcium chloride is used as a desiccant in the laboratory. desiccant - a substance used to absorb moisture from the air and create a dry atmosphere. Calcium chloride monohydrate spontaneously absorbs a second molecule of water when exposed to moist air.

Hydrates Deliquescent Compounds Deliquescent - compound that removes sufficient water from the air to dissolve completely and form solutions. Some compounds are so hygroscopic that they become wet when exposed to normally moist air.

Pellets of sodium hydroxide are deliquescent. For this reason, containers of NaOH should always be tightly stoppered. The solution formed by a deliquescent substance has a lower vapor pressure than that of the water in the air. Hydrates

Key Concepts and Key Equation Substances that dissolve most readily in water include ionic compounds and polar covalent compounds. All ionic compounds are electrolytes because they dissociate into ions. The forces holding the water molecules in hydrates are not very strong, so the water is easily lost and regained. mass of water mass of hydrate percent by mass H 2 O =  100%

CHEMISTRY & YOU Why are some sunsets red? The atmosphere contains particles of water and dust. As sunlight passes through the particles, it is scattered. However, not all wavelengths are scattered to the same extent.

Suspensions What is the difference between a suspension and a solution?

A suspension differs from a solution because the particles of a suspension are much larger and do not stay suspended indefinitely. Suspensions The particles in a typical suspension have an average diameter greater than 1000 nm. By contrast, the particle size in a solution is usually about 1 nm. suspension - mixture from which particles settle out upon standing.

Suspensions A solution is a homogeneous mixture. Suspensions are heterogeneous because at least two substances can be clearly identified.

Suspensions The difference between a solution and suspension is easily seen when the type of mixture is filtered. The small size of the solute particles in a solution allows them to pass through filter paper. The particles of a suspension can be removed by filtration.

Explain why a mixture of sand and water can be separated by filtration, but a mixture of salt and water cannot. A mixture of sand and water is a suspension, and a mixture of salt and water is a solution. The particles in the sand mixture are much larger than the ions in the salt mixture. The sand particles are too large to pass through filter paper; the ions are not.

Colloids What distinguishes a colloid from a suspension and a solution?

Colloids A colloid is a heterogeneous mixture containing particles that range in size from 1 nm to 1000 nm. The particles in a colloid are spread, or dispersed, throughout the dispersion medium, which can be a solid, liquid, or gas.

Colloids The first substances to be identified as colloids were glues. Some Colloidal Systems System TypeExample Dispersed phase Dispersion medium GasLiquidFoamWhipped cream GasSolidFoamMarshmallow Liquid EmulsionMilk, mayonnaise LiquidGasAerosolFog, aerosol SolidGasSmokeDust in air SolidLiquidSols, gels Egg white, jelly, paint, blood, starch in water, gelatin

Colloids Colloids have particles smaller than those in suspensions and larger than those in solutions. These intermediate-sized particles cannot be retained by filter paper as are the larger particles of a suspension. They do not settle out with time.

Colloids The Tyndall Effect You cannot see a beam of sunlight unless the light passes through particles of water (mist) or dust in the air. These particles scatter the sunlight. Similarly, a beam of light is visible as it passes through a colloid.

Colloids The Tyndall Effect The scattering of visible light by colloidal particles is called the Tyndall effect. Flashlight Solution Colloid Suspension

Colloids Suspensions also exhibit the Tyndall effect. The particles in solutions are too small to scatter light. Flashlight Solution Colloid Suspension

CHEMISTRY & YOU What would be the ideal conditions to see a red sunset? A misty or foggy evening would be ideal for seeing a red sunset. There would be a large number of particles to scatter the sunlight.

Colloids Brownian Motion Flashes of light, or scintillations, are seen when colloids are studied under a microscope. This happens because the particles reflecting and scattering the light move erratically.

Colloids Brownian Motion The chaotic movement of colloidal particles, which was first observed by the Scottish botanist Robert Brown (1773–1858), is called Brownian motion.

Colloids Coagulation Colloidal particles also tend to stay suspended because they become charged by adsorbing ions from the dispersing medium onto their surface. Adsorption means to adhere to a surface.

