Introduction to Liquid State

Liquid state

Liquids can flow, they can acquire the shape of their container and their volume is fixed. It is on the basis of these properties that we say that a particular substance is a liquid. Various physical properties of liquids provide an insight into their structure. Some of the properties are:

• Liquids can be characterized by many properties.
• The volume of any given sample of liquid is fixed at a given temperature.
• The cohesive forces due to inter molecular force of attraction confine liquid to a definite volume.
• Liquids show very poor compressibility in compression to gases.
• Different liquids diffuse each other rapidly due to some freedom of motion of the liquid molecule. Without movement, the process of diffusion would not be possible.
• Liquids have their definite boiling points and freezing points.
• liquids have their definite surface tension and viscosity.
• Liquids have definite vapour pressures at a given temperature. The vapour pressure of the liquid having low boiling point greater than that having a high boiling point at a given temperature.

With the detail discussion of properties of liquids is here:

1. Volume: Liquids have definite volume. The intermolecular force is stronger than that of the gas. Hence, they do not expand to occupy all the available space i.e. the given mass of a liquid has a fixed volume.
2. Density: Liquid molecules are more densely packed than the gaseous molecules. So, the density of a liquid is greater than that of a gas under similar condition. The density of a liquid decreases with increase in temperature.
3. Compressibility: Liquids are less compressible than gases because there is a little free space to be compressed.
4. Evaporation: Molecules of a liquid are held by attractive force but a few molecules always have a tendency to escape from the surface which is known as evaporation. Hence, evaporation is defined as the process of conversion of liquid molecules into the vapour form from the surface of the liquid at the room temperature. During evaporation, the surface molecules, which have higher kinetic energy than the bulk molecules, escape, and the slow moving molecules are left behind. Thus, the average kinetic energy of the molecules left in the liquid state is lowered which causes cooling of the liquid. i.e., cooling is caused by evaporation. Evaporation depends on upon temperature, strength of the intermolecular force, the surface area of the container and the intermolecular hydrogen bond in the liquid molecules.
5. Vapour pressure: The pressure exerted by the vapours of the liquid in the equilibrium state with its liquid form at a given temperature is known as vapour pressure. At the equilibrium state, the concentration of gaseous molecules becomes constant.
   Vapour pressure of a liquid depends on upon:
   a) Nature of liquid
   b) Temperature
   c) Presence of impurities
   A plot of temperature units and vapour pressure of different liquids is given below:
   

   

When the temperature increases a lot of liquid molecules are changed into gaseous molecules and as a result, vapour pressure rises. Thus, it is concluded that vapour pressure of a liquid increases with increasing temperature.
6. Boiling point: The temperature at which vapour pressure of a liquid becomes equal to the atmospheric pressure is known as the boiling point. The boiling point of a liquid is higher at plains and sea level than at the higher altitudes because as the height of the earth's surface increases, atmospheric pressure will go on decreasing.
7. Surface tension: Due to an imbalance of one-sided down ward force, the surface molecules are under tension. This effect is known as surface tension. Thus, the surface tension is defined as force per unit length acting perpendicular to the tangential line on the surface.The unit of surface tension is dynes cm^-1. Surface tension governs the physical properties of a liquid. The following phenomena are the outcomes of the surface tension.

a) The liquid drops have the spherical shape: The effect of surface tension is to reduce the surface area of the liquid to the minimum. Since a sphere has the minimum surface area for a given volume of the liquid, the inward pull or the surface tension is responsible for the spherical shape of the liquid drops.

b) The rise or fall of a liquid in a capillary tube: If a capillary tube is dipped into a liquid, the liquid rises into the capillary tube to a certain height. This phenomenon takes place because the adhesive force ( force of attraction between polar water and glass molecules) is greater than the cohesive force of liquid ( force of attraction between the liquid molecules) i.e. surface tension. The rising stops when this adhesive force is balanced by the weight of the meniscus upward.

On the other hand water droplets over a waxed paper take a spherical shape since surface tension ( or cohesive force) dominates the adhesive force between the polar water molecules and non-polar wax molecules. If a capillary tube is dipped into mercury, a cohesive force of mercury is greater than the adhesive force between glass and mercury. When water rises in plant bodies from the root or stem.

c) The efficiency of tooth paste: The tooth pastes and mouth washes consist of the substances which lower the surface tension. Due to this, the pastes spread over the surface they come in contact with. This increases their efficiency.

d) Cleansing action of soaps and detergents: The soaps and detergents lower the interfacial tension between water and grease or dirt. This facilities the mixing of water and dirt. Consequently, dirt can easily be removed.

