In this post we are discussing about solutions, colloids and suspensions for class 9.
Solutions
A lump of sugar when placed in water disappears and the water becomes sweet. Where is sugar gone? Why does the water taste sweet? The sugar placed in water gets dissolved in it but remain unchanged. This is an example of a solution. Sharbat is a solution of sugar in water. Sea water is another example of a solution.
What is a solution?
Solution may be defined as a homogeneous mixture of two or more substances is called a solution.
When a small amount of sodium chloride is dissolved in water, a homogeneous mixture so obtained is called a solution of sodium chloride in water.
The solution of sodium chloride in water is homogeneous because the properties of the solution are the same throughout the solution. This means that if we take out 1 ml samples from different parts of the solution, it will have the same degree of salty taste and the same concentration.
It means all 1 ml samples of this solution will contain the same number of sodium chloride in water molecules.
Some other common solutions are:
(i) Sugar solution
(ii) Copper sulphate solution
(iii) Vinegar
What is true solution?
A homogeneous mixture of two or more miscible liquids is also called a solution, e.g., ethanol dissolves in water to give a solution of ethanol in water.
Aqueous solutions of low molecular mass solutes are true solutions. A solution in which size of the solute particles is about 10-10 m, is called a true solution.
In a true solution, the solute particles and the solvent molecules cannot be distinguished even under a microscope.
In a true solution, the solute particles are trapped into the spaces between the solvent molecules. For example, solutions of sodium chloride, copper sulphate, glucose, etc. in water are true solutions .
Homogeneous solution of a solid into another solid is called as solid solution. Common metal alloys are solid solutions. The gold used in making jewellery is a solid solution containing gold and small quantities of copper and silver.
What is meant by solute and solvent?
A solution is obtained when a substance is dissolved into another substance. The substance which is dissolved to obtain a solution is called solute, whereas the substance in which the solute is dissolved is called solvent.
For example, when salt is dissolved in water to obtain the salt solution, then salt is the solute and water is the solvent.
Generally, the component present in larger amount in a solution is called solvent whereas the component present in smaller amount is called the solute.
Most commonly, solutes are solids and solvents are liquids. However, solutes may also be liquids and gases. In some solutions, solvent may be a gas or a solid.
For example,
| Solution | Name of solvent | Phase of solvent | Name of solute | Phase of solute |
|---|---|---|---|---|
| Aqueous solution of sugar | Water | Liquid | Sugar | Solid |
| Aqueous solutions of urea | Water | Liquid | Urea | Solid |
| Aqueous solution of ammonium chloride | Water | Liquid | Ammonium chloride | Solid |
| Aqueous solution of ethyl alcohol | Water | Liquid | Ethyl chloride | Liquid |
| Soda water | Water | Liquid | Carbon dioxide | Gas |
What are aqueous solutions?
Solutions in which water is the solvent are called aqueous solutions. For example when sugar is dissolved in water the solution obtained is called an aqueous solution of sugar.
An aqueous solution of any solute is described by writing the word ‘aq’ immediately after the name of the solute, thus the aqueous solution of sugar is described by sugar (aq).
What are non aqueous solutions?
The solution obtained by dissolving a solute in any solvent other than water are non aqueous solution . Non-aqueous means without water.
For example, if a solute is dissolved in any solvent such as benzene, ethyl alcohol, ether, carbon
disulphide, carbon tetrachloride, acetone, chloroform, etc, then the solution obtained is called non aqueous solution.
Some commonly used non aqueous solutions are:
(i) solution of sulphur in carbon disulphide
(ii) solution of iodine in carbon tetrachloride
(iii) solution of iodine in alcohol, i.e., tincture of iodine.
Why is water called as universal solvent?
Water can dissolve a wide variety of substances, that’s why, water is called universal solvent. Plants use this property of water for taking minerals from the soil.
What are the characteristics of a true solution?
A true solution shows the following characteristics:
(i) A true solution is a homogeneous mixture, i.e., in a true solution the solute and solvent molecules cannot be distinguished even under a microscope. The composition and properties of a true solution are the same throughout.
(ii) In a true solution the solute particles are very small of the order of about 10-10 m.
(iii) A true solution is clear and transparent.
(iv) A true solution does not scatter light.
(v) The components of a true solution, the solute and the solvent, cannot be separated by filtration because both solute particles and the solvent molecules are very small as compared to the pores in the filter paper.
