What is liquid solution

We can write. We can calculate these values from the solution concentrations using Daulton's Law as follows. The vapour composition curve can be plotted as shown in the figure below. It is really two plots, one the straight line is the Vapour pressure of solution versus Liquid composition x A and the other, the curved line is the same Vapour pressure of solution, but plotted as a function of Vapour composition y A.

It could be thought of as pulling the Liquid line to the right towards the more volatile liquid A. Horizontal tie lines join the two curves such that for any given Vapour pressure the liquid composition x A and the the corresponding vapour composition y A can be determined as indicated by the arrows in the figures. Normally, we don't carry out experiments as constant temperature as seemed to be indicated in the previous two figures and the corresponding discussion.

To do so would involve complicated pressure measuring devices, sealed rigid containers and constant temperature devices. We can much more easily do a measurement of temperature at fixed pressure say one bar as a function of mole fraction. We would thus get a plot of boiling point of solution as a function of mole fraction of the solution. To this, we can add a plot of the vapour composition. This curve can be calculated using concepts much like those discussed above for the constant temperature case.

The resulting curve seen below is shifted towards the higher vapour pressure component just as it was in the diagram above.

In this case, since we already know that the vapour pressure is not a linear function of Temperature c. However, for an ideal solution the curvature of the line is only slight.

Let's explore the tie line in further detail. The graph of "T vs. The diagram below is a blowup of the tie-line region of the previous figure; the blue line represents the liquid solution composition, the green line is the vapour composition. The vertical axis is Temperature and the horizontal axis is Mole fraciton of component A in a two-component mixture of A and B.

The vertical purple line represents the overall mole fraction of the system both liquid and vapour phase. The vertical position of the tie-line represents the temperature of the system. According to the lever rule which was first developed for real levers but works here too , the length of the segment times the number of moles of the segment for one side equals the length times moles of the other side.

This makes sence if we look at the graph. If L 1 is shorter than L 2 as illustrated , then the overall composition of the system is closer to that of the liquid that to that of the vapour phase. That means most of the moles of material is in the liquid phase. Example: a closed system containing two volatile miscible liquids A and B is allowed to reach equilibrium. The total number of moles of the system is 1. At equilibrium, 0. What is the ratio of lengths of the line segments L 1 and L 2 in a tie-line diagram as pictured above?

So, the ratio of the two line segment lengths will be 2. If we were to collect all the vapour above the liquid at the boiling point and then condense it we would have a liquid that was higher in the more volatile component than the starting material. If we then re-boil this liquid, we again increase the more volatile component in the resulting distillate. With repetitive steps of boiling, condensing, boiling again, we can eventually completely separate the two components. This would require, however an infinite number of steps.

We have a more complicated situation in the case of two liquids, A and B, that mix completely but where the strengths of the intermolecular forces differ significantly. There are two possibilities:. Since the intermolecular forces holding a liquid together determines the vapour pressure and hence the boiling point of a liquid , we can predict that in the former case 1 , the expected boiling point of the solution should be higher than that of either pure liquid while in the latter case 2 , the solution will boil at a lower temperature than the boiling point of either of the two pure liquids.

Consider a solution of benzene and ethanol. Benzene and ethanol are fully miscible but the intermolecular forces in the solution are less than that in the individual liquids. Since the forces holding the molecules are less, the energy temperature needed to break those forces are less.

Thus, we expect that there will be a minimum in the boiling point curve see the figure below. This is called an azeotropic mixture and the particular point on the boiling point curve is called the azeotrope.

A maximum boiling azeotrope happens when the intermolecular forces of the mixture are stronger than the individual liquids. This results in a mixture with a higher boiling point lower vapour pressure than the individual. In this case, vapour in equilibrium with the liquid have compositions away from the azeotropic mixture composition, towards pure liquid.

If you cool a solution sufficiently, it will freeze. Allowing for the freezing to occur slowly enough and the solid which crystallizes out will be pure. The temperature at which the solution starts freezing depends on the composition of the solution. Take, for example, a mixture of acetic acid and water. For the purposes of the following illustration, I wish to clarify the distinction between the word state and the word phase.

A state is one of three states, solid, liquid or gas. No distinction is given as to the material in that state. The intersection between the red-yellow border and the purple-yellow border represents the eutectic point. This represents the lowest melting point composition for this solution. For Acetic acid, water, that point is at a temperature of Below this temperature, any mixture of ice and acetic acid is solid.