Effect of Temperature on Surface Tension

The surface tension of a liquid decreases with rising in temperature because surface tension depends on upon the strength of the intermolecular force of liquids. Intermolecular force weakens when the temperature is increased. So, the surface tension of hot water is less than that of cold water. This is why the clothes are washed more efficiently in hot water than in cold water.

8. Viscosity: Viscosity is defined as the internal resistance to the flow of a liquid which one layer offers to another layer trying to pass over it. The liquids flowing slowly ( e.g. glycerine, honey) are said to have higher viscosity i.e. are viscous whereas those flowing easily (e.g. water, alcohol) are said to have low viscosity i.e. are less viscous. The coefficient of viscosity (η) is defined as the force in dyne per square cm required to maintain a difference of velocity of 1 cm / sec (unit velocity) between two parallel layers of the liquid held at a distance of 1 cm apart.
The unit of viscosity is poise (P)
1P = 1 dyne cm^-2 sec

Viscosity depends on:
(a) The nature of liquid: Viscosity is related to the intermolecular force in the liquid. If the intermolecular force is large, the viscosity will be high. For example, glycerine is viscous liquid because its intermolecular forces are strong due to hydrogen bonding.

(b) Temperature: Viscosity of a liquid decreases with increases in temperature because the kinetic energy of the molecules will increase at high temperature which overcomes the intermolecular forces.

Solution

A solution is defined as a perfectly homogeneous mixture of two mixed components. The two components of a solution are called solute and solvent.

Solute: The component of the solution which gets dissolved in another substance is called solute.

Solvent: The component of the solution which dissolves the solute is called solvent i.e., the solvent is the media for the solute to be dissolved. For example, when glucose is added to water, a solution is formed in which water is the solvent and glucose is the solute.

Types of Solution

a) Unsaturated solution: The strength of a solute in the solution is called concentration. If the concentration of the solution can be changed by adding solute at the same temperature, the solution is called unsaturated solution. In this type of solution, more solute can be dissolved at the temperature

b) Saturated solution: The solution whose concentration can't be changed by adding extra solute at the same temperature and pressure is called saturated solution. Such type of solution can dissolve no more solute at a temperature.

c) Super saturated solution: When the saturated solution prepared at higher temperature is allowed to cool, the dissolved solute particles will appear. The solution is called super saturated solution. On adding more solute to a saturated solution, the precipitation occurs so that the remaining solution will be saturated at the given temperature.

Strength of Solution

The amount of solute present in the solution determines the strength of the solution. Strength of solutions can be expressed in any of the following terms:

1. Percentage by weight: The weight of a solute in gram which has been dissolved in 100 gram of the solution is called percentage by weight of the solute.
   $$Percentage\; by\; weight =\frac{weight\; of\; solute}{weight\; of\; solution}\times100$$
2. Mole fraction: The mole fraction of a solution indicates the fraction of moles of solute / solvent in the solution. It is defined as the ratio of the number of the moles of solute / solvent and some of the number of moles of solute and solvent present in the solution.
   Let n is the number of solute and N be the number of moles of solvent.
   Mole fraction of solute = \(\frac{n}{n+N}\)
   Mole fraction of solvent = \(\frac{N}{n+N}\)
3. Normality: The equivalent weight of a substance expressed in gram is called gram equivalent. Normality of a solution is defined as the number of gram equivalent of a solute present in one liter of its solution. It is denoted by N.
   $$Normality = \frac{Number\; of\; gram\; equivalents\; of\; solute}{Volume\; of\; solution\; in\; liter}$$
   $$i.e., Normality = \frac{Weight\; of\; solute\; in\; gram}{Equivalent\; weight\; of\; solute}\times\frac{1000}{Volume\; of\; solution\; in\; ml}$$
   A solution having 1 gram equivalent of the dissolved solute in 1 liter of its solution is called unit normal solution. For example, if 53 gram of Na₂CO₃is dissolved in 1 liter of its solution, then the solution is the normal solution i.e., its normality is 1N.
   The solution in which \(\frac{1}{10}\)the gram equivalent of a solute is dissolved per liter of its solution is called decinormal solution i.e., the solution will have \(\frac{N}{10}\) strength.
4. Molarity: The molecular weight of substances expressed in term of the gram is called gram mole or simply, mole. The number of moles of a solute present in one liter of its solution is called molarity of the solution. It is denoted by M.
   $$Molarity = \frac{Number\; of\; gram\; moles\; of\; solute}{Volume\; of\; solution\; in\; liter}$$
   $$i.e., Molarity = \frac{Weight\; of\; solute\; in\; gram}{Molecular\; weight\; of\; solute} \times\frac{1000}{Volume\; of\; solution\; in\; ml}$$
   A solution having 1 gm mole of the dissolved solute in 1 liter of its solution is called unit molar solution i.e., the strength of such solution will be 1M.
5. Molality: The number of moles of solute present in one kilogram of solvent is called molality of the solution. It is denoted by m.
   $$Molality = \frac{Number\; of\; gm\; moles\; of\; solute}{Weight\; of\; solvent\; in\; gm}\times1000$$
   A solution having 1 gm mole of the dissolved solute in 1 kg of the solvent is called molal solution and the strength of such solution will be 1m.
6. Gram per liter: The number of grams of solute present in one liter of its solution is called gram per liter.
   $$Gram/ liter = \frac{Weight \;of \;solute\; in\; gm}{Volume\; of\; solution \;in\; ml}\times1000$$
   $$Gram / liter = Normality \times {equivalent\; weight = Molarity\times{ molecular\; weight}}$$
7. Percentage by volume: This is the mass of solute in gm dissolve in 100 ml of the solution.
   $$Percentage\; by\; volume = \frac{Weight\; of\; solute\; in\; gm}{Volume \;of\; solution\; in\; ml}\times 100$$

Solubility

The maximum amount of solute in gram that can be dissolved in 100 gram of the solvent to make a saturated solution at a temperature is called the solubility of the solute at that temperature.

i.e.,$$ Solubility =\frac{Weight \;of\; solute\; in\; gm}{Weight \;of \;solvent\; in \;gm} \times 100$$

Effect of Temperature on Solubility: Solubility Curve

Solubility curve is defined as the curve obtained by plotting the solubility of a substance at a different temperatures against these temperatures. While plotting the solubility curve, temperatures are taken in the X-axis and solubilities are taken in the Y-axis.

Classification of Solubility curves
The nature of the solubility curve depends on the nature of a salt. There are two types of solubility curves.

1. Continuous solubility curve: The solubility curve which shows regular increase or decrease in the solubilities with temperature change is known as continuous solubility curve. These types of curves are given by anhydrous salts, acids and bases like NaCl, KNO₃, KCl, NaNO₃, HCl, NaOH etc.

2. Discontinuous solubility curve: The solubility curve which shows that the solubility increases or decreases irregularly with temperature is called discontinuous solubility curve. These types of curves are given by hydrated salts like CuSO₄.5H₂O, CaCl₂.6H₂O, Na₂CO₃.10H₂O etc. In the case of sodium sulphate, its solubility increases up to 32.4°C. After this temperature, the solubility decreases. At this temperature, there is a break in the curve. This temperature is the transition temperature at which the deca-hydrated phase changes into the anhydrous phase.

At the transition temperature, the solubility of a solute is maximum. If the saturated solution at the transition temperature is cooled, then the hydrated solute gets crystallized out. If it is heated, then the anhydrous form will separate out.

The formation of the discontinuous curve from hydrated salts is due to the change of hydrated phase of the compound when the temperature is varied.

Information Obtained from Solubility Curve

From the solubility curves, the following information can be obtained.

• It explains the effect of temperature on the solubility of the substance.
• It indicates the solubility of a substance at a particular temperature.
• It informs about the nature of substance wether it is hydrous or anhydrous.
• It helps in comparing the solubilities of different substances.
• It shows a change in the composition of a solute.

Bibliography:

Acharya, Sitaram and Pradyuman Wagley. Principles of Chemistry. Second edition. Kathmandu: Buddha Academic Publishers and Distributors Pvt. Ltd., 2006 year.