SECTION 1. TYPES OF MIXTURES CHEMISTRY CHAPTER 12 SOLUTIONS SECTION 1. TYPES OF MIXTURES A solution is a homogeneous mixture of two or more substances in a single phase. The dissolved particles are very small (atoms, molecules, ions), and do not settle out. Does not scatter light.

Cannot be separated by filtration. Solutions – properties (cont.) Cannot be separated by filtration. Ex. – sugar in water. When sugar dissolves, all its molecules become uniformly distributed among the water molecules. The solid sugar is no longer visible.

The dissolving medium in a solution (present in larger amount) is called the solvent, and the substance dissolved in a solution (present in lesser amount) is called the solute.

Solutions may exist as gases, liquids, or solids Solutions may exist as gases, liquids, or solids. There are many possible solute-solvent combinations between gases, liquids, and solids. example: Alloys are solid solutions in which the atoms of two or more metals are uniformly mixed. Brass is made from zinc and copper. Sterling silver is made from silver and copper.

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Particle Models for Gold and Gold Alloy

Heterogeneous mixtures Large particles that settle out on standing. Suspensions Heterogeneous mixtures Large particles that settle out on standing. Can be separated by filtration. May scatter light. Ex: muddy water

Ex: milk, shaving cream, smoke, fog Colloids Heterogeneous Particles that are intermediate in size between those in solutions and suspensions Does not settle out – particles kept suspended by the constant movement of surrounding molecules Scatters light Ex: milk, shaving cream, smoke, fog

example: a headlight beam is visible from the side on a foggy night Tyndall Effect Many colloids look similar to solutions because their particles cannot be seen. The Tyndall effect occurs when light is scattered by colloidal particles dispersed in a transparent medium. example: a headlight beam is visible from the side on a foggy night The Tyndall effect can be used to distinguish between a solution and a colloid.

Colloids

Properties of Solutions, Colloids, and Suspensions

Any soluble ionic compound, such as NaCl Solutes: Electrolytes Versus Nonelectrolytes A substance that dissolves in water to give a solution that conducts electric current is called an electrolyte: Any soluble ionic compound, such as NaCl Polar molecular compounds that dissociate to give ions (ex.: HCl forms H3O+ and Cl-).

A substance that dissolves in water to give neutral solute molecules is a nonelectrolyte. The solution does not conduct electric current. Ex. - sugar

Electrical Conductivity of Solutions

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SECTION 2. THE SOLUTION PROCESS Factors That Increase the Rate of Dissolution (Dissolving) They increase the rate at which solute and solvent can interact: Increasing surface area Stirring or shaking Heating. At higher temperatures, collisions between solvent molecules and solute are more frequent and of higher energy.

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Solubility For every combination of solvent with a solid solute at a given temperature, there is a limit to the amount of solid that can be dissolved. The point at which this limit is reached depends on the nature of the solute, the nature of the solvent, and the temperature.

Particle Model for Soluble and Insoluble Substances

Particle Model for Soluble and Insoluble Substances

When a solute is first added to a solvent, solute molecules leave the solid surface and move about at random in the solvent. As more solute is added, more collisions occur between dissolved solute particles. Some of them return to the crystal. When maximum solubility is reached, molecules are returning to the solid form at the same rate at which they are going into solution.

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Saturated Versus Unsaturated Solutions A solution that contains the maximum amount of dissolved solute is described as a saturated solution. If more solute is added to a saturated solution, it falls to the bottom of the container and does not dissolve. A solution that contains less solute than a saturated solution under the same conditions is an unsaturated solution.

Mass of Solute Added Versus Mass of Solute Dissolved

Supersaturated Solutions When a saturated solution is cooled, the excess solute usually comes out of solution. Sometimes the excess solute does not separate, and a supersaturated solution is produced. This contains more dissolved solute than a saturated solution contains under the same conditions.

The solubility of a substance is the amount of that substance required to form a saturated solution with a specific amount of solvent at a specified temperature. example: The solubility of sugar is 204 g per 100 g of water at 20°C.

Solubility of Common Compounds

Solubility of Common Compounds

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Solubility varies greatly with the type of compounds involved. Solute-Solvent Interactions Solubility varies greatly with the type of compounds involved. “Like dissolves like” is a rough but useful rule for predicting whether one substance will dissolve in another. Polar and ionic solutes are more soluble in polar solvents such as water or ethanol.

Nonpolar solutes are more soluble in nonpolar solvents such as benzene or toluene. Examples: LiCl is soluble in water but not in toluene. Fats are soluble in chloroform but not in water.

Dissolving Ionic Compounds in Aqueous Solution Aqueous solutions (in water) of ionic compounds: the dissolving process is known as hydration. Positive ions interact with the oxygens of water (slightly negative), while negative ions interact with the positive ends.

Ex. – lithium chloride

SECTION 3. CONCENTRATION OF SOLUTIONS The concentration of a solution is a measure of the amount of solute in a given amount of solvent or solution. Concentration is a ratio: any amount of a given solution has the same concentration. Concentration may be expressed in several different ways.

Concentration Units

1. Molarity Molarity is the number of moles of solute in one liter of solution (solvent plus solute). The symbol for molarity is M. The concentration of a one molar NaOH solution is written 1 M NaOH.

Ex. – what is the concentration of a solution of HCl prepared by adding 3 moles of HCl to H2O and giving a final volume of 6 liters? = 3 mol/6 L = 0.5 mol/L = 0.5 M

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The symbol for molality is m. (ex.: 1 m NaOH) 2. Molality [note – will not be tested] Molality is the concentration of a solution expressed in moles of solute per kilogram of solvent. A solution that contains 1 mol of solute dissolved in 1 kg of solvent is a “one molal” solution. The symbol for molality is m. (ex.: 1 m NaOH)

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3. % by mass Ex. – 25 g sugar in 75 g solution: How much solvent is there?

3. % by mass Ex. – 25 g sugar in 75 g solution: How much solvent is there? 75 g – 25 g = 50 g

% by volume (used for liquids) Ex. – 25 mL of ethanol is added to enough water to make 100 mL of solution. What is the percent by volume of ethanol?

mass per volume Ex. – a sugar solution contains 26 g sugar in 0.50 L of solution. What is the concentration in g/L?