Solutions

Agenda Day 64 – Introduction to Solutions Lesson: PPT Handouts: 1. Solution Handout Text: 1. P Solutions HW: 1. P. 381 # 1-9

MATTER Heterogeneous mixture Is it uniform throughout? No Homogeneous Yes Can it be separated by physical means? Pure Substance Homogeneous Mixture (Solution) Can it be decomposed into other substance by a chemical process? Element Compound Noyes No yes

States of matter in solution Example of solutions gas in gasair ( N 2, O 2, Ar, CO 2, other gases) gas in liquidsoda pop (CO 2 in water) liquid in liquidgasoline (a mixture of hydrocarbon compounds) solid in liquidFiltrated sea water ( NaCl and other salts in water) gas in solid H 2 in platinum or palladium Naturally absorbed alloy liquid in solid dental amalgams (mercury in silver) solid in solid alloys ( brass, (Cu/Zn), sol-der (Sn/Pb), Steel (Fe/C ))

Definitions Solutions are made up of at least two components: 1.SOLVENT - The substance that does the dissolving and is usually in greater proportion. ( Often will indicate the phase of the solution). 2.SOLUTE - the substance that is dissolved and is usually in smaller proportion A solution that is composed of a high percentage of solute is said to be concentrated A solution with a low percentage of solute is said to be dilute.

SOLUBILITY The SOLUBILITY of a substance in a solvent is the maximum amount of the solute which will dissolve in a fixed quantity of solvent at a specific temperature The solubility of a substance changes with temperature and is dependent on the nature of the solute and solvent. This is because a high temperature means H 2 O molecules are moving faster (keeping more solid molecules suspended).

Solutions may be classified on the basis of the amount of solute dissolved in the solvent UNSATURATED SOLUTION - A solution in which more solute can be dissolved while the temperature remains constant. ( solvent volume is unchanged) SATURATED SOLUTION - A solution in which no more solute can be dissolved while the temperature remains constant. ( in a given volume of solvent) A saturated solution represents an equilibrium: the rate of dissolving is equal to the rate of crystallization. The salt continues to dissolve, but crystallizes at the same rate so that there “appears” to be nothing happening.

Dissolving a salt... A salt is an ionic compound - usually a metal cation bonded to a non-metal anion. The dissolving of a salt is an example of equilibrium. The cations and anions are attracted to each other in the salt. They are also attracted to the water molecules. The water molecules will start to pull out some of the ions from the salt crystal.

At first, the only process occurring is the dissolving of the salt - the dissociation of the salt into its ions. However, soon the ions floating in the water begin to collide with the salt crystal and are “pulled back in” to the salt. (precipitation) Eventually the rate of dissociation is equal to the rate of precipitation. The solution is now “saturated”. It has reached equilibrium.

Solubility Equilibrium: Dissociation = Precipitation In a saturated solution, there is no change in amount of solid precipitate at the bottom of the beaker. Concentration of the solution is constant. The rate at which the salt is dissolving into solution equals the rate of precipitation. Dissolving NaCl in water Na + and Cl - ions surrounded by water molecules NaCl Crystal

SUPERSATURATED SOLUTION SUPERSATURATED SOLUTION - A solution which contains more dissolved solute than it would normally at a given temperature and specific volume of solvent. Supersaturated solutions are unstable. The supersaturation is only temporary, and usually accomplished in one of two ways: 1.Warm the solvent so that it will dissolve more, then cool the solution. 2.Evaporate some of the solvent carefully so that the solute does not solidify and come out of solution.

Water Polar substances -are ones with an unequal distribution of charge on the molecule These molecules interact with other polar substances because of dipole interactions “like dissolves like” Water is a polar solvent and is known as the universal solvent since it is able to dissolve a great variety of substances [its solutions are known as aqueous (aq)]

Aqueous Solutions Water acts to dissolve both molecular and ionic substances through intermolecular dipole interactions. In some instances water can also form hydrogen bonding when the solute has potential for hydrogen bonding. Water can cause some molecular substances to ionize by H-bonding to water or by LDF ie HCl (g) + H 2 O (l) > H 3 O + (aq) + Cl – (aq) (hydronium) Water causes ionic substances to dissociate ie NaCl (s) > Na + (aq) + Cl – (aq)

