SOLUTIONS

Chapter 13

What is a solution?

In chemistry, a solution is a homogenous mixture composed of two or more substances. In such a mixture, a solute is dissolved in another substance, known as a solvent. A common example is a solid, such as salt or sugar, dissolved in water,a liquid.

A mixture is a substance made by combining two or more different materials in such a way that no chemical reaction occurs. The objects do not bond together in a mixture. A mixture can usually be separated back into its original components. Some examples of Mixtures are oil, ocean water and soil.

A suspension is a heterogenous fluid containing solid particles that are sufficiently large for sedimentation.

A colloid is a homogeneous solution with intermediate particle size between a solution and a suspension.

What is concentration?

Solvent: The substance in which the solute dissolves to make the solution.

Solute: The substance dissolved in a solution.

Aqueous: Describes a solution in which the solvent is water.

Concentration: The quantity of solute in a specific quantity of solvent or solution.

Molarity: A concentration unit, expressed in moles of solute per liter of solution.

Molarity(M) = moles of solute/liters of solution=mol/L

Class Practice

What is the molarity of a potassium chloride solution that has a volume of 400 ml and contains 85 g of KCl?

Sodium thiosulfate, Na2S2O3, is used as a fixer when developing photographic film. What is the mass in grams of Na2S2O3 needed to make 100ml of a 0.250M solution?

Home work

Page 475 Practice problems do all odd #.

Separating mixtures

There are many ways to separate mixtures into their components.

distillation

Magnetic separation

filtering

chromatography

Different ways of separating mixture 1 A magnet can be used to separate IRON from SAND. How it works: The magnet sticks to the iron but not to the

sand. 3. Filtering (filtration) can be used to separate a solid (or

suspension) from a liquid. How it works: The liquid (and anything dissolved in the

liquid) passes through holes in the filter paper but the solid particles are too big and get stuck.

Example: Filtration would be used to separate the dirt from some salty water.

4. Evaporation can be used to separate a dissolved SOLUTE from a SOLUTION

Example: Evaporation would be used to obtain some pure salt from salty water.

How it works: When salty water is warmed the water evaporates leaving behind crystals of salt.

Distillation is a method of separating chemical substances based on differences in their volatilities in a boiling liquid mixture. Example: Used to separate crude oil into more fractions for specific uses such as transport, power generation and heating.

Chromatography is a physical method of separation in which the components to be separated are distributed between two phases, one which is the stationary (stationary phase) while the other (the mobile phase) moves in a definite direction..

Gas Solubility

Gases can dissolve in liquids: Gases may dissolve in liquids, for example, carbon dioxide or oxygen in water.

Gas solubility depends on pressure: A soda bottle can be considered to be saturated since there is some soda above the liquid level and some is in the liquid itself.In an unopened soda bottle there is much higher partial pressure of CO2 in the neck of the bottle, above the liquid than there is in the air outside the bottle. Due to this difference in the pressure the CO2 dissolves in the liquid.

Gas solubility decreases with increasing temperature: After the soda bottle is opened the soda is allowed to warm, the soda forms fewer bubbles and tastes flat.

Liquids and miscibilities

Miscible: Indicates liquids that will dissolve in each other.

Immiscible: Indicates liquids that will not dissolve appreciably in each other example oil and water.

Polarity of the solute and solvent Polar solvents dissolve polar compounds,

and non polar solvents dissolve non polar compounds example when a sugar cube having polar sucrose molecules is dissolved in water a polar solvent.

Hydrogen bonding : High solubility is expected when hydrogen bonds form between solute and solvent molecules.

Two compounds that are both polar or both non polar are likely to be miscible.

Two compounds that can form hydrogen bonds with each other are likely to be miscible.

Solubility principles at work

Dry cleaners use solubility principles to remove stains. Commonly used dry cleaning solvent is tetrachloro ethylene C 2Cl4.

Vitamin C dissolves in water. Some sources of vitamin C are,

Vitamin A dissolves in oils and fats. Vitamin A affects vision because the human body uses it to make a pigment called visual purple, one of the vital light sensitive molecule in the retina.

