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Experimentally it is found that the solubility of most compounds depends strongly on temperature and, if a gas, on pressure as well. Figure 13.9 “Solubilities of Several Inorganic and Organic Solids in Water as a Function of Temperature” shows plots of the solubilities of several organic and inorganic compounds in water as a function of temperature. Solubility may increase or decrease with temperature; the magnitude of this temperature dependence varies widely among compounds. The variation of solubility with temperature has been measured for a wide range of compounds, and the results are published in many standard reference books. Fractional crystallization is a common technique for purifying compounds as diverse as those shown in Figure 13.9 “Solubilities of Several Inorganic and Organic Solids in Water as a Function of Temperature” and from antibiotics to enzymes. The solubility of gases in liquids decreases with increasing temperature, as shown in Figure 13.10 “Solubilities of Several Common Gases in Water as a Function of Temperature at Partial Pressure of 1 atm”.
The decrease in the solubilities of gases at higher temperatures has both practical and environmental implications. Heating the solution decreases the solubility of CO2, which escapes into the gas phase above the solution.
Calcium carbonate (CaCO3) deposits in hot water pipes can significantly reduce pipe capacity. In thermal pollution, lake or river water that is used to cool an industrial reactor or a power plant is returned to the environment at a higher temperature than normal.
As the data in Table 13.6 “Henry’s Law Constants for Selected Gases in Water at 20°C” demonstrate, the concentration of a dissolved gas in water at a given pressure depends strongly on its physical properties. Gases that react chemically with water, such as HCl and the other hydrogen halides, H2S, and NH3, do not obey Henry’s law; all of these gases are much more soluble than predicted by Henry’s law. Due to the low Henry’s law constant for O2 in water, the levels of dissolved oxygen in water are too low to support the energy needs of multicellular organisms, including humans.
B Use Henry’s law to calculate the solubility, expressed as the concentration of dissolved gas. The solubility of most substances depends strongly on the temperature and, in the case of gases, on the pressure. The solubility of a solid may increase or decrease with increasing temperature, whereas the solubility of a gas decreases with an increase in temperature and a decrease in pressure.
Use the kinetic molecular theory of gases discussed in Chapter 10 “Gases” to explain why the solubility of virtually all gases in liquids decreases with increasing temperature.
An industrial plant uses water from a nearby stream to cool its reactor and returns the water to the stream after use. One manufacturer’s instructions for setting up an aquarium specify that if boiled water is used, the water must be cooled to room temperature and allowed to stand overnight before fish are added. Using a carbonated beverage as an example, discuss the effect of temperature on the “fizz.” How does the “foaminess” of a carbonated beverage differ between Los Angeles, California, and Denver, Colorado? A common laboratory technique for degassing a solvent is to place it in a flask that is sealed to the atmosphere and then evacuate the flask to remove any gases above the liquid.
Evacuating the flask to remove gases decreases the partial pressure of oxygen above the solution. What volume of CO2 would be released by warming 750 g of water saturated with CO2 from 0°C to 20°C?
The effects of a competitive inhibitor can be reduced by increasing the substrate concentration. These are not similar to the substrate and they do not bind to the active site of the enzyme. In the presence of a non-competitive inhibitor, increasing the substrate concentration cannot prevent the inhibitor from binding to the enzyme as the two bind to different sites.
The product of the last reaction of the metabolic pathway will bind to a site other than the active site of the enzyme that catalyses the first reaction.
There is a clear advantage in using end-product inhibition for controlling metabolic pathways.
The World Meteorological Organisation’s annual Greenhouse Gas Bulletin highlights an alarming increase in atmospheric CO2 concentration between 2012 and 2013. The 2014 bulletin reported that the global average amount of carbon dioxide in the atmosphere hit 396 parts per million in 2013, an increase of 3 ppm from 2012.
Oksana Tarasova, chief of the atmospheric research division at the WMO, offered further insight into the data and the complexities of climate research. The WMO researchers noted that the increase in CO2 last year was due not only to greater emissions but also to a reduced carbon uptake by the Earth’s biosphere. It’s also important to note the WMO data shows there was an 34% increase in the warming impact on the climate over the last 24 years because carbon dioxide and other greenhouses gases such as methane and nitrous oxide can survive for many years in the atmosphere.
Clayton Browne is a writer, editor and translator with a background in business and finance, technology and the social sciences. As we shall see, the ability to manipulate the solubility by changing the temperature and pressure has several important consequences. Although the solubility of a solid generally increases with increasing temperature, there is no simple relationship between the structure of a substance and the temperature dependence of its solubility. Chemists are often able to use this information to separate the components of a mixture by fractional crystallizationThe separation of compounds based on their relative solubilities in a given solvent., the separation of compounds on the basis of their solubilities in a given solvent. For the technique to work properly, the compound of interest must be more soluble at high temperature than at low temperature, so that lowering the temperature causes it to crystallize out of solution. Attractive intermolecular interactions in the gas phase are essentially zero for most substances. Anyone who routinely boils water in a teapot or electric kettle knows that a white or gray deposit builds up on the inside and must eventually be removed.
