Why solutions are important

Solutions can involve all physical states—gases dissolved in gases the air around us , solids dissolved in solids metal alloys , and liquids dissolved in solids amalgams—liquid mercury dissolved in another metal such as silver, tin or copper. This chapter is almost exclusively concerned with aqueous solutions, substances dissolved in water. A volcanic dam impounds the lake waters. A pocket of magma lies beneath the lake and leaks carbon dioxide into the water, changing it into carbonic acid.

Nyos is one of only three known exploding lakes to be saturated with carbon dioxide in this way. A good example is milk. In addition to water, milk contains principally fat , sugar , and casein. The sugar is truly dissolved in the water.

The fat and the casein are fine particles held in suspension. If the milk stands for a while, the fat particles rise to the top as cream. Duration 2 years 1 year 24 days 15 minutes 5 months 27 days. Other uncategorized cookies are those that are being analyzed and have not been classified into a category as yet.

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The greater the partial pressure of a gas, the more of that gas will dissolve in the liquid. In the health field, this means that we can expose the body to higher pressures of oxygen to increase the amount of oxygen in the blood.

Or vice versa, we can remove toxic gases like carbon monoxide from the air to which a patient is exposed, to remove it from the solution of blood. So, this is really just a simple relationship between the amount of gas that dissolves in the liquid and the partial pressure of the gas above the liquid solution. The constant in that equation is also affected by three other factors.

The first is temperature. If we leave a glass of tap water out for a long time, bubbles form as the water warms up to room temperature because the air dissolved in the water is actually more soluble when the water is colder.

It also depends on what gas is being dissolved and what solvent or liquid is being used. Therefore, if we dive down with compressed air for a while, nitrogen begins to dissolve significantly into our blood under the increased pressure of the water. In order to prevent death if this occurs, a diver needs to get to a hospital quickly to get repressurized in a chamber, so that the bubbles will redissolve and the pressure can be released slowly.

This is important for lakes, streams, and other water reservoirs. The fish and other creatures that live in these bodies of water need certain levels of oxygen to survive. As the water moves around, and comes in constant contact with the air above the surface, a level of oxygen is maintained that allows fish to survive. However, in the ocean, where the depths of the water can be miles deep, only the water near the surface will be in contact with the air, while the water farther down will have little oxygen dissolved in it and can support very little aquatic life.

In a fish tank, we have to use a bubbler to force enough oxygen to dissolve in the water so certain types of fish will have enough oxygen to survive. Deep Sea Divers. However, we can also have an effect by changing the solvent. There is an experimental approach to respiratory medicine, with applications in deep-sea diving, that hopes to put this into practice.

It is theorized that applying liquid perfluorodecalin to wounds would accelerate healing by saturating the tissue with oxygen. It is also hoped that breathing perfluorodecalin into the lungs would be useful in treating pulmonary or cardiac trauma, as this compound was originally developed by the Navy for deep-sea diving.

Chemists typically work with solutions that are dilute , meaning that they consist mostly of solvent with only a little solute. In these cases, the mole fraction of the solvent is high and may be close to one. Of course, it is not actually one, because then we would have a pure substance instead of a solution. Many physical properties of solutions depend only on the concentration or amount of the solute in the solution, not on the identity of the solute.

In these, the concentration can be measured as a mole fraction, molarity or molality. These properties are called colligative properties and include:. These four properties are bound together by the fact that they each vary in proportion to the amount of solute and not the type of solute present. Vapor Pressure Depression The vapor pressure above a pure volatile liquid will always be higher than a solution of that liquid with some solute dissolved in it.

An example would be an aqueous solution of sodium chloride: The sodium chloride will not evaporate to form a gas above the solution, but the water will. Because the solvent has a solute dissolved in it, the mole fraction of the solvent is less than one, so the partial pressure is less than the vapor pressure of pure solvent.

The amount by which it is less is called the vapor pressure depression. The more concentrated the solute, the greater the vapor pressure depression. Figure This means that if we take a glass of water from the tap and some saltwater from the ocean and leave them sitting out, the saltwater will evaporate much more slowly than the pure water.

As the saltwater evaporates, the concentration of the salt will rise, the vapor pressure of the water will keep falling, and it will take longer and longer to evaporate the same amount of water.

Osmotic Pressure When a membrane or similar barrier allows the passage of some molecules but not others, it is called a semipermeable membrane. If a semipermeable membrane is placed in a solution, generally the solvent flows freely across the membrane and the solute is blocked. When this happens, solvent will flow through the membrane toward the side on which there is a higher concentration of solute.

The sidebar on how cells behave in solutions shows some of the interesting effects of osmotic pressure. See the Osmotic Pressure and Cells sidebar. In Figure , we have placed solvent in a jar and set a glass tube in it with a semipermeable membrane at the bottom. Some solute particles are dissolved in the solvent inside the tube, where they are confined because they cannot pass through the semipermeable membrane.

Because the solution in the tube is more concentrated, solvent will flow through the membrane into the tube to try to dilute it. The height of this column of water can be related to what we call its osmotic pressure , which has forced the liquid up to that height. Osmotic Pressure A. When solvent is free to flow across a semipermeable membrane, it will flow into the solution to dilute it somewhat. We can counteract this flow by placing a plunger on the liquid in the tube and pressing down.

Pushing down on the plunger will force pure water out through the membrane. This pressure can affect cells in the body making them swell or shrink, but it has a practical industrial use: reverse osmosis. If a pressure higher than the osmotic pressure of solution is applied to it, as by pushing down on a plunger, pure water can be forced out of the semipermeable membrane.

