In this article, we shall study two types of solutions first, those obeying Raoult’s law called ideal solutions and those not obeying Raoult’s law called non-ideal solutions.

Ideal Solutions:

The solutions which obey Raoult’s law over the entire range of concentration are known as ideal solutions.

The ideal solutions have two important properties. The enthalpy of mixing of the pure components to form the solution is zero and the volume of mixing is also zero, i.e. ΔmixH = 0 and ΔmixV = 0. It means that no heat is absorbed or evolved when the components are mixed. Also, the volume of solution would be equal to the sum of volumes of the two components.

At the molecular level, ideal behaviour of the solutions can be explained by considering two components A and B. In pure components, the intermolecular attractive interactions will be of types A-A and B-B, whereas in the binary solutions in addition to these two interactions, A-B type of interactions will also be present.

If the intermolecular attractive forces between the A-A and B-B are nearly equal to those between A-B, this leads to the formation of an ideal solution.

A perfectly ideal solution is rare but some solutions are nearly ideal in behaviour. The solution of n-hexane and n-heptane, bromoethane and chloroethane, benzene and toluene, chlorobenzene and bromobenzene etc. fall into this category. Most of the dilute solutions behave as ideal solutions.

Notes:

The process of separation of one liquid from another liquid (binary mixture) having different boiling points by distillation is called fractional distillation. The separation is possible when the vapour phase has a different composition from that boiling liquid mixture. Thus the components of an ideal solution can be separated by fractional distillation.

Examples of ideal solutions:

• All dilute solutions
• benzene + toluene
• n-hexane + n-heptane
• chlorobenzene + bromobenzene
• ethyl bromide + ethyl iodide
• n-butyl chloride + n-butyl bromide

Non-ideal Solutions:

When a solution does not obey Raoult’s law over the entire range of concentration, then it is called a non-ideal solution.

The vapour pressure of such a solution is either higher or lower than that predicted by Raoult’s law. If it is higher, the solution exhibits a positive deviation and if it is lower, it exhibits a negative deviation from Raoult’s law.

Positive Deviation:

The cause for these deviations lies in the nature of interactions at the molecular level. In case of positive deviation from Raoult’s law, A-B interactions are weaker than those between A-A or B-B, i.e.

In this case, the intermolecular attractive forces between the solute-solvent molecules are weaker than those between the solute-solute and solvent-solvent molecules. This means that in such solutions, molecules of A (or B) will find it easier to escape than in the pure state. This will increase the vapour pressure and result in positive deviation.

Mixtures of ethanol and acetone behave in this manner. In pure ethanol, molecules are hydrogen-bonded. On adding acetone, its molecules get in between the host molecules and break some of the hydrogen bonds between them. Due to the weakening of interactions, the solution shows a positive deviation from Raoult’s law.

In a solution formed by adding carbon disulphide to acetone, the dipolar interactions between solute-solvent molecules are weaker than the respective interactions among the solute-solute and solvent-solvent molecules. This solution also shows positive deviation.

For solution showing positive deviation from Rault’s law ΔmixH > 0 and ΔmixV > 0,

Examples of solutions showing positive deviation from Raoult’s law

• acetone + ethanol
• acetone + CS2
• water + methanol
• water + ethanol
• CCl4 + toluene
• CCl4 + CHCl3
• acetone + benzene
• CCl4+ CH3OH
• cyclohexane + methanol

Negative Deviation:

In the case of negative deviations from Raoult’s law, the intermolecular attractive forces between A-A and B-B are weaker than those between A-B and leads to decrease in vapour pressure resulting in negative deviations.

An example of this type is a mixture of phenol and aniline. In this case, the intermolecular hydrogen bonding between phenolic proton and lone pair on the nitrogen atom of aniline is stronger than the respective intermolecular hydrogen bonding between similar molecules. Similarly, a mixture of chloroform and acetone forms a solution with a negative deviation from Raoult’s law. This is because chloroform molecule is able to form a hydrogen bond with acetone molecule as shown.

