Unit 6: Solutions

Unit 6: Solutions

Introduction

Chemical reactions usually take place in mixtures dissolved in water. In this unit we will learn the definition and nature of solutions, the factors affecting the rate at which substances dissolve, calculations and units of concentration. We will also discuss the increase in boiling point and freezing point of solutions, and experimental methods of separating substances in solution.

Solutions.

A solution is a homogeneous mixture. The substance that does the dissolving is called the solvent and the substance that is dissolved is called the solute. The substance in the largest amount is usually the solvent but not always. For example, it is possible to dissolve 180 g of sugar in 100 g of water. Solvents and solutes can be any state of matter. Below is a table listing many combinations that make up a solution.

Solution

/

Solvent

/

Solute

/
salt water / water / salt
flavored drink
(like Kool-ade) / water / sugar, dye,
and flavoring
rubbing alcohol
(70% isopropanol) / isopropanol / water
carbonated water / water / carbon dioxide gas
unpolluted air / nitrogen (72%) / oxygen (22%), argon (1%), carbon dioxide (0.1%), and water vapor (varies < 5%)
brass / copper / zinc or tin
pewter / tin / silver, lead
bronze / copper

Every part of a solution has the same composition with the solute particles evenly distributed. This is why the first cup of flavored drink has the same flavor and sweetness as the last cup. Moreover, solutions with solvents of liquid and gas are clear since the solutes are uniformly mixed and the particles of the solutes are now widely separated within the solvent.

Particles in Solution

When ionic compounds are dissolved in water the cation and anion separate and each is surrounded by water molecules. In a water molecule, the electron density is concentrated around the oxygen, so the molecule has a partial negative charge over oxygen and a partial positive charge near the hydrogens. The negative oxygens on water will surround each cation to dissolve it. Likewise, the positive hydrogens of water will surround each anion. See the animation of the process at http://mw2.concord.org/public/student/solution/dissolve.html & there is a lesson at http://molit.concord.org/database/activities/186.html .

Water is a polar molecule,
with positive and negative ends

http://commons.wikimedia.org/wiki/File:Watermolecule.png

NaCl dissolves in water and sodium is surrounded by water, solvated.

http://commons.wikimedia.org/wiki/File:Na%2BH2O.svg

See the animation of the process at http://mw2.concord.org/public/student/solution/dissolve.html & there is a lesson at http://molit.concord.org/database/activities/186.html .

Polar molecules behave much the same with the positive end of the molecule attracted to the negative oxygen of water and the negative end of the polar molecule attracted to the positive hydrogens of water. However, nonpolar molecules do not dissolve, since the attraction between water molecules is strong (including H-bonding) the nonpolar molecules are excluded.

The solute must be evenly distributed in the solvent to be a solution. Muddy water that separates after a long period of time is called a suspension of clay and water. Whipped cream is a mixture with evenly distributed milk fats and sugar in water. This mixture is called a colloid. Colloids are very much like solutions except the dissolving particles are so large that the mixture is not clear like in a solution.

Factors Affecting the Rate of Dissolving,

There are four factors affecting the rate of dissolving and three of them are ways you already know increase the rate of dissolving. 1) Agitation. Agitation means stirring, swirling, or shaking a solution. The more the solvent and the solute are mixed actively and not allowed to remain still, the faster the solution will form. 2) Increasing heat. Most solid and liquid solutes will dissolve when heated (which increases molecular motion). 3) Decrease surface area. By reducing the size of the pieces of solid being dissolved the process is faster.

All these factors affect how fast the ions, molecules, or atoms in a solute are broken apart and surrounded by the solvent. To see how this works picture a solid ionic compound arranged in its crystal lattice where each ion is connected to the surrounding ions, except for those ions on the outer surfaces where there are fewer connections. Water, the most common solvent, moves randomly and starts to connect to the different ions. At some point the water molecules, which can surround either positive or negative ions, provide a more stable system for the ion than the surrounding ions, because some of these ions are the same charge that repel the ion and some are opposite charges that attract the ion. The ion is surrounded by water and leaves the area. New water molecules move in and surround the next ion.