Conductivity in Solutions

Conductivity is a measure of water’s ability to conduct electrical current. Measurements of conductivity provide a general indication of water quality.

Electrical conductivity is a measure of a material's ability to conduct an electric current. When an electrical potential difference is placed across a conductor, its movable charges flow, giving rise to an electric current. The conductivity σ is defined as the ratio of the current density to the electric field strength :

Electrolytes

What are electrolytes? Electrolytes are substances that become

ions in solution and acquire the capacity to conduct electricity.

Non electrolyte is a substance that does not dissociate into ions and so in solution it is a nonconductor of electricity.

Non electrolyte in solution

Weak electrolyte

Strong electrolyte

Tap water conducts electricity: You should avoid water when you are

using electricity. Unlike distilled water that does not conduct enough electricity to light a bulb in the conductivity apparatus tap water contains various ions from dissolved minerals. Water sources like water from a well has more concentration of salts than the surface water such as lakes and rivers.

While swimming in the pool, get out of the pool if a thunderstorm strikes. Chlorinated water conducts electricity.

Dissociation: A process in which a compound separates into fragments, such as simpler molecules,atoms,radicals or ions.

Hydration: The process by which water molecules surround each ion as it moves into solution.

Factors affecting solubility

Temperature:The solubility of a given solute in a given solvent often depends on temperature. For around 95% of solid solutes, the solubility increases with temperature, but gaseous solutes exhibit more complex behavior. As the temperature is raised gases usually become less soluble in water, but more soluble in organic solvents.

Pressure:Henry’s Law states that the solubility of a gas is directly proportional to the pressure of that gas, which may be written as p=kc

where k is a constant. Polarity: "Like dissolves like" This indicates that

a solute will dissolve best in a solvent that has a similar polarity to itself. For example, a very polar (hydrophilic) solute such as urea is highly soluble in highly polar water, less soluble in fairly polar methanol, and practically insoluble in non-polar solvents such as benzene.

Colligative properties

Colligative properties of solutions are properties that depend upon the concentration of solute molecules or ions, but not upon the identity of the solute. Colligative properties include

1. freezing point depression: the difference between the freezing point of a pure solvent and that of a solution.

2. boiling point elevation:the difference between the boiling point of a solution and that of the pure solvent.

3.vapor pressure lowering, and osmotic pressure.

The concentration of the solute affects freezing and boiling point changes: The more particles there are the greater the freezing point depression and the boiling point elevation. Based on the number of moles of solute particles produced, 1 mole of sodium chloride is expected to be twice as effective dissolved in water as 1 mol of sucrose,

C₁₂H₂₂O₁₁(s)H₂O- C₁₂H₂₂O₁₁ (aq) NaCl(s)-H₂ONa+(aq)+Cl-(aq)

Emulsions

An emulsion is a mixture of two immiscible substances. One substance is dispersed in the other Examples of emulsions include butter and margarine, espresso, mayonnaise, the photo-sensitive side of photographic film, and cutting fluid for metal working. In butter and margarine, a continuous liquid phase surrounds droplets of water (water-in-oil emulsion). Emulsification is the process by which emulsions are prepared.

Surfactant

What is soap?: A sodium or potassium salt of a long chain fatty acid. If the skin has accumulated oil dirt and bacteria then to remove this one must first emulsify the oil by scrubbing then stabilize it with an emulsifier such as soap.

Only then can the soap and oily dirt emulsion be rinsed away from the skin.

The soap ion dissolves the oil, forming a tiny sphere called a micelle, while the other end on the outside of the micelle dissolves in the surrounding water. The oil droplet stays suspended in the water and can be easily washed away. The characteristic property of a surfactant is its ability to form a layer between two dissimilar phases.

Soap forms micelle which is the bubble.A thin film of water is coated on both surfaces by soap molecules.

Hard water destroys soap’s surfactant abilities.

Detergents outperform soaps in hard water.

Homework

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