This decreases the escaping tendency of molecules for each component and consequently the vapour pressure decreases resulting in a negative deviation from Raoult’s law. This decreases the escaping tendency of molecules for each component and consequently the vapour pressure decreases resulting in a negative deviation from Raoult’s law.

For solution showing negative deviation from Rault’s law ΔmixH < 0 and ΔmixV < 0,

Examples of solutions showing negative deviation from Raoult’s law

• acetone + aniline
• acetone + chloroform
• methanol + acetic acid
• water + nitric acid
• chloroform + diethyl ether
• water + HCl
• acetic acid + pyridine
• chloroform + benzene

Azeotropes:

Azeotropes are binary mixtures having the same composition in the liquid and vapour phase and boil at a constant temperature. Some liquids on mixing, form azeotropes. In such cases, it is not possible to separate the components by fractional distillation. There are two types of azeotropes called minimum boiling azeotrope and maximum boiling azeotrope.

The solutions which show a large positive deviation from Raoult’s law form minimum boiling azeotrope at a specific composition. For example, ethanol-water mixture (obtained by fermentation of sugars) on fractional distillation gives a solution containing approximately 95% by volume of ethanol. Once this composition, known as azeotrope composition, has been achieved, the liquid and vapour have the same composition, and no further separation occurs.

The solutions that show large negative deviation from Raoult’s law form maximum boiling azeotrope at a specific composition. Nitric acid and water is an example of this class of azeotrope. This azeotrope has the approximate composition, 68% nitric acid and 32% water by mass, with a boiling point of 393.5 K.

Related Topics

Solutions and Their Colligative Properties

• Solutions and Their Types
• Solutions of Solids and Liquids
• Concentration of Solution
• Numerical Problems on Percentage by Mass and Volume
• Numerical Problems on Mole Fraction
• Numerical Problems on Molarity
• Numerical Problems on Molality
• Short Cuts For Above Numerical Problems
• Solutions of Gases in Liquid
• Lowering of Vapour Pressure
• Numerical Problems on Lowering of Vapour Pressure
• Elevation in Boiling Point and Depression in Freezing Point
• Osmosis and Osmotic Pressure

For More Topics of Chemistry Click Here

The post Ideal and Non-Ideal solutions appeared first on The Fact Factor.

In this article, we shall study solid and liquid solutions and the concept of solubility curves.

A solution of Solids in Liquids: 

The dissolution of any substance in a liquid to form a solution is governed by the basic principle that the solute-solvent interaction is either similar to or greater than solute-solute and solvent-solvent interaction. Let us consider the dissolution of an ionic solid in water. There is an interionic attraction between the ions of solid. In this case, the cations and anions of the solid get attracted by the opposite ends of water dipoles.

If the ion-dipole interaction forces are stronger than interionic attraction, the ions are pulled out of crystal lattice and they pass into solution. Formed ions are called hydrated ions.

Molecular solids dissolve in the water on account of their capacity to form hydrogen bonds with water molecules.

The energy required to dismantle 1 mol of the crystal lattice is called lattice energy (Δ_LH). The energy released during hydration is called hydration energy (Δ_HydroH). Mathematically, 

Δ_SolH = Δ_LH + Δ_HydroH.

• Case – I:  Δ_HydroH ≥ Δ_LH:Dissolution occurs and Δ_SolH < 0
• Case – II:  Δ_HydroH marginally less than Δ_LH: Dissolution occurs and Δ_SolH > 0
• Case – III:  Δ_LH too large to compensate Δ_HydroH: Dissolution does not occur.

Factors Affecting Solubility of a Solids in Liquids:

• Nature of solutes: Polar solvent (e.g. water) dissolves polar solute (e.g. NaCl, KCl) and non-polar solvents (CCl₄, CS₂) dissolve non-polar solutes (e.g. I₂, S₈).
• Temperature: The solubility of a solid in a liquid is significantly affected by temperature changes. Consider the equilibrium of solute particles dissolving and crystallizing.