Colloids Brownian motion is caused by collisions of the molecules of the dispersion medium with the small, dispersed colloidal particles. These collisions help prevent the colloidal particles from setting. Brownian Motion

Colloids Coagulation All the colloidal particles in a particular colloidal system will have the same charge, although the colloidal system is neutral. The repulsion between the like-charged particles prevents the particles from forming heavier aggregates that would have a greater tendency to settle out.

Colloids Coagulation A colloidal system can be destroyed or coagulated by the addition of electrolytes. The added ions neutralize the charged colloidal particles. The particles can clump together to form heavier aggregates and settle out from the dispersion.

Colloids Emulsions An emulsion is a colloidal dispersion of a liquid in a liquid. An emulsifying agent is essential for the formation of an emulsion and for maintaining the emulsion’s stability.

Colloids Oils and greases are not soluble in water. However, oils and greases readily form a colloidal dispersion if soap or detergent is added to the water. Emulsions

Colloids One end of a large soap or detergent molecule is polar and is attracted to water molecules. The other end of the soap or detergent molecule is nonpolar and is soluble in oil or grease. Soaps and other emulsifying agents allow the formation of colloidal dispersions between liquids that do not ordinarily mix. Emulsions

Colloids This table summarizes the properties of solutions, colloids, and suspensions. Properties of Solutions, Colloids, and Suspensions Property System SolutionColloidSuspension Particle type Ions, atoms, small molecules Large molecules or particles Large particles or aggregates Particle size0.1–1 nm1–1000 nm1000 nm and larger Effect of lightNo scatteringExhibits Tyndall effect Effect of gravity Stable, does not separate Unstable, sediment forms Filtration Particles not retained on filter Particles retained on filter UniformityHomogeneousHeterogeneous

Calculate the percent by mass of water in washing soda, sodium carbonate decahydrate (Na 2 CO 3  10H 2 O). Hydrates

Calculate the percent by mass of water in epsom salt, magnesium sulfate heptahydrate (MgSO 4  7H 2 O).

Solution Formation Granulated sugar dissolves faster than sugar cubes, and both granulated sugar and sugar cubes dissolve faster in hot tea or when you stir the tea.

Solution Formation Composition of solvent and solute determine whether or not a substance will dissolve. Factors that affect how fast a substance dissolves include: Agitation (stirring or shaking) Temperature Particle size of the solute

Solution Formation Agitation If the contents of the glass are stirred, the crystals dissolve more quickly. The dissolving process occurs at the surface of the sugar crystals. Stirring speeds up dissolving because fresh solvent is continually brought in contact with the surface of the solute.

Solution Formation Agitation Agitation (stirring or shaking) affects only the rate at which a solid solute dissolves. It does not influence the amount of solute that will dissolve. An insoluble substance remains undissolved regardless of how vigorously or for how long the solvent/solute system is agitated.

Solution Formation Temperature also influences the rate at which a solute dissolves. Sugar dissolves much more rapidly in hot tea than in iced tea. Most solids dissolve faster at higher temperatures. Temperature

Solution Formation Temperature At higher temperatures, the kinetic energy of water molecules is greater than at lower temperatures, so the molecules move faster. At higher temperatures, solvent molecules move faster, resulting in more frequent collisions between solute and solvent molecules, resulting in faster dissolving.

Solution Formation Particle Size of the Solute The rate at which a solute dissolves also depends upon the size of the solute particles. The smaller particles in granulated sugar expose a much greater surface area to the colliding water molecules.

Solution Formation Particle Size of the Solute The dissolving process is a surface phenomenon. Smaller particles dissolve faster because more surface area of the solute is exposed, speeding up the rate of dissolving.

Which of the following will not speed up the rate at which a solid solute dissolves? A.Increasing the temperature B.Stirring the mixture C.Crushing the solute D.Decreasing the temperature

Which of the following will not speed up the rate at which a solid solute dissolves? A.Increasing the temperature B.Stirring the mixture C.Crushing the solute D.Decreasing the temperature

Solubility What is happening in this figure? Particles move from the solid into the solution. Some dissolved particles move from the solution back to the solid. When two processes occur at the same rate, no net change occurs in the overall system (equilibrium).

Solubility Such a solution is said to be saturated. saturated solution contains the maximum amount of solute for a given quantity of solvent at a constant temperature and pressure.