This is one of the common ways, not involving filters or evaporation, to purify water. Hypertonic, Hypotonic, and Isotonic Solutions. An erythrocyte, or red blood cell, is sensitive to the osmotic pressure of its environment. The membrane of the cell is a semipermeable membrane. In a hypertonic environment, where there is more dissolved salt outside the cell than inside, water flows out of the cell and it shrivels up.

In a hypotonic environment, with more dissolved particles inside the cell than outside, the water flows in causing the cell to swell up. If this goes far enough, the cell will burst. In an isotonic environment, the flow of water across the membrane is balanced and the cell has its normal size and shape. Plant cells do not have to worry about bursting when placed in a hypotonic solution, because they have strong cell walls on the outside of the cell membrane that prevent that type of rupture.

Humans are not so fortunate. Why does our skin become wrinkled when we take baths or swim for long periods of time. What has actually happened is that we were placed in pure or nearly pure water that was hypotonic relative to the inside of our skin cells, and they all began to swell with water.

And now, with our skin increasing in size, it starts to buckle or wrinkle like a carpet that is too big for a room. If we leave the water, our cells will pump out that water, it will evaporate, and our skin will return to normal.

However, if we stay in water for too long, our skin cells will be permanently destroyed. This is why swimmers who are crossing large bodies of water, like the English Channel, wear wet suits or coat their bodies with grease to prevent the water from coming in contact with their skin.

Boiling Point Elevation As mentioned earlier, vapor pressure increases as we raise the temperature. This means that we can compensate for the vapor pressure depression by increasing the temperature. For a particular decrease in vapor pressure due to the presence of dissolved solutes, there will be some increase in temperature that will raise the vapor pressure back up to what it was for the pure solvent.

This is important for understanding the next colligative property, boiling point elevation. Boiling Point Elevation A solution has a higher boiling point than the corresponding pure solvent. Dissolving a solute in the solvent decreases the mole fraction of the solvent relative to what it would be if it were pure. This decreases the vapor pressure at any given temperature. Since the boiling point is the temperature at which the vapor pressure equals the atmospheric pressure, the solution must therefore be heated to a higher temperature to reach this equivalence.

When we increase the temperature of a liquid, the vapor pressure above it increases. At some point, the vapor pressure of the liquid equals the pressure of the atmosphere above it. When this happens, the liquid stops merely evaporating from the surface of the liquid and begins to bubble vigorously and rapidly turn into the vapor phase. We can compensate for this, though, by heating the water to a higher temperature until the vapor pressure increases to equal atmospheric pressure again. This phenomenon is called boiling point elevation.

Freezing Point Depression While dissolving a solute in solvent increases the boiling point, dissolving a solute in solvent decreases the freezing point. As the mole fraction of solvent goes down, the freezing point of the solution goes down as well. Freezing Point Depression Just as adding solute to a solvent raises the boiling point, it also lowers the freezing point. This means that solutions will melt at a colder temperature than the pure solvent.

Author: Michael Pereckas, 3 December This phenomenon is called freezing point depression , and it is what makes salt water freeze at a lower temperature than pure water. This is why we salt roads in the winter: By lowering the freezing point of the water, any ice will melt at a higher temperature. For salt in water, freezing point depressions are more dramatic than are boiling point elevations. Purification by Filtration When making coffee, we use a filter to separate the coffee solution from the solid particles the coffee grounds.

When camping, we pump water from a stream through a filter to remove dirt particles and microorganisms like Giardia, which leaves us with clean drinking water as the purified product. Right: National Park Service. Often, we want to remove a solute from a solution, either because the solute is desirable and we want to extract it and use it, or because the solute is undesirable and we want to purify the solvent by getting rid of it.

There are a number of ways to do this. Filtration Filtration is a familiar process for anyone who has made coffee. Ground coffee beans are mixed with hot water, and the molecules in the beans dissolve into solution. But the solid residue of grounds is still mixed in with the coffee. In this way, the coffee is purified from the grounds. Chromatography The pigments in leaves have been separated by paper chromatography.

The pigments that have relatively more affinity for the solvent than the paper have migrated upward faster than those that have relatively more affinity for the paper than the solvent. The result is that the various molecules of different colors are spread out into the pattern seen here.

Author: Dominikmatus, 9 January If the molecules of solute can be turned into solid crystals, filtration can be used to remove the solute from a solution. This method of analysis involves precipitating the solute out as a solid. For example, if we had a solution of silver nitrate, but we wanted to collect the silver, we could add sodium chloride to the solution.

It would form insoluble silver chloride precipitate, and then we could filter the solution and collect the precious silver that is now separated from the solution it was originally in. Another method of separating and purifying compounds from solutions is called extraction. Some solutes are more soluble in some solvents as compared to others. For example, if we were to make a strong cup of tea, there would be some caffeine, some flavors, and some brown colorant molecules that could be extracted from the tea leaves into the solution of tea in water.

But it turns out that the caffeine is very soluble in organic solvents. If we shook together the tea and an organic solvent, the caffeine would move into the organic solvent and leave behind the flavors and colors in the water. We can now say that we have extracted the caffeine from the tea using an organic solvent. Chromatography Chromatography is a way of separating the components out of mixtures or solutions.

In chromatography, a solution is passed over or through a solid material. Different solute molecules will have differing degrees of molecular attraction for the solid material and so they will move though it at different speeds.

The more attracted a molecule is to the solid material, the less quickly it will move. The different rates at which the solutes pass through the solid material are what allow them to separate. In chromatography, the solvent is called the mobile phase , because it migrates through the solid material, and the unmoving material is called the stationary phase. With an understanding of solutions chemistry, we can see how to dissolve one substance into another, separate solvents from solutions, and purify them.

These processes are central to many modern industries, from oil refining to the development of new pharmaceutical products.