Solute + Solvent ⇌ Solution

It is a dynamic equilibrium and follows Le Chatelier,s Principle. If in a nearly saturated solution, the dissolution process is endothermic (Δ_solH > 0), the solubility should increase with the rise in temperature if it is exothermic (Δ_solH < 0) the solubility should decrease.

• Pressure: Pressure does not have any significant effect on so the ability of solids in liquids. It is so because solids and liquids are highly incompressible and practically remain unaffected by changes in pressure.

A solution of Solids in Solids: 

A solid solution of two or more metals or metals or metals with one or more non-metal is called an alloy.

Types of Solid Solutions:

• Substitutional: Solute
  atoms occupy the regular lattice sites of the parent metal (solvent).
  Substitutional solid solutions can be random (Cu-Ni) or ordered (Cu-Au).
• Interstitial: Solute
  atoms occupy the interstitial positions (Steel – C solute atoms in Fe).

Examples of Alloys:

Amalgams:

Alloys of mercury with other metals are called amalgams. This property of mercury is used for extraction of metals like silver and gold from their ores. These metals are further recovered by distillation of mercury.

A solution of Liquids in Liquids:

Evolution or Absorption of Heat During Formation of Solution:

When one liquid is dissolved in another, two processes should take place simultaneously. In the first, the solvent molecules move apart to accommodate the solute molecules while in the second, the solute molecules separate to get accommodated in the space provided by the solvent. For this process energy and absorbed. At the end of the formation of the solution, the solute and solvent molecules are brought together. This results in the release of energy. The net of the two energies gives the enthalpy of solution.

If there is a strong attraction between the solute and the solvent molecules, the heat released is more than the heat absorbed and more energy is released in the final step. Thus with the concept of energy, there are thee possibilities.

• Case – I: There is the absorption of heat (e.g. formation of a
  solution of ethyl alcohol and water).
• Case – II: There is an evolution of heat (e.g. formation of a solution
  of acetone and water)
• Case – III: There is neither absorption nor evolution of heat (e.g.
  formation of a solution of benzene and carbon tetrachloride)

Effect of Relative Solubility of Solute in Solution:

When two liquids are mixed, there are three possibilities.

• Case – I: The two components are almost immiscible. It happens when
  one liquid is polar and other in non-polar. e.g. a solution of water and
  benzene, a solution of carbon tetrachloride and water, a solution of
  benzene and alcohol.
• Case – II: The miscibility of the component may be partial. This
  happens when the intermolecular attraction of one liquid is different from the
  intermolecular attraction of other liquid. e.g. a solution of water and
  ether, a solution of phenol and water, a solution of nicotine and
  water.
• Case – III: The two components may be completely miscible. It happens
  when the two liquids have the same nature. Either both are polar (e.g. a
  solution of alcohol and water) or both are non-polar (e.g. a solution of
  benzene and hexane, a solution of benzene and toluene).

Characteristics of miscible liquids:

• They are chemically alike.
• Both have the same nature.
• Dipole-dipole interaction is stronger.
• Molecular sizes of liquids are approximately the same.

Solubility Curves:

Solubility is measured by determining the maximum mass of a solute that can be dissolved in 100 g of a solvent at a given temperature. A solubility curve is a graph giving the relationship between the temperature and solubility of a particular solute. Thus the solubility curves give the idea about what mass of solute will dissolve in 100g (or 100mL) of water over a range of temperatures.

Solutes whose solubility curves move upward with increased temperature are typically solids because the solubility of solids increases with increased temperature. The steeper the incline of a solute, the more soluble the solute is because it doesn’t take as much of a temperature increase to dissolve the substance. This also means that more of the solute can be dissolved versus another substance at the same temperature. From the solubility curve, we can see that the solubility of KNO₃ rises sharply with the increase in temperature. In solutions involving liquids and solids typically more solute can be dissolved at higher temperatures.

Solutes whose solubility curves move downward with increased temperature are typically gases because the solubility of gases decreases with increased temperature.

The flatter the line, the less soluble the solute it, because it takes a larger temperature in order for a solute to dissolve compared to a
steeper inclined substance. The solubility of NaCl almost remains constant.