In the presence of calcium ions, the carbonate ions precipitate as insoluble calcium carbonate, the major component of boiler scale.
These deposits, called boiler scale, form when dissolved CO2 is driven into the gas phase at high temperatures.


Because of the reduced solubility of O2 at higher temperatures (Figure 13.10 “Solubilities of Several Common Gases in Water as a Function of Temperature at Partial Pressure of 1 atm”), the warmer water contains less dissolved oxygen than the water did when it entered the plant. Many marine species that are at the southern limit of their distributions have shifted their populations farther north. In contrast, the solubility of gases increases as the partial pressure of the gas above a solution increases.
When the concentration of dissolved gas molecules has increased so that the rate at which gas molecules escape into the gas phase is the same as the rate at which they dissolve, a dynamic equilibrium has been established, as depicted here.
Although the gas concentration may be expressed in any convenient units, we will use molarity exclusively. For a series of related substances, London dispersion forces increase as molecular mass increases.
For example, HCl reacts with water to give H+(aq) and Cl?(aq), not dissolved HCl molecules, and its dissociation into ions results in a much higher solubility than expected for a neutral molecule. For example, bubbles of CO2 form as soon as a carbonated beverage is opened because the drink was bottled under CO2 at a pressure greater than 1 atm.
To increase the O2 concentration in internal fluids, organisms synthesize highly soluble carrier molecules that bind O2 reversibly.
Over a period of time, dead fish start to appear downstream from the plant, but there is no evidence for any leaks of potentially toxic chemicals into the stream. When the water is cooled to room temperature, it initially contains very little dissolved oxygen. According to Henry’s law, the solubility of any gas decreases as its partial pressure above the solution decreases. When the substrate binds to the active site, this changes the shape of the active site and only then does it perfectly fit the substrate. This required energy is called the activation energy of the reaction and it is needed to break bonds within the reactants. This means that when a competitive inhibitor binds to the active site of an enzyme, it prevents the substrate from binding to the active site.
More substrate would successfully bind to the active site than inhibitor and therefore reducing the effect of the inhibition. Instead they bind to a different site on the enzyme and change the conformation of the active site. Therefore, no matter how high the concentration of substrate is, some of the enzymes will still be inhibited. When ATP accumulates it binds to a site other than the active site on the enzyme phosphofructokinase.
When there is an excess of end-product, the whole metabolic pathway is shut down as the end product inhibits the first enzyme of the pathway. The WMO argues that the significant increase in CO2 makes clear the pressing need for a global climate treaty. On average, close to half of all greenhouse gas emissions are absorbed up by the oceans, plants and living organisms. 1998 was the last time there was a reduction in the biosphere’s ability to absorb carbon, and that year saw huge global forest fires, together with El Nino conditions. Many compounds (such as glucose and CH3CO2Na) exhibit a dramatic increase in solubility with increasing temperature. In fact, the magnitudes of the changes in both enthalpy and entropy for dissolution are temperature dependent. In addition, the impurities must be more soluble than the compound of interest (as was KBr in this example) and preferably present in relatively small amounts.
The same phenomenon occurs on a much larger scale in the giant boilers used to supply hot water or steam for industrial applications, where it is called “boiler scale,” a deposit that can seriously decrease the capacity of hot water pipes (Figure 13.11 “Boiler Scale in a Water Pipe”). Fish and other aquatic organisms that need dissolved oxygen to live can literally suffocate if the oxygen concentration of their habitat is too low.
In 2005, the eelgrass, which forms an important nursery habitat for fish and shellfish, disappeared from much of the bay following record high water temperatures. This point is illustrated in Figure 13.12 “A Model Depicting Why the Solubility of a Gas Increases as the Partial Pressure Increases at Constant Temperature”, which shows the effect of increased pressure on the dynamic equilibrium that is established between the dissolved gas molecules in solution and the molecules in the gas phase above the solution.
This equilibrium is entirely analogous to the one that maintains the vapor pressure of a liquid.
Thus among the elements of group 18, the Henry’s law constants increase smoothly from He to Ne to Ar.
When the bottle is opened, the pressure of CO2 above the solution drops rapidly, and some of the dissolved gas escapes from the solution as bubbles. For example, human red blood cells contain a protein called hemoglobin that specifically binds O2 and facilitates its transport from the lungs to the tissues, where it is used to oxidize food molecules to provide energy.
The components of a mixture can often be separated using fractional crystallization, which separates compounds according to their solubilities. Why does the temperature of the solvent usually decrease substantially during this process?
Allowing the water to stand overnight allows oxygen in the air to dissolve, so that the fish will not suffocate. Consequently, dissolved oxygen escapes from solution into the gas phase, where it is removed by the vacuum pump. As the substrate binds it changes the shape of the active site and this weakens the bonds in the substrate and therefore reduces the activation energy.
The maximum rate of reaction or a level very close to the maximum rate of reaction can be reached. Succinate is found in the Krebs cycle of aerobic respiration and binds to the active site of the dehydrogenase enzyme. The substrate may still be able to bind to the active site however the enzyme is not able to catalyse the reaction or can only do so at a slower rate.