(vi) The solute particles in a true solution do not settle down.
What are the advantages of preparing/using solutions?
In a solution, the solute and the solvent may be present in the form of molecules or ions or both. Solutions are prepared for the following purpose:
(a) For carrying out reactions: For a reaction to take place, the reacting molecule or ion must come closer to each other. In solutions, the reactants can come closer easily. So reactions in solution take place at a faster rate because the reactants in solution are available in the form of molecules or ions.
(b) For dissolving medicines: Certain medicines are given to patients in the form of their solution in suitable aqueous or non-aqueous solvents such as alcohol. For example, saline glucose solution contains glucose and sodium chloride in water. This solution is administered to a patient suffering from dehydration.
Concentration of a solution
In chemistry, most reactions are carried out in solutions. In order to perform calculations, it is necessary to know concentrations of the solutions.
The quantity of solute dissolved in a given mass or volume of the solution or the solvent is called concentration of the solution.
How is the concentration of a solution described?
There are two simple ways to express the concentration of a solution which are:
(a) Mass by mass percent
(b) Mass by volume percent
Mass by mass percent (w/w):
The mass of solute in grams per 100 grams of the solution is called mass percent of solute in the solution.
For example, a 7% (w/w) solution of sugar means that 7 g of sugar is present in 100 g of solution.
Mathematically, mass percent of solute in solution is expressed as:
Both the mass of solute and solution must be expressed in same units, i.e., either in grams or kilograms.
When a solute is present in trace (very small) quantity, then the concentration is expressed in terms of ppm (parts per million).
A solution containing 5 g of solute in 106 grams of a solution is said to have a concentration of 5 ppm. It is also expressed in volume ratio for composition of gaseous mixtures, as 10 ml in 106 ml corresponds to 10 ppm.
Mass by volume percent (w/v):
The mass of solute in grams per 100 ml of the solution is called mass by volume percent of solute in solution.
For example, a 7% (w/v) solution of sugar means that 7 g of sugar is present in 100 ml of solution.
Mathematically, mass by volume percent of solute in solution is expressed as:
The unit of mass by volume percent must be expressed in g/mL or Kg/L.
Question on concentration of solution
Q. A solution is prepared by dissolving 30 g of sodium chloride in 250 g of water. What is the mass by mass percent of sodium chloride in the solution. Also find the mass by volume percent of sodium chloride in the solution, if the density of solution given is 1.12 g/ml.
Sol: Mass of solute (sodium chloride) = 30 g
Mass of solvent (water) = 250 g
Mass of solution = Mass of solute + Mass of solvent = 30 g + 250 g = 280 g
Mass by mass percent = (30 g/280 g)*100 = 10.71%
Volume of solution = Mass of solution/Density of solution = (280 g)/(1.12 g/ml) = 250 ml
Mass by volume percent = (30 g/250 ml)*100 = 12%
Try yourself: A solution is prepared by dissolving 70 g of sodium chloride in 500 g of water. What is the mass by mass percent of sodium chloride in the solution. Also find the mass by volume percent of sodium chloride in the solution, if the density of solution given is 1.55 g/ml.
What is an unsaturated solution?
A solution in which more solute could be dissolved at any fixed temperature is called an unsaturated solution.
For example, a solution of sugar in which some more sugar could be resolved without changing its temperature is called an unsaturated solution of sugar.
What is a saturated solution?
A solution in which no more solute can be dissolved at any fixed temperature is called a saturated solution.
In other words, a solution which contain the maximum possible amount of solute at any given temperature is called a saturated solution.
In a saturated solution, dissolved and undissolved solutes are in equilibrium with each other.
Any solution containing more solute than that required to prepare a saturated solution, at any fixed temperature, is called supersaturated solution. The state of supersaturation is a meta stable state.
How to prepare a saturated solution of any substance?
(i) A saturated solution of any substance in water can be prepared as follows:
(ii) Take about 100 ml of water in a beaker.
(iii) Keep this water at the desired temperature by using a water bath.
(iv) Add a small quantity of the given substance, i.e., solute, into the water and dissolve it by stirring.
(v) Add a little more substance and stir to dissolve it.
(vi) Continue dissolving more and more solute until a little solute is left undissolved at the bottom of the beaker.
(vii) Keep the solution so prepared at the desired temperature for a few minutes.
(viii) The supernatant solution is the saturated solution of the given substance in water at that temperature.
