Subject: Chemistry
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:
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.
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.
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.
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:
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.,$$ Solubility =\frac{Weight \;of\; solute\; in\; gm}{Weight \;of \;solvent\; in \;gm} \times 100$$
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.
From the solubility curves, the following information can be obtained.
Bibliography:
Acharya, Sitaram and Pradyuman Wagley. Principles of Chemistry. Second edition. Kathmandu: Buddha Academic Publishers and Distributors Pvt. Ltd., 2006 year.
i.e., Molarity = \(\frac{Weight of solute in gram}{Molecular weight of solute}\) \;times \(\frac{1000}{Volume of solution in ml}\)
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