What phase transitions will occur as the cooling process progresses. We use these properties in our every-day experiences, for instance, in the radiators of automobiles, we put a mixture of ethylene glycol and water. Even if it freezes, it will do so slowly, lowering the freezing point as it does and creating a slushy mixture rather than a single solid phase.

Thus, even at extremely cold temperatures such as those found in northern Canada, the radiator 'coolant' mixture will flow through the engine and not plug it up. Look at the diagram again. At first, the acetic acid dissolves into the water. As the proportion of acetic acid increases, we reach the point where the dashed line crosses into the purple region. Beyond the purple-yellow border, we would see crystals of acetic acid sitting in the bottom of the beaker.

The solution would be saturated at equilibrium and no matter how much more acetic acid solid we add to the beaker, there will be no further net increase in the amount that will dissolve.

However, if we raise the temperature to room temp, we would see the rest of the acetic acid dissolve as we cross back into the yellow region on the phase diagram. Hence, we can use the diagram to determine the solubility concentration at equilibrium of acetic acid in water for any particular temperature. All phase diagrams of this sort have the same features.

The pure liquids have characteristic melting points and the eutectic point represents the lowest melting point composition of the solution. In organic chemistry, we often use the properties of solutions to tell if we have properly separated out a desired compound. For example, in the synthesis experiments, which you do in the lab, you test the purity of the crystals you make by measuring their melting point. If your crystals melt at the correct temperature at a well-defined temperature, then your crystals are probably close to pure.

If, on the other hand, they melt over a large temperature range or well below the correct melting point, you can be sure that your crystals are not very pure. Sometimes, the components being mixed to form the solutions have melting points that are very different. Take for example, the mixing of water and a salt such as KCl.

The clarity observation allows us to confidently say that the apple juice is a single phase and therefore it is a solution whereas the milk is not a single phase and therefore, it is not a single solution.

Gas phase solutions are easily formed from any mixture of gases since the molecules of the gas so rarely interact with each other. In the liquid phase, the molecules are close enough that intermolecular forces become important. For example, hexane and heptane are two non-polar liquids. The intermolecular forces in each of these pure liquids are primarily dispersion forces, due to temporary dipoles. These are quite weak forces. However, the intermolecular forces that would exist between hexane and heptane would also be primarily dispersion in nature.

Hence, a liquid solution will form. The two liquids are said to be completely miscible in each other. If the forces of one of the molecules for its own kind is much greater than for the other a solution may not form. Take, for example, Water and hexane. Water is a polar molecule and in addition, it bonds to other water molecules with hydrogen bonds.

Those are two stronger and strongest of the intermolecular forces compared to dispersion forces. Hexane, on the other hand cannot get involved in either of these two types of interactions and so will not mix with the water.

These two liquids are said to be immiscible in each other. In the solid phase, not only are the intermolecular forces very well defined, but the crystals of solid form rigid arrangements of atoms whose spacing is quite regular. In order for a second type of molecule to fit, it must be of similar size and shape to the host molecules or atoms.

There are several common methods for reporting the composition of solutions that we are dealing with. The particular method we use depends largely on the use to which we will put it. In most relatively dilute solutions where we need quick, easy calculations that relate the number of moles in solution to the volume, we use molarity.

Concentration, in Molarity can be calculated as:. Be careful with equations. Students often mix up variable symbols used in equations with unit symbols used in the calculations.

This is a case in point. The equation here does not have the letter M as a variable. Upper case M is used as a variable elsewhere to represent molar mass so it should not be used in this equation to represent concentration. The variable C is used to represent concentration of any units and here, C M stands for concentration in molarity.

A sample of 0. What is the molar concentration of the solution? We can use the equation above to solve this with one caveat. The volume in the equation is supposed to be litres of solution but the volume given in this example is litres of the solvent. We cannot just use one volume in place of the other as a general rule. If we make the assumption that the change is negligible, i. We could have alternatively my actual preference simply figured out how to do this using dimensional analysis.

Since we know the final units of concentration we want are moles per litre, we simply divide the number of moles of solute by the volume of solution in Litres and presto! The one drawback to using molarity is that the volume of solvent is not necessarily the volume of solution and hence, we must measure the amount of solute before mixing but measure the volume of solution after mixing and then calculate.

Molarity concentrations are very useful for experiments where we are making volumetric measurements. Titrations are a prime example of an experiment where molarity is the most convenient unit to use. In a titration, we measure a volume of solution added from a burette and can quickly calculate the number of moles we have added.

