SOLUBILITY

 

Week:                                                                   Date:                                                     Time:

Period:                                 Duration: 1 HR 20 MIN.                                                  Average age of learners: 16YEARS

Subject:                               CHEMISTRY                                                                         Class: SS TWO

Topic:               SOLUBILITY

Sub topic:  

Reference materials:

(1) ESSENTIAL CHEMISTRY, TONALD PUBLISHERS, I. O ODESINA

(2) NEW SCHOOL CHEMISTRY, AFRICAN FIRST PUBLISHERS, OSEI YAW ABABIO

(3) INTERNET

Instructional materials: WATER , SALT

Entry behavior: The students have been taught energy change and chemical reaction.

Behavioural objective: At the end of the lesson the students should be able to:

1.       Define solute, solvent, and solution.

2.       Explain factors that affect solubility.

3.       Differentiate between saturated and supersaturated

4.       Solve problems on solubility

CONTENT

SOLUBILITY

In chemistry, solubility is the ability of a substance to dissolve; the quality of being soluble. "The solubility of a solute in a solvent at a particular temperature is the number of grams of the solute necessary to saturate 100gm of the solvent at that temperature."

Basic concept

SOLUTE: This is the minor component in a solution, dissolved in the solvent.

SOLVENT: The liquid in which a solute is dissolved to form a solution. It is a major component in a solution.

SOLUTION: It is a homogenous mixture composed of only one phase. In such mixture, a solute is a substance dissolved in another substance known as a solvent. The solvent does the dissolving.

                Solution  =  Solute  +   Solvent

The maximum amount of a substance dissolved in a given volume of solvent is called Solubility. Often the solubility in water is expressed in moldm-3.

SOLUBILITY OF DIFFERENT SUBSTANCES

The solubility of ionic compounds in aqueous solutions is wide and varied. Some compounds are highly soluble (NaCl), some compounds are moderately soluble(SnI2) and some compound are highly insoluble (AgCl2).

Determining which compound are soluble and not soluble in a water medium is not simple task. However, a set of rules that help in this regard can be stated. These rules are known as “solubility rules”. Below is a set of solubility rules:

Solubility Compounds

Exception

1. almost all salts of Na+, K+, and NH4+

 

2. all salts of Cl-, Br-, and I-

Halides of Ag+, Hg+ and Pb2+

3. compound containing F-

Fluorides of Mg2+, Ca2+, Sr2+, Ba2+, Pb2+

4. salt of nitrate,NO3-, chlorate,ClO3-, perchlorate,ClO4-,acetate, CH3CO2-.

 

5. salt of sulphate, SO42-

Sulphates of Sr2+, Ba2+, Pb2+

Insoluble compound

All salt of: carbonate,CO32-, phosphate,PO42-, oxalate, C2O42-, chromate,CrO42-, sulphide, S2-, most metal hydroxide and oxide (OH-)

Exception

Salt of NH4+ and the alkali metal cation.

FACTORS THAT AFFECT SOLUBILITY

There are three (3) main factors that control solubility of a solute: I. temperature   II. nature of solute or solvent 

III. pressure.

EFFECT OF TEMPERATURE: Generally in many cases solubility increases with the rise in temperature and decreases with the fall of temperature but it is not necessary in all cases. However , we must follow 2 behaviours:

I.                    In endothermic process solubility increases with the increase in temperature and vice-versa e.g solubility of potassium nitrate (KNO3) increases with the increase in temperature.

II.                  In exothermic process, solubility decreases with the increase in temperature e.g solubility of calcium oxide (CaO) decreases with the increase in temperature. Gases are more soluble in cold solvent than in hot solvent.

NATURE OF SOLUTE AND SOLVENT: Solubility of a solute in a solvent purely depends on the nature of both solute and solvent. A polar solute dissolved in polar solvent. Solubility of a non-polar solute in a solvent is large. A polar solute has low solubility or insoluble in a non-polar solvent.

EFFECT OF PRESSURE: The effect of pressure is observed only in the cases of a gases. An increase in pressure increases of solubility of a gas in a liquid. For example, carbondioxide is filled in cold drink bottles such as coca-cola, sprite etc.