When a solution is agitated (stirred, shaken, swirled, etc), the process of moving the surrounded ions away from the undissolved substance proceeds faster and the solid is dissolved more rapidly. If the solution is heated the molecules of water and the particles of the solid move more rapidly, both of which help break up of the crystal lattice and move the solvated ions away from the dissolving substance.

Increasing the surface area of a solid involves adding more surfaces to the solute. Breaking or cutting the solid creates new surfaces, so smaller pieces have greater surface area than a larger piece of the same mass. These smaller pieces provide more places for the solvent to interact with the solute. This increases the rate of dissolving since more areas are exposed to and engage in solute particles being surrounded by and dissolved by the solvent.

Some nice simulations of the dissolving process are shown at: http://www.mhhe.com/physsci/chemistry/essentialchemistry/flash/molvie1.swf &

http://molo.concord.org/database/activities/186.html

Pressure the Fourth Factor

Besides agitation, temperature, and surface area, the fourth factor affecting the rate of dissolving is pressure. But pressure fundamentally affects only gases, because solids and liquids are not compressible, so pressure has very little affect on them. For a gas, an increase in pressure will increase the solubility of the gas. This is why bottles of soda are so stiff. The high pressure inside the bottle is necessary to maintain the high solubility of the carbon dioxide in the carbonated water and the fizz you expect from a fresh bottle of soda. Once the bottle has been opened and the pressure is released, the amount of carbon dioxide decreases. If the cap is left off the bottle the amount of carbon dioxide continues to decrease until the soda is flat, indicating most of the carbon dioxide has left the solution.

Concentration

Solutions are described in terms of concentration. The laundry detergent is Ultra because it is more concentrated, the bug spray is more effective because it has a higher concentration of repellant, the water is more pure because the concentration of dissolved solids is low, and your allergies are triggered by a higher concentration of pollen in the air. Concentration is used for solutions because the total amount of solute is dissolved in either a little bit of solvent or a lot of solvent and that matters. Streams dissolve a lot of water and carry it down to the ocean to make it salty, but there is no salty taste in streams because the stream only dissolves a small bit of salt at a time. It is concentrated in the ocean, where the amount of salt increases and the amount of water remains essentially constant.

Concentration can be described in a variety of ways: parts per million or billion (or trillion), mass per volume, volume per volume, percent of solute to solution, moles of solute per liter of solution, or moles of solutes per kilogram of solution.

ppm or ppb

The EPA (Environmental Protection Agency) monitors our water and air carefully and our water services have elaborate purification systems that make sure that the water we get in our taps is clean and safe. In fact the water from most municipalities only has small amounts of solutes like iron ions, Fe3+, calcium ions, Ca2+, sodium ions, Na+, and chlorine ions, Cl–, that are measured in parts per million, ppm, or parts per billion, ppb.

The picture of 100 dots is 10 times 10. The one black dot represents 1 part per hundred. Below each cluster of dots is 10 blue dots, so this is a picture of 1000 particles. The one black dot represents 1 part per thousand (the black dot is 1.5 times bigger than the other dots to emphasize its location). To get to 1 part per million, there would have to be 1000 similar pictures of 1000 dots all blue except for the 1 black dot already shown (imagine stacking 1000 papers of the picture on top of each other). To represent 1 part per billion, it would take 1,000,000 similar pictures of 1000 dots.

The parts per million and billion are ratios of the mass of the solute to total mass of the solvent. The density of water is assumed to be 1.00 g/mL just like pure water since very little solute is in the solution.