You can prepare saturated solution of a substance in any solvent, other than water, by the same method. In such a case, instead of water, take the solvent in a beaker.
In the case of volatile solvent, such as alcohol, use a stoppered flask in place of beaker.
Suspension
A suspension is a heterogeneous mixture in which very fine particles, having size 10-7 m or more, of a solid are dispersed in any medium, either liquid or gas. In any suspension, fine particles of the solid remain suspended in the medium.
Muddy pond water, smoke coming out of chimney of a factory, chalk-water mixture are examples of suspension.
What are the properties of suspension?
Following are the characteristic properties of suspension:
(a) Heterogeneous nature: A suspension is a heterogeneous system.
(b) Visibility: The particles in a suspension can be seen with naked eyes or under a simple microscope.
(c) Particle size: In a suspension, the size of the particle is of the order of 10-7 m or larger.
(d) Sedimentation: The particles in a suspension have a tendency to settle down. Very fine particles, however, remain suspended in the medium.
(e) Separation by filtration: Larger particles in a suspension can be separated from the liquid or air by filtration.
Colloid
A, homogeneous-looking, heterogeneous mixture in which particles having size between 1 nm and 100 nm (1 nm = 10-9 m, nm: nanometer) are dispersed in a continuous medium is called a colloid. Colloids are commonly called sols.
In a colloid, particles of the dispersed phase may be 10 to 1000 times the size of a small molecule. The continuous medium in a colloid is called dispersion medium and the particle form the dispersed phase. The particles of the dispersed phase are commonly called colloidal particles.
Example are milk, gum, blood, toothpaste, jelly, etc.
Fog is a common colloid in which water droplets are the dispersed phase which are dispersed in air which is the dispersion medium.
Depending upon the nature of dispersed phase and dispersion medium, the colloids may be classified into various types. Some common types of colloids are given below:
What are the properties of colloid?
Some characteristic properties of a colloid are described below:
(a) Heterogeneous nature: A colloid (also called as sol) is heterogeneous in nature. The particles in a colloid can be seen only with a powerful microscope.
(b) Particle size: The size of the particle in a colloid lies between 10-9 m and 10-7 m, i.e., 1 nm and 100 nm.
(c) Separation of colloidal particles by filtration: Colloidal particles can easily pass through the pores of a filter paper. Therefore, colloidal particles cannot be separated by ordinary filtration.
(d) Stability: Colloids are unstable unless stabilised by any suitable stabiliser. The colloidal particles tend to come together and settle down.
(e) Brownian movement of colloidal particles: When seen under a microscope, the colloidal particle in a colloid are seen to be moving in a random fashion, that is zig-zag motion. This zig-zag motion of particle in a colloid is called Brownian movement.
Brownian movement in a colloid is shown in the given figure below:
(f) Light scattering by colloidal particles – Tyndall effect: When a strong beam of light is passed through a colloid kept in a dark room, then the path of the light beam through the colloid become visible. This occurs due to the scattering of light by the colloidal particles. The scattering of light by the particle in a colloid is called tyndall effect.
The tyndall effect is shown in the figure given below:
True solution do not scatter light. So, true solution do not show tyndall effect.
(g) Electrophoresis: Many colloidal particles have electrical charges on them, i.e., colloidal particle may have positive or negative charge on them. So, when electric current is passed through a colloid, the colloidal particle move towards the oppositely charged electrode. The movement of colloidal particle under the influence of electric field is called electrophoresis. Electrophoresis is shown in the given figure below:
What are the applications of colloid?
Due to their unique properties, colloids find many applications in our daily life and industry. Some typical application of colloids are described below:
(i) In our food: Many items in our food contain colloidal materials. For example, milk, starch, proteins, fruit-jellies are colloidal in nature.
(ii) In medicines: A large number of medicines and pharmaceutical preparations are colloidal in nature. Some of them are in fact emulsions. Such colloidal medicines are easily absorbed by the body tissues and, are more effective. For example, colloidal gold, calcium, silver are used in medicines or as ointments.
Colloids are important in understanding certain natural processes. For example, raw natural rubber, called latex, and clay in soil are colloidal in nature. Formation of deltas in the sea at the mouth of a river is due to the precipitation or coagulation of the colloidal clay in the river water.
How does a true solution differ from a colloid and a suspension?
The difference between true solution, a colloid and suspension is given in the table below:
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