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Solutes can be solids, liquids or gases before they are dissolved. The power of a solvent to dissolve solutes depends principally on its chemical proper%es.

1 Solu%ons Liquids can make weak intermolecular bonds to chemically diﬀerent molecules called solutes. The solutes enter the liquid (called a solvent) as individual par%cles and the mixture is called a solu)on. We say the solute has been dissolved by the solvent. Solutes can be solids, liquids or gases before they are dissolved. The power of a solvent to dissolve solutes depends principally on its chemical proper%es. If the solute molecules are strongly aeracted to each other it will be diﬃcult to dissolve them in the solvent. The ability of a solvent to dissolve a solute depends largely on the rela%ve strengths of the solvent- solvent aerac%on, the solute- solute aerac%on and the solvent- solute aerac%on. Water is an extremely eﬀec%ve solvent.

2 Concentra%on There are a few different ways of talking about how much solute is dissolved in a solvent but, in chemistry, the most common reference is molarity (M or mol dm 3 ), as we have seen before. Another reference is the molality (m or mol kg 1 ) which refers to the number of moles of solute dissolved in one kilogram of solvent. This measure is commonly used when calcula%ng some physical proper%es across different temperatures where the density of a solvent (and hence its volume) may change but the mass of the solvent does not. There is also percentage by weight which is simply the weight of solute as a percentage of the weight of the whole solu%on (usually for solid solutes) and percentage by volume, which is the added volume of the solute as a percentage of the volume of the solu%on (usually for liquid solutes).. Parts per million (ppm) is a common concentra%on unit used for very small concentra%ons and is similar in use to percentages (parts per hundred). Common examples is for 1 mg of solute in one litre of water ( 1kg H 2 O) or 1 mg of material in 1 kg of soil. Solu%ons containing a lot of solute are known as concentrated solu%ons, and solu%ons with only a liele solute are known as dilute solu%ons. Note that these terms are rela%ve, so a 1M solu%on might be considered dilute in one experiment but concentrated in another. Solu%ons that cannot dissolve any more solute are known as saturated solu%ons.

3 Solubility Solubility, the ability of a solute to dissolve in a solvent, is an important concept in many areas of chemistry. It is easy to dissolve sucrose (table sugar) in water, for instance, so we say that sucrose is soluble in water. It is very difficult to dissolve calcium carbonate (Ca 2+ CO 3 2 ) in water and we say that it is insoluble. However, the real picture is more complex than that. Sucrose has a maximum solubility in water of about 130 g dm 3. If we put some sucrose in water, it will dissolve up un%l this maximum. When the maximum is reached, we say the solu%on is saturated and no more sugar will dissolve. If we remove some of the water (for example by evapora%on) sucrose will precipitate as a solid to maintain the maximum concentra%on. If we dilute a saturated solu%on with more water, more sucrose will dissolve un%l either the maximum solubility is reached or all the sucrose has dissolved. Note that the terms soluble and insoluble are also rela%ve, so a research chemist would say that calcium carbonate is insoluble in water (15 mg dm 3 ), while a soil chemist might consider it to be soluble (compared with quartz at 3 mg dm 3 ).

4 Consequences of Solubility In a simple laboratory case, the solubility of a compound determines what solvents we can use. However, in applica%ons to environmental science and engineering, solubility has important implica%ons. Here are some examples: CO 2 dissolves in water to make a weak acid called carbonic acid (H 2 CO 3 ), which lowers the ph of the solu%on. Calcium carbonate (CaCO 3 ) is the main mineral in limestone and insoluble in water. However, it is a weak base and so it can react with carbonic acid to make a salt, Ca(HCO 3 ) 2, which is more soluble in water. This enhanced solubility allows acidic water to dissolve CaCO 3 with serious consequences: Rainwater, which contains H 2 CO 3, dissolves limestone to create limestone caves. When the CO 2 is released from dripping water, CaCO 3 is deposited on the ceilings and floor as stalac%tes and stalagmites. However, increased CO 2 from people visi%ng the caves creates more soluble Ca(HCO 3 ) 2, meaning that the visitors are slowly dissolving the stalac%tes and stalagmites. The dissolu%on of limestone can also create sinkholes with catastrophic consequences. Coral is based on CaCO 3 skeletons. Increasing atmospheric CO 2 means that the sea is increasing in acidity from increasing H 2 CO 3. This is slowly destroying coral reefs. Concrete incorporates large amounts of CaCO 3. Carbonic acid can dissolve this CaCO 3, significantly weakening the structure (interes%ngly, the reverse process can heal specially prepared concretes).

5 Solubility and Temperature Increasing temperature usually increases the solubility of solids and decreases the solubility of gases. A simple explana%on for this is that the increase in temperature makes the solutes prefer the more disordered state: solid > solu%on for solids and solu%on > gas for gases. (Note that the real explana%on is a liele more complicated than this). A sudden change in solubility, such as that for Na 2 SO 4, indicates a chemical change (in this case, Na 2 SO 4.10H 2 O > Na 2 SO 4 ).

When they dive deep in the sea, the high pressure causes more nitrogen (N2) to dissolve in their blood stream.

6 Solubility and Pressure Increasing pressure also increases solubility, though it is usually only signiﬁcant for gaseous solutes. This solubility- pressure rela%onship causes problems for deep sea divers. When they dive deep in the sea, the high pressure causes more nitrogen (N2) to dissolve in their blood stream. When they ascend, the lower pressure causes the nitrogen to come back out of solu%on. If this happens too quickly, the nitrogen forms bubbles in their blood vessels and %ssues, leading to extreme pain and even death.