The EPA regulates the amount of substances that are allowed in pure air and water. For air the amounts of pollutants are closely watched because many people have difficulty breathing even when traces of certain substances are in the air. The limit for contamination in air can be very small and are reported as parts per billion. Ozone is one of the primary pollutants and at a level of 115 parts per billion the air is unhealthy for many people and all children especially if they are active. When ozone reaches a level of 400 parts per billion the air is unhealthy for all people. Another pollutant from cars, buses, trucks, and many other oil powered systems is nitrogen dioxide. Nitrogen dioxide is responsible for the brown-red color in a city sunset. This gas is hazardous when it reaches 1.6 parts per million.

Curious. Look here for air pollutants and then look at the calculator that converts the Air Quality Index, AQI, to units of concentration: http://www.epa.gov/air/urbanair/ & http://www.airnow.gov/index.cfm?action=resources.aqi_conc_calc .

Aqueous (or water) solutions can also have particles in small amounts that are listed best using ppm. For example, many ions in bottled water are listed in concentrations of ppm. While many bottled waters are filtered tap water (Aquifina, Dasani) others use the water purified by cities for their bottled water. Other bottled waters are can be nearly mineral free spring waters. Mineral water is water whose unusual taste is favored by many people to compliment their food instead of simply for hydration. Perrier, from Vergeze, France, contains 150 ppm calcium, 320 ppm bicarbonate (hydrogen carbonate, HCO3–), and 4.2 ppm magnesium (plus other ions); Avita, from Northern Michigan, contains 46 ppm calcium, 151 ppm bicarbonate, and 10.3 ppm magnesium; and Spa Reine, from Belguim, contains 3.5 ppm calcium, 11 ppm bicarbonate, and 1.3 ppm magnesium. All these different waters taste different and people favor one bottle over another because of the small amounts of ions that are dissolved in the different waters.

Our tap water is regulated by the EPA so many tap water systems provide very similar concentrations of ions even though the taste can be quite different. Many of these substances are measured in concentration units of ppm, but sometimes the concentration unit used is µg per L (a microgram, µg, is one millionth of a gram). Here is a list of drinking water contaminate limits published by the EPA: http://www.epa.gov/safewater/contaminants/index.html .

Lead and arsenic are commonly known to be dangerous in drinking water. The maximum amount of lead allowed in drinking water is 0.015 µg Pb per liter of solution. The maximum amount of arsenic is 0.010 µg As per liter of solution.

Some consumer products are sold with the concentration listed as volume of solute per volume of solution. Isopropanol is listed as 70% isopropanol, which means that its concentration is 70 mL of isopropanol for each 100 mL of solution. Lugol’s solution, which is an iodine solution used to disinfect cuts and scraps, is 2.2% solution of iodine/iodide.

In chemistry, the amount we are interested in is the mole, so chemicals are often sold in concentrations listing the moles per liter of solution. The strong acid, hydrochloric acid, is sold in 12 moles per liter solution. The unit for moles per liter is called molar and has a label M. So hydrochloric acid is sold in 12 M concentration. Sulfuric acid, one of the top chemicals in the world is sold as 18 M concentration. Calcium hydroxide a strong base, but it is also insoluble, so its highest concentration is 0.00105 M.

Calculating Concentration

Parts per million is a ratio calculation that determines the amount in a million based on the given mass of solute and solution (if the solvent is water, the density of the solution is 1.000 g/mL, since there is very little solute so the density is the density of water). For example, “Determine the ppm of lead in a 1 L solution that contains 0.0015 g of lead.” {Note that this solution is 100,000 times greater than the EPA amount mentioned above}

1 L• 1.00 g/mL = 1000 mL • 1 g/mL = 1000 g of solution

Known Unknown

0.0015 g Pb = x gives x = 1.5 ppm

1000 g solution 1,000,000

Another example, “Find the maximum amount of PCB’s, polychlorinatedbiphenyls, (widely used chemicals that were banned in 1979) in 2 L, if the EPA limit is 0.5 ppb in drinking water.”