The maximum rate of reaction will always be lower in the presence of a non-competitive inhibitor. When it binds to the allosteric site it acts as non-competitive inhibitor and changes the conformation of the active site. Therefore less of the end product gets produced and by inhibiting the first enzyme it also prevents the formation of intermediates. Because the solubility of a compound is ultimately determined by relatively small differences between large numbers, there is generally no good way to predict how the solubility will vary with temperature.
According to the temperature curves in Figure 13.9 “Solubilities of Several Inorganic and Organic Solids in Water as a Function of Temperature”, both compounds dissolve in water at 80°C, and all 50 g of KBr remains in solution at 0°C. Because heat is released when these new attractive interactions form, dissolving most gases in liquids is an exothermic process (?Hsoln < 0). The problem is not a uniquely modern one: aqueducts that were built by the Romans 2000 years ago to carry cold water from alpine regions to warmer, drier regions in southern France were clogged by similar deposits. Because the warm, oxygen-depleted water is less dense, it tends to float on top of the cooler, denser, more oxygen-rich water in the lake or river, forming a barrier that prevents atmospheric oxygen from dissolving. Presumably, decreased oxygen levels decreased populations of clams and other filter feeders, which then decreased light transmission to allow the eelsgrass to grow. Because the concentration of molecules in the gas phase increases with increasing pressure, the concentration of dissolved gas molecules in the solution at equilibrium is also higher at higher pressures.
Values of the Henry’s law constants for solutions of several gases in water at 20°C are listed in Table 13.6 “Henry’s Law Constants for Selected Gases in Water at 20°C”.
Henry’s law also explains why scuba divers have to be careful to ascend to the surface slowly after a dive if they are breathing compressed air.
The concentration of hemoglobin in normal blood is about 2.2 mM, and each hemoglobin molecule can bind four O2 molecules. Filling the flask with nitrogen gas and repeating this process several times effectively removes almost all of the dissolved oxygen. The inhibitor is called a competitive inhibitor as it competes with the substrate for the active site. Often, the product of the last reaction in the pathway inhibits the enzyme that catalyses the first reaction of the pathway.
When the levels of the end product decrease, the enzymes start to work again and the metabolic pathway is switched on. An active member of his community in Austin, Texas, Clayton is also a member of the Austin-area Salvation Army Advisory Board.
Conversely, adding heat to the solution provides thermal energy that overcomes the attractive forces between the gas and the solvent molecules, thereby decreasing the solubility of the gas.The phenomenon is similar to that involved in the increase in vapor pressure of a pure liquid with increasing temperature, as discussed in Chapter 11 “Liquids”. The chemistry behind the formation of these deposits is moderately complex and will be described in more detail in Chapter 17 “Solubility and Complexation Equilibriums”, but the driving force is the loss of dissolved CO2 from solution.
The complex relationships in ecosystems such as the Chesapeake Bay are especially sensitive to temperature fluctuations that cause a deterioration of habitat quality.
Although London dispersion forces are too weak to explain such a large difference, O2 is paramagnetic and hence more polarizable than N2, which explains its high solubility. At the higher pressures under water, more N2 from the air dissolves in the diver’s internal fluids. The temperature of the solvent decreases because some solvent evaporates as well during this process.
The reason for this is that it explains why some enzymes can bind to many different substrates.
In exothermic reactions the energy released by the new bonds formed is greater than the activation energy. Once the inhibitor is released from the allosteric site, the active site returns to its original conformation and the substrate is able to bind again. In the case of vapor pressure, however, it is attractive forces between solvent molecules that are being overcome by the added thermal energy when the temperature is increased. Additionally, most fish and other nonmammalian aquatic organisms are cold-blooded, which means that their body temperature is the same as the temperature of their environment.
If the diver ascends too quickly, the rapid pressure change causes small bubbles of N2 to form throughout the body, a condition known as “the bends.” These bubbles can block the flow of blood through the small blood vessels, causing great pain and even proving fatal in some cases. Synthetic oxygen carriers based on fluorinated alkanes have been developed for use as an emergency replacement for whole blood. The heat that is required to evaporate some of the liquid is initially removed from the rest of the solvent, decreasing its temperature. If the shape of the active site changes when a substrate binds, this allows many different but similar substrates to bind to the one enzyme. Thus fractional crystallization allows us to recover about 75% of the original CH3CO2Na in essentially pure form in only one step.
Calcium bicarbonate [Ca(HCO3)2] is rather soluble in water, but calcium carbonate (CaCO3) is quite insoluble. Temperatures substantially greater than the normal range can lead to severe stress or even death.
Unlike donated blood, these “blood substitutes” do not require refrigeration and have a long shelf life.
Cooling systems for power plants and other facilities must be designed to minimize any adverse effects on the temperatures of surrounding bodies of water. Their very high Henry’s law constants for O2 result in dissolved oxygen concentrations comparable to those in normal blood.



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