Solu%ons Have Elevated Boiling Points The vapour pressure of the solvent in a solu%on is lower than the vapour pressure of the pure solvent because the solute molecules absorb some energy from the

7 Solu%ons Have Elevated Boiling Points The vapour pressure of the solvent in a solu%on is lower than the vapour pressure of the pure solvent because the solute molecules absorb some energy from the pre- vapour molecules of solvent. For a liquid to boil, its vapour pressure must be equal to atmospheric pressure (or the surrounding pressure). The lower vapour pressure of solu%ons means that we need a higher temperature to boil a solu%on. Note that this change in boiling point is usually very small.

Solu%ons Have Depressed Mel%ng Points Solute molecules also prevent solvent molecules from freezing into a solid crystal and this lowers the mel%ng point of the solvent.

Mel%ng points can be lowered by several degrees. This image from the BBC series Frozen Planet shows a brinicle forming under polar sea ice.

8 Solu%ons Have Depressed Mel%ng Points Solute molecules also prevent solvent molecules from freezing into a solid crystal and this lowers the mel%ng point of the solvent. This is why the sea can remain unfrozen even if rivers are frozen. Eventually, the solvent freezes as a pure solvent, gradually pushing the solute molecules into an ever increasing concentra%on. Mel%ng points can be lowered by several degrees. This image from the BBC series Frozen Planet shows a brinicle forming under polar sea ice. As ice forms at the surface, salt is frozen out and forms a stream of dense, extremely cold, high concentra%on salt solu%on. This dense solu%on falls to the sea bed but, since the rest of the sea water does not have so much salt and is not so cold, a hollow tube of ice forms around the falling solu%on. There is a remarkable video of this phenomenon at hep://

Osmo%c Pressure These considera%ons of solute par%cles help us to understand osmo)c pressure. Plants use osmo%c pressure to accomplish their feats of strength.

9 Osmo%c Pressure These considera%ons of solute par%cles help us to understand osmo)c pressure. Plants use osmo%c pressure to accomplish their feats of strength. In osmosis, a semi- permeable membrane allows only water molecules to pass through. Solute par%cles reduce the number of par%cles at the membrane interface. This means that water molecules are more likely to pass through the membrane from the low concentra%on side than from the high concentra%on side. This results in a net ﬂow of water from the low concentra%on to the high concentra%on side. and aeract solvent molecules away from the membrane In fact, water will move un%l the concentra%on on both sides of the membrane is equal. The net ﬂow of water allows plants to push through tarmac.

Emulsions and Colloids Emulsions are a special kind of solu%on. Molecules called surfactants consist of a water- loving hydrophilic head group and one or two water- ha%ng hydrophobic/lipophilic tails.

10 Emulsions and Colloids Emulsions are a special kind of solu%on. Molecules called surfactants consist of a water- loving hydrophilic head group and one or two water- ha%ng hydrophobic/lipophilic tails. The head group dissolves in water and the tail dissolves in oils. In solu%on, surfactants form nanoscale capsules called colloids, which are hydrophilic on the outside and hydrophobic on the inside (or vice versa). Soap and detergent emulsions can therefore dissolve hydrophobic dirt inside the colloids and carry it away into solu%on.

11 Past Exam Ques%ons A) i) A student accurately measures the boiling point of tap water and 8inds it to be 101 C. What is the most likely explanation for this high boiling point? B) ii) Fish can have dif/iculty breathing at elevated temperatures. What is the most likely explanation for this problem? B) i) An aqueous solution of formaldehyde (HCHO) is labelled as being 37% by weight. What is the concentration of this solution in molar units (M)? (refer to the data sheet for necessary information) ii) Given that formaldehyde is a gas, should the solution be stored at low or high temperatures to maintain its maximum concentration? Why? iii) Could this solution be stored at 1 C and still be ready for immediate use as a liquid? Why? iv) If the solution partially froze, what would happen to the concentration of formaldehyde in the remaining liquid?

12 Answers A) i) A student accurately measures the boiling point of tap water and 8inds it to be 101 C. What is the most likely explanation for this high boiling point? Impurities in the water have raised the boiling point of the water. ii) Fish can have dif/iculty breathing at elevated temperatures. What is the most likely explanation for this problem? There is less dissolved oxygen in higher temperature water. B) i) An aqueous solution of formaldehyde (HCHO) is labelled as being 37% by weight. What is the concentration of this solution in molar units (M)? (refer to the data sheet for necessary information) 1 dm 3 37% HCHO solution weighs 1000 ml x 1.01 g ml 1 = 1010 g mass of HCHO = 1010 g x 0.37 = 373 g M r HCHO = 2(1.01 g mol 1 ) g mol g mol 1 = 30.0 g mol g 30.0 g mol 1 = 12.5 mol (12.45 mol) Molarity = 12 mol dm 3 (12 mol dm 3 is acceptable) ii) Given that formaldehyde is a gas, should the solution be stored at low or high temperatures to maintain its maximum concentration? Why? Formaldehyde solution should be stored at low temperatures because gases are less soluble at high temperatures. iii) Could this solution be stored at 1 C and still be ready for immediate use as a liquid? Why? Yes, because the solute will lower the freezing point of the solvent (water). iv) If the solution partially froze, what would happen to the concentration of formaldehyde in the remaining liquid? It would increase.

13 Reading and Problems Reading: Chapter 14: , 14-6, 14-7, (14-8), 14-9, 14-11, 14-12, 14-15, Problems: Chapter 14: 14, 23, 36, 91

Lecture Presentation. Chapter 12. Solutions. Sherril Soman, Grand Valley State University Pearson Education, Inc.

Lecture Presentation. Chapter 12. Solutions. Sherril Soman, Grand Valley State University Pearson Education, Inc. Lecture Presentation Chapter 12 Solutions Sherril Soman, Grand Valley State University Thirsty Seawater Drinking seawater can cause dehydration. Seawater Is a homogeneous mixture of salts with water Contains