Solutions This kind…

Section 15.1 Forming Solutions 1. To understand the process of dissolving 2. To learn why certain substances dissolve in water 3. To learn qualitative terms describing the concentration of a solution 4. To understand the factors that affect how fast a solid dissolves Objectives

Section 15.1 Forming Solutions What is a solution? Solution – homogeneous mixture Solvent – substance present in largest amount Solutes – other substances in the solution Aqueous solution – solution with water as the solvent

Section 15.1 Forming Solutions Example State of Solution Original State of Solute State of Solvent Air, natural gasGas Antifreeze in water Liquid BrassSolid Carbonated water (soda) LiquidGasLiquid Seawater, sugar solution LiquidSolidliquid

Section 15.1 Forming Solutions A. Solubility – Ionic Substances Ionic substances breakup into individual cations and anions.

Section 15.1 Forming Solutions A. Solubility of Ionic Substances Polar water molecules interact with the positive and negative ions of a salt.

Section 15.1 Forming Solutions Solubility of Polar Substances Ethanol is soluble in water because of the polar OH bond.  Polar water molecule Polar ethanol Molecule 

Section 15.1 Forming Solutions A. Solubility – Polar Substances Why is solid sugar soluble in water? Polar substances are hydrophilic. Polar OH bonds in sugar molecule

Section 15.1 Forming Solutions A. Solubility – Substances Insoluble in Water Nonpolar oil does not interact with polar water  nonpolar substances are hydrophobic. Water-water hydrogen bonds keep the water from mixing with the nonpolar molecules.

Section 15.1 Forming Solutions A. Solubility – How Substances Dissolve A “hole” must be made in the water structure for each solute particle. The lost water-water interactions must be replaced by water-solute interactions. “like dissolves like” – polar solutes dissolve in polar solvents, nonpolar solutes dissolve in nonpolar solvents

Section 15.1 Forming Solutions What kind of bear is hydrophilic? A Polar Bear!

Section 15.1 Forming Solutions Solution Composition: Concentration Amounts of substances can vary in different solutions. –Concentration - amount of solute in relation to solvent –Qualitative measures of concentration concentrated – relatively large amount of solute dilute – relatively small amount of solute

Section 15.1 Forming Solutions Solution Composition: Concentration Which solution is more concentrated? B

Section 15.1 Forming Solutions Solution Composition: Concentration Which solution is more concentrated? Neither

Section 15.1 Forming Solutions The solubility of a solute is limited. Saturated solution – contains as much solute as will dissolve at that temperature Unsaturated solution – has not reached the limit of solute that will dissolve at that temperature Solution Composition: How much solute can you dissolve in the solvent?

Section 15.1 Forming Solutions Interpreting Solubility Curves At 40 o C, about 38g NaCl can be dissolved in 100g water. The solution will then be saturated. How much Ba(NO 3 ) 2 can be dissolved in 100g water at 80 o C? At 20 o C, a solution with less than 38g NaCl in 100g water would be unsaturated.

Section 15.1 Forming Solutions Supersaturated solution – occurs when a solution is saturated at an elevated temperature and then allowed to cool but all of the solid remains dissolved Contains more dissolved solid than a saturated solution at that temperature Unstable – adding a crystal causes precipitation B. Solution Composition: An Introduction

Section 15.1 Forming Solutions Making a Supersaturated Solution Ex.: At 20 o C, a solution containing more than 19g Na 2 SO 4 dissolved in it would be supersaturated. At 20 o C, a solution containing ~19g Na 2 SO 4 dissolved in it would be saturated. To dissolve more solute, the temperature has to be increased. If we dissolve more solute at a higher temperature, then cool the solution slowly so the extra solute stays dissolved in it, the solution will be supersaturated.

Section 15.1 Forming Solutions Supersaturated Solution YouTube Video

Section 15.1 Forming Solutions Factors Affecting the Rate of Dissolving Increasing the number of collisions between solute and solvent particles increases the rate (speed) at which a solid dissolves. Three factors that speed up dissolving: –Increasing Surface area – dissolving occurs at the surface  the greater the surface area exposed to solvent, the faster the dissolving process Ex.: A sugar cube will dissolve faster if it is ground to small crystals, increasing the total surface area. –Stirring – removes newly dissolved particles from the surface of the solid and continuously exposes the surface to fresh solvent. –Increasing Temperature – higher temperatures cause molecules to move more rapidly; in addition, most solids are more soluble at higher temperatures*. Ex.: More sugar will dissolve in hot water than in cold water. *For gases dissolved in water, the opposite is true: the solubility of gases in water typically decreases as the temperature increases.

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