Related Topics

Solutions and Their Colligative Properties

• Solutions and Their Types
• Concentration of Solution
• Numerical Problems on Percentage by Mass and Volume
• Numerical Problems on Mole Fraction
• Numerical Problems on Molarity
• Numerical Problems on Molality
• Short Cuts For Above Numerical Problems
• Solutions of Gases in Liquid
• Ideal and Non-ideal Solutions
• Lowering of Vapour Pressure
• Numerical Problems on Lowering of Vapour Pressure
• Elevation in Boiling Point and Depression in Freezing Point
• Osmosis and Osmotic Pressure

For More Topics of Chemistry Click Here

The post Solid and Liquid Solutions appeared first on The Fact Factor.

In this article, we shall study the concept of solutions and their types based on phases, composition, etc.

The Terminology of Solutions:

• Solution: A solution is a homogeneous mixture of two or more than two or more components. A solution has a single phase. The constituent particles of solution can not be separated by filtration, settling or centrifugal action.
• Solvent: The component of a solution which dissolves the other component in itself is called a solvent. It is the larger component of the solution. For example, a solution of sugar in water is solid in the liquid. Here, sugar is the solute and water is the solvent.
• Solute: The component of the solution which dissolves in the solvent is called the solute. The solute is the smaller component of the solution. For example, a solution of sugar in water is solid in the liquid. Here, sugar is the solute and water is the solvent.
• Homogeneous Solution: If the composition and properties are uniform throughout the mixture, the solution is called a homogeneous solution.
• If the solution consists of two chemical components, the solution is called a binary solution. If it contains three or four chemical components it is called a ternary or quaternary solution.
• Soluble Substance: A substance that dissolves in a solvent is said to be soluble in that solvent. A soluble substance is able to dissolve in a solvent because attractive forces between the solvent and solute particles are strong enough to overcome the attractive forces holding the solute particles together.
• Insoluble Substance: A substance that does not dissolve in a solvent is called insoluble in that solvent.
• Miscible Liquids: Two liquids that are soluble in each other are said to be miscible such as water and vinegar, coffee and cream
• Immiscible Liquids: Liquids that are not soluble in each other are immiscible such as vegetable oil and vinegar, petrol and water.
• The Process of Dissolving: Solvent particles surround solute particles to form a solution in a process called solvation. If the solvent is water the process is known as hydration. During this process, there is a change in energy observed by an increase or decrease in temperature.
• Like dissolve Like: Polar dissolves polar & nonpolar dissolves nonpolar.
• Aqueous Solution: Solutions having water as a solvent are called aqueous solutions.

The concept of Solubility:

When a substance (solute) gets dissolved in a liquid (solvent) to form a solution it means solute-solvent interaction is greater than solute-solute. The solubility of a substance is its maximum amount that can be dissolved in a specified amount of solvent. It depends upon the nature of the solute and solvent as well as temperature and pressure.

Let us consider the effect of these factors in a solution of a solid or a gas in a liquid. When a solid solute is added to the solvent, some solute dissolves and its concentration increases in solution. This process is known as dissolution. Some solute particles in solution collide with the solid solute particles and get separated out of solution. This process is known as crystallization. A stage is reached when the two processes occur at the same rate. Under such conditions, the number of solute particles going into the solution will be equal to the solute particles separating out and a state of dynamic equilibrium is reached. At this stage, the concentration of solute in solution will remain constant under the given conditions, i.e., temperature and pressure. A similar process is followed when gases are dissolved in liquid solvents.

Such a solution in which no more solute can be dissolved at the same temperature and pressure is called a saturated solution. An unsaturated solution is one in which more solute can be dissolved at the same temperature. The maximum amount of solute that dissolves completely in a given amount of solvent at constant temperature is called the solubility of the solute.

Factors that affect solubility:

• The solubility of a solid in liquid increases with the increase in temperature. Whereas, the solubility of a gas in the liquid decreases with the increase in temperature.
• A particular volume of solvent can dissolve some maximum amount of solute. Thus the rate of dissolving decreases with the increase in the amount of solute.
• Stirring or agitating solution increases the rate of dissolving. Agitation increases the rate by bringing fresh solvent into contact with more solute
• Dissolving occurs at the surface of a solid, by increasing the surface area we can increase the rate of dissolving.