Dissolving process in water Na + Cl - 1. Overcome attractive forces in solid 2. Hydration of solute Orientation of water molecules around solute

Types of attractive forces Na + Cl - For water: dipole-dipole For NaCl (s): ion-ion For hydrated ion: ion-dipole

How do I get sugar (molecular compound) to dissolve faster in my iced tea? Stir, and stir, and stir Add sugar to warm tea then add ice Grind the sugar to a powder Fresh solvent contact and interaction with solute Greater surface area, more solute-solvent interaction Faster rate of dissolution at higher temperature

Aqueous Solutions How do we know ions are present in aqueous solutions? Solutions that form ions are known as electrolytes and will conduct electrical current. HCl, MgCl 2, and NaCl are strong electrolytes. They dissociate completely (or nearly so) into ions. Solutions that do not form ions are known as non-electrolytes. e.g: sugar, ethanol, ethylene glycol

Types of solutes Na + Cl - Strong Electrolyte - 100% dissociation, all ions in solution high conductivity

Types of solutes CH 3 COOH CH 3 COO - H+H+ Weak Electrolyte - partial dissociation, molecules and ions in solution slight conductivity

Types of solutes sugar Non-electrolyte - No dissociation, all molecules in solution no conductivity

Electrolytes in the Body Make your own g sugar One liter of warm water Pinch of salt 200ml of sugar free fruit squash Mix, cool and drink Carry messages to and from the brain as electrical signals. Maintain cellular function with the correct concentrations electrolytes

Solvents Non polar substances have an equal distribution of charge and interact with other nonpolar substances because of London (dispersion) force interactions. Other popular solvents: Alcohol, e.g. I 2(al) - antiseptic Acetic Acid, e.g. glues and solvents

SOLUBILITY IN WATER Solids usually have a higher solubility in water at higher temperatures Gases always have a higher solubility in water at lower temperatures. This is because when gas molecules are moving faster they are able to escape from the liquid surface. Think of cold soda vs. warm soda. Halogens and oxygen are only slightly soluble in water but because they are so reactive, even in small concentrations they are often very important in solution reactions

Miscible vs. Immiscible Nonpolar liquids do not dissolve in water to any large degree but instead form a separate layer. Theses liquids are said to be IMMISCIBLE in water Some liquids made up of small polar molecules with the ability for form hydrogen bonds dissolve completely in water and are said to be MISCIBLE. Polar liquids usually have a higher solubility in water at a higher temperature Elements that do not react with water generally have a low solubility in water. Note: there are always exceptions to these generalized statements

Solubility of Solutes in Water Temperature Solubility, g/100 mL water Most solids (endothermic hydration) Some solids (exothermic hydration) All gases

The Nature of Molecules

Properties of Water 1. Water has a high specific heat. - A large amount of energy is required to change the temperature of water. 2. Water has a high heat of vaporization. - The evaporation of water from a surface causes cooling of that surface. 3. Solid water is less dense than liquid water. - Bodies of water freeze from the top down. 4. Water is a good solvent. - Water dissolves polar molecules and ionic compound. 28

Properties of Water 5. Water organizes nonpolar molecules. - hydrophilic: “ water-loving ” - hydrophobic: “ water-fearing ” Water causes hydrophobic molecules to aggregate or assume specific shapes. 6. Water can form ions. H 2 O (l)  OH -1 (aq) + H +1 (aq) hydroxide ionhydrogen ion 29

Why oil and water don’t mix ++ –– ++ ++ –– ++ ++ –– ++ ++ –– ++ ++ –– ++ ++ –– ++ ++ –– ++ ++ –– ++ ++ –– ++ The non-polar substance is pushed away. If it were moving faster it might break through the attractive forces. Solubility is a balance between speed and attraction. Also, the more similar the strength of their dipoles the more likely two compounds are to mix.

Colour Changing Milk Explanation: Milk contains fat which is a non polar substance. Food colouring is a polar solution. Therefore when the food colouring is added to the milk, there is no interaction between the two solutions. However, when soap is added, there is an observable change. Soap is a special substance having both a polar and a non polar end. The polar end interacts with the food colouring, and the non polar end interacts with the non polar fat in the milk solution.