TYPES OF SOLUTION

I.                    SATURATED SOLUTION: A solution that can hold no more of the solute at a particular temperature is said to be a saturated solution at that temperature.

II.                  UNSATURATED SOLUTION: Is a solution which contains less amount of solute than is required to saturate it at that temperature.

III.                SUPERSATURATED SOLUTION: Solution that is more concentrated than a saturated solution. If a crystal of solute is added to this solution, the excess of solute crystallizes.

IV.                AQUEOUS SOLUTION: A solution of any substance in which solvent is water.

PROBLEMS INVOLVING SOLUBILITY

Solubility  =    X  

Solubility is expressed in term of moldm-3 and is defined as the number of moles of solute that forms a saturated solution in one dm3.

Example

1.       When 50cm3 of a saturated solution of sugar (molar mass of 342.0g) at 40oC was evaporated to dryness; 34.2g of dry solid was obtained. What is the solubility of sugar at 40oC.

Solution

Mass of dry solid = 34.2g; volume of saturated solution =50cm3; molar mass of sugar = 342.0g

Solubility  =    X  

Solubility  =    X    = 2.0 moldm-3 of solution.

2.       An empty evaporating dish weighs 14.32g when partly filled with a saturated solution of potassium trioxonitrate (v) in water at 15oC, the dish weighs 35.70g. after the solution has been evaporated to dryness, the dish and its content weigh 18.60g. Calculate the solubility in moldm-3 of KNO3 in water at 15oC (K=39, N=14, O= 16)

Solution

Mass of saturated solution = 35.70 – 14.32g = 21.38g

Mass of dry salt   =  18.60 – 14.32g = 4.28g

Density of solution KNO3 = 1gdm-3

Volume of solution of KNO3 = 21.38cm3

Molar mass of KNO3 = K + N + 3O = 39 + 14 +(3X16) = 101gmol-1

21.32cm3 of solution contains 4.28g

1000cm3 of solution will contains  1000/21.3X4.28g = 200.75g

Solubility of KNO3 at 15oC in gdm-3 = 200.75g

Solubility of salt =    X  

 

Solubility  =    X    = 1.98 moldm-3 of solution.

SOLUBILITY GRAPHS OR CURVES

If the solubility of a solute in a given solvent are plotted against their respective temperature, a solubility graph or curve showing the effect of temperature on the solubility of the substance will be obtained.

APPLICATION OF SOLUBILITY CURVE

I.                    Solubility curves enable pharmacist to determine the amounts of solid drug that must be dissolved in a given quantity of solvent to give a prescribed drug mixture.

II.                  It enables chemists and research workers to determine the most suitable solvents to be used at various temperature for the extraction of essential chemicals.

III.                With the help of solubility curve, a given mixture of solute can be separated or purified by fractional crystallization.

PRESENTATION

i.The teacher explains solubility and its basic concept

ii. The teacher explains factors that affect solubility.

iv. The students mention factors that affect solubility .

v. The teacher explains types of solution.

vi. The teacher leads the students in solving mathematical problems in solubility.

EVALUATION

 The teacher evaluates the lessons by asking the following questions:-

1.       Define solute, solvent, and solution.

2.       Explain factors that affect solubility.

3.       Differentiate between saturated and supersaturated

ASSIGNMENT

1a. Distinguish between saturated solution and unsaturated solution.

b. state two ways by which a saturated solution could be made to dissolve more solute.

c. state one factor that could affect the solubility of a solid in a liquid

Solution

Saturated solution

Unsaturated solution

1. cannot dissolve any more solute at that temperature

Can dissolve more solute at that temperature

Contains undissolved solute at that temperature

Does not contain undissolved solute at that temperature

b. i. increasing the temperature ii. Increasing the volume of solvent.

c. –Temperature – Nature of solvent –Volume of solvent –Nature of solute

2.A saturated solution of volume 10cm3 yielded 0.06g of its dry salt at 25oc. Calculate the solubility of the salt in gdm-3.

 

 

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