TYpes of Solutions:

On the Basis of Size of Solute Particles:

Depending upon the size of solute particles, the solutions are broadly classified into three types.

• Corse Solution: When the size of constituent particles is relatively bigger, then the solution is called a coarse solution e.g. a mixture of salt and sugar.
• Colloidal Solution: A colloidal solution is formed when the size of the particles dispersed in the solvent is in the range of 10^-7 cm to 10^-4 cm. The colloidal particles carry a charge of the same nature, which is important for the stabilization of the solution e.g. ferric hydroxide sol
• True Solution: A true solution is defined as a homogeneous mixture of two or more substances, the composition of which is not fixed and may be varied within a certain limit. The size of the particles dispersed in the solvent is less than 10^-4 cm. e.g. Solution of common salt in water.

Characteristics of True Solutions

• A solution is a
  homogeneous mixture
• The size of
  solute particles in the solutions is extremely small. It is less than 1 nm in
  diameter.
• The particles of
  a solution cannot be seen even with a microscope.
• The particles of
  a solution pass through the filter paper. Thus filtration cannot separate the
  solution.
• It is very
  stable. The particles of solute present in a solution do not separate out on
  keeping.
• A true solution
  does not scatter light (because its particles are very small).

On the Basis of Phases of Solute and Solvent:

Depending upon the phases of solute and solvent the solutions are broadly classified into three types.

Gaseous Solutions:

• Solute phase: solid, solvent phase: gas, Also known as solid in gas solutions e.g. iodine in air
• Solute phase: liquid, solvent phase: gas, Also known as a liquid in gas solutions e.g. Chloroform in nitrogen
• Solute phase: gas, solvent phase: gas, also known as gas in gas solutions e.g. Mixture of non-reacting gases (O₂+ N₂)

Liquid Solutions:

• Solute phase: solid, solvent phase: liquid, Also known as solid in liquid solutions e.g. sugar in water
• Solute phase: liquid, solvent phase: liquid, Also known as a liquid in liquid solutions e.g. ethanol in water
• Solute phase: gas, solvent phase: liquid, Also known as gas in liquid solutions e.g. carbon dioxide in water

Solid Solutions:

• Solute phase: solid, solvent phase: solid, Also known as solid in solid solutions e.g. Alloys like brass, bronze
• Solute phase: liquid, solvent phase: solid, Also known as a liquid in solid solutions e.g. Amalgam of mercury with metal
• Solute phase: gas, solvent phase: solid, Also known as gas in solid solutions e.g. pumice stone, H₂ gas in palladium

Sometimes solutions of liquid in gas and solid in a gas are not considered as solutions because the mixture may not be homogeneous.

On the Basis of Concentrations:

• Saturated solution: A saturated solution is defined as the solution that contains a just amount of dissolved solute necessary for establishing equilibrium between dissolved solute and undissolved solids.
• Unsaturated solution: An unsaturated solution is defined as a solution in which more solute can be dissolved at the same temperature.
• Supersaturated solution: A supersaturated solution is defined as a solution in which excess solute is dissolved than required for the formation of a saturated solution

Related Topics

Solutions and Their Colligative Properties

• Solutions and Their Types
• Solutions of Solids and Liquids
• Concentration of Solution
• Numerical Problems on Percentage by Mass and Volume
• Numerical Problems on Mole Fraction
• Numerical Problems on Molarity
• Numerical Problems on Molality
• Short Cuts For Above Numerical Problems
• Solutions of Gases in Liquid
• Ideal and Non-ideal Solutions
• Lowering of Vapour Pressure
• Numerical Problems on Lowering of Vapour Pressure
• Elevation in Boiling Point and Depression in Freezing Point
• Osmosis and Osmotic Pressure

For More Topics of Chemistry Click Here

The post Solutions and Their Types appeared first on The Fact Factor.