Solubility In a saturated solution, a state of dynamic equilibrium exists between the solution and any undissolved solute, provided that the temperature remains constant.

Solubility solubility of a substance is the amount of solute that dissolves in a given quantity of a solvent at a specified temperature and pressure to produce a saturated solution. Solubility is usually expressed as grams of solute per 100 g of solvent (g solute/100 g H 2 O). Sometimes the solubility of a gas is expressed in grams per liter of solution (g/L).

Solubility unsaturated solution contains less solute than a saturated solution at a given temperature and pressure If additional solute is added to an unsaturated solution, the solute will dissolve until the solution is saturated.

Solubility miscible liquids dissolve in each other in all proportions. Ex: water and ethanol immiscible - Liquids that are insoluble in each other Ex: oil and water

Factors Affecting Solubility Solubility for most solid substances increases as the temperature of the solvent increases. Interpret Graphs Temperature (°C) Solubility (g/100g H 2 O) For a few substances, solubility decreases with temperature.

Solubilities of Substances in Water at Various Temperatures Solubility (g/100 g H 2 O) SubstanceFormula0°C0°C20°C50°C100°C Barium hydroxideBa(OH) —— Barium sulfateBaSO — Calcium hydroxideCa(OH) — 0.07 Potassium chlorateKClO Potassium chlorideKCl Sodium chlorideNaCl Sodium nitrateNaNO Aluminum chlorideAlCl Silver nitrateAgNO Sucrose (table sugar)C 12 H 22 O HydrogenH2H OxygenO2O Carbon dioxideCO Interpret Data

Factors Affecting Solubility supersaturated solution contains more solute than it can theoretically hold at a given temperature. The crystallization of a supersaturated solution can be initiated if a very small crystal, called a seed crystal, of the solute is added.

Factors Affecting Solubility The rate at which excess solute deposits upon the surface of a seed crystal can be very rapid. The solution is clear before a seed crystal is added. Crystals begin to form immediately after the addition of a seed crystal. Excess solute crystallizes rapidly.

Factors Affecting Solubility The effect of temperature on the solubility of gases in liquid solvents is opposite that of solids. Solubility for most gases is greater in cold water than in hot. Temperature

Factors Affecting Solubility Changes in pressure have little effect on the solubility of solids and liquids, but pressure strongly influences the solubility of gases. Gas solubility increases as the partial pressure of the gas above the solution increases. Pressure

Factors Affecting Solubility Pressure Carbonated beverages are a good example. These drinks contain large amounts of carbon dioxide (CO 2 ) dissolved in water. Dissolved CO 2 makes the liquid fizz

Factors Affecting Solubility Pressure The drinks are bottled under a high pressure of CO 2 gas, which forces larger amounts of the gas into solution.

Factors Affecting Solubility Pressure When the container is opened, the partial pressure of CO 2 above the liquid decreases. Immediately, bubbles of CO 2 form in the liquid and escape from the open bottle.

Factors Affecting Solubility How is the partial pressure of carbon dioxide gas related to the solubility of CO 2 in a carbonated beverage? The relationship is described by Henry’s law, which states that at a given temperature, the solubility (S) of a gas in a liquid is directly proportional to the pressure (P) of the gas above the liquid. Pressure

Factors Affecting Solubility As the pressure of the gas above the liquid increases, the solubility of the gas increases. As the pressure of the gas decreases, the solubility of the gas decreases. Pressure

Explain why an opened container of a carbonated beverage is more likely to go flat sitting on the counter than in the refrigerator. The solubility of a gas in a liquid increases with decreasing temperature. More carbon dioxide will remain in solution at the colder temperature found in the refrigerator.

Factors Affecting Solubility What factors affect the solubility of a substance? Temperature affects the solubility of solid, liquid, and gaseous solutes in a solvent; both temperature and pressure affect the solubility of gaseous solutes.

Key Concepts Factors that determine how fast a substance dissolves are stirring, temperature, and surface area. In a saturated solution, a state of dynamic equilibrium exists between the solution and any undissolved solute, provided that the temperature remains constant. Temperature affects the solubility of solid, liquid, and gaseous solutes in a solvent; both temperature and pressure affect the solubility of gaseous solutes.