In conclusion, I repeat: use dimensional analysis to figure out how to do this rather than memorizing these equations. In such cases, molarity may not be a useful unit set to use. An alternate unit set for concentration is molality. Molality is not a volumetric unit and would not be useful for situations where we need to measure volumes of liquid solutions.

However, it is very useful in situations where we simply need to create solutions of known concentrations.

The units of molality are moles of solute per kilogram of solvent. We use the shortcut of italicized lower case m for molal.

This set of units means we can quickly measure gravimetrically both the solute and the solvent, mix them together and get a solution with an easily calculated concentration in units of molality. Here, C m is the variable representing the concentration in molality lower case m n is the moles of solute, as it was in the definition of molarity and the variable m is the mass of solvent in kg.

Notice that the letter m is used in two ways here. As a variable, m represents the mass of the solvent in kg, but as a unit, m is the symbol for molality. What is the molal concentration of a solution formed by adding 0.

We need the number of moles, n , of the solute, oxalic acid. We can use the molar mass of oxalic acid to convert g to moles of oxalic acid. Scales like Molarity and molality are only useful in the case of relatively dilute solutions where one of the species is clearly the most abundant termed the solvent and the other is in relatively small proportions the solute.

Most of the concentration range of solutions is not accessible using this type of terminology. Which is the solute? Which is the solvent? For this concentration variable we use the Greek letter chi , not capital X , which is equivalent of our letter C.

Mole fraction of a component i in a mixture of multiple components I is the number of components is defined as. The sum of all mole fractions must always equal one,. In reality, we can never get this to occur but we can find solutions where the forces are very close to equal. One example of a mixture which forms nearly ideal solutions is hexane and heptane.

They are both non-polar and therefore, can interact only using dispersion-type intermolecular forces. Consider a mixture of hexane A and heptane B.

Since both of these liquids are volatile, we expect that the solution too will have a vapour pressure. The vapour will be made up of a mixture of the two gases. For Ideal solutions, we can determine the partial pressure component in a vapour in equilibrium with a solution as a function of the mole fraction of the liquid in the solution.

The Vapour that collects over the solution will have a composition that is not necessarily the same as that of the liquid. The more volatile component evaporates easier and so will have a higher mole fraction in the vapour phase than it does in the liquid phase. We can write. The vapour composition curve can be plotted as shown in the figure below.

It is really two plots, one the straight line is the Vapour pressure of solution versus Liquid composition x A and the other, the curved line is the same Vapour pressure of solution, but plotted as a function of Vapour composition y A. It could be thought of as pulling the Liquid line to the right towards the more volatile liquid A. Horizontal tie lines join the two curves such that for any given Vapour pressure the liquid composition x A and the the corresponding vapour composition y A can be determined as indicated by the arrows in the figures.

To do so would involve complicated pressure measuring devices, sealed rigid containers and constant temperature devices. We can much more easily do a measurement of temperature at fixed pressure say one bar as a function of mole fraction. We would thus get a plot of boiling point of solution as a function of mole fraction of the solution. To this, we can add a plot of the vapour composition. This curve can be calculated using concepts much like those discussed above for the constant temperature case.

The resulting curve seen below is shifted towards the higher vapour pressure component just as it was in the diagram above. In this case, since we already know that the vapour pressure is not a linear function of Temperature c.

However, for an ideal solution the curvature of the line is only slight. The diagram below is a blowup of the tie-line region of the previous figure; the blue line represents the liquid solution composition, the green line is the vapour composition.

The vertical axis is Temperature and the horizontal axis is Mole fraction of component A in a two-component mixture of A and B. The vertical purple line represents the overall mole fraction of the system both liquid and vapour phase. The vertical position of the tie-line represents the temperature of the system. According to the lever rule which was first developed for real levers but works here too , the length of the segment times the number of moles of the segment for one side equals the length times moles of the other side.

This makes sense if we look at the graph. On this basis solutions can be divied into following three types -. Solutions in which solvent is present is gasesous state are called Gasesous solution. Gaseous solutions can be divided into following three types on the basis of phases of solute and solvent. Solutions in which solute and solvent both are gases; are called Gas-gas Solutions.

For example - solution mixture of nitrogen and oxygen, solution mixture of carbon dioxide and nitrogen, solution mixture of carbon dioxide and oxygen, etc. Solutions, in which solute is in liquid state and solvent is in gaseous state, are called Liquid-Gas Solution.

For example - solution mixture of chloroform in nitrogen gas. Solutions, in which solute is in solid state and solvent is in gasesous state, are called Solid-Gas Solutions. For example - Solution mixture of camphor in nitrogen gas.