ST. LAWRENCE SCHOOL-SESSION 2020-2021 Class-IX: CHEMISTRY Chapter 3: Water PART - III Smita Das This particular document would cover the summarized gist of chapter 3 based on various key points with granular details. KEY POINTERS: CONCENTRATION OF A SOLUTION: It is the amount of solute dissolved in a given quantity of that solution. It can be expressed in many ways for e.g. 1- Mass percent: It is defined as the mass of the solid solute in grams present in hundred grams of solution. It is mostly used when solute is solid and solvent is liquid. Concentration of solution = Mass of the solute x 100 Mass of the solution (Solute + Solvent) 2- Volume Percent: It is defined as the volume of the solution in ml present in 100 ml of solution. It is mostly used when solute and solvent are liquids. Volume percent= Volume of the solute x 100 Volume of the Solute + Volume of the Solvent SOLUBILITY: The amount of solute that dissolves in 100 grams of solvent to form a saturated solution at a particular temperature is called solubility. 1- Insoluble: A substance that has negligible solubility is called insoluble. For eg. Silver Chloride. 2- Sparingly Soluble: A substance that has more than negligible but less than high solubility. For eg. Calcium Hydroxide. 3- Soluble: A substance having high solubility. For eg. Sodium Chloride. Solubility= Mass of the solute x 100 Mass of the Solvent FACTORS AFFECTING SOLUBILITY: The rate of solubility of a solid in a liquid depends on the following factors: 1- Size of solute particles: The smaller the size of the solute particles, the greater is its total surface area exposed to the solvent hence the greater is the solubility of that solute. 2- Stirring: This brings more of the solvent in contact with the solute and thus increases the rate of formation of solution. 3- Temperature: The solubility of a gas in the liquid always decreases with rise in temperature but the solubility of most solids in water usually increases with rise in temperature. SOLUBILITY CURVES: It is a line graph that plots changes in solubility of a solute in a solvent against changing temperature. ANOMALOUS SOLUBILITY: There are some salts whose solubilities first increase and then decrease with rise in temperature. For eg. Na2SO4 . 10H2O (Glauber’s Salt) APPLICATIONS OF SOLUBILITY CURVES: 1- Shape of the curve indicates how the solubility of the given substance in a solvent varies with change in temperature. The solubility of a substance at particular temperature can be determined from the curve. 2- Solubilities of different solutes in a solvent at different temperatures can be compared. 3- The effect of cooling of hot solutions of different substances can be found from the curves. EFFECT OF PRESSURE AND TEMPERATURE ON SOLUBILITY OF GASES IN WATER (Liquids): 1- Pressure: At any given temperature the mass of gas dissolved by a fixed volume of liquid is directly proportional to the pressure on the surface of the liquid. This law is known as Henry’s Law. For eg: The solubility of Carbon dioxide in water under normal atmospheric pressure is low but if it is subjected to high pressure a lot more of carbon dioxide gas dissolves in it (Soda Water) 2- Temperature: An increase in temperature of water causes a decrease in solubility of gases in it.For eg. Once water is boiled it loses its taste. CRYSTALS AND CRYSTALLIZATION: A homogenous solid of definite geometrical shape with sharp edges is known as CRYSTAL. The process by which crystals of a substance are obtained by cooling a hot , saturated solution is known as CRYSTALLIZATION. In a laboratory the crystals maybe obtained by the following methods: 1- By cooling a hot, saturated solution. 2- By slowly evaporating a hot, saturated solution. 3- By cooling a fused mass. 4- By sublimation. WATER OF CRYSTALLIZATION: The fixed amount of water that is associated with hydrated crystals which is an integral part of the crystal is known as water of crystallization. STUDENTS! NOW LET’S TEST YOUR KNOWLEDGE: Here comes the assignment (Note: Please ensure you solve each question in your notebook, based on which you will be accessed later on.) Q-1) Define the following terms: 1- Solubility 2- Henry’s law 3- Crystallization 4- Water of Crystallization Q-2) Normally, solubility of a crystalline solid increases with temperature. Does it increase uniformly in all cases? Name a substance whose solubility: a- Increases rapidly with temperature. b- Increases gradually with temperature. c- Increases slightly with temperature. d- Initially increases, then decreases with rise in temperature. .