Water: The Universal Solvent

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Water: The Universal Solvent

• Polarity makes water special.

• O atom is an electron “hog” in the covalent bond it forms with 2 H atoms.

• Water has a positive end and a negative end.

• This makes it a good solvent for ionic solids.

OH H

Slightly positive

Slightly negative

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Solutions

• Same as a homogeneous mixture

• Solvent = substance that takes in, or dissolves, another substance

• Solute = substance that is taken in, or dissolved by the first substance

• The 3 phases of matter can be paired together in 9 ways to make solutions.

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Solutions and phases of matter

Solute (dissolved) (in) Solvent Example

Gas Gas O2 in N2 - Air

Gas Liquid CO2 in water

Gas Solid Hydrogen fuel storage

Liquid Gas Water in air

Liquid Liquid Acid in water (vinegar)

Liquid Solid Amalgam dental fillings (Hg in Ag)

Solid Gas Mothballs in air

Solid Liquid Salt water

Solid Solid Metal alloys - jewelry

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Factors which affect solution rate and solubility

• Solution rate:– Temperature– Particle size– Movement

• Solubility:– Temperature– Pressure

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Solubility

• Solids: Increased temp increases solubility.• Gases: Solubility in water decreases when

temperature increases.• Gases: Increased pressure increases

solubility.• More fizz in a soda bottle if you store with

lid on (increased pressure) and in the refrigerator (decreased temperature).

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Concentration• Relationship between solute and solvent:• Concentration = volume solute/volume solvent• Relative concentration of solutions:1. Unsaturated: More solute can be dissolved at a given

temp. and pressure. May be dilute or concentrated.2. Saturated: Maximum solute a solvent will dissolve at

a given temp. and pressure.3. Supersaturated: More than the maximum solute a

solvent will dissolve at a given temp. and pressure. (Usually obtained by heating a saturated solution to very high temp and cooling slowly.)

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Concentration• Concentration is the amount of solute dissolved in

an amount of solution.

• Molar concentration is called Molarity, or M.

• M = moles of solute

volume of solution in L

• If 2.0 moles NaCl is dissolved in 1 L of solution, its concentration is “two molar” and is expressed as 2.0 M NaCl.

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Steps to make a 1 M solutionExample: 1 M NaOH solution

1. Calculate the mass needed. 1 mol NaOH 40.01 g NaOH = 40.01 g NaOH

1 mol NaOH 2. Measure this amount of NaOH into a beaker, then

dissolve in some solvent to dissolve it.3. Pour the solution into a 1.0 L volumetric flask.4. Rinse the weighing beaker with more solvent and add

to the flask.5. Add solvent until the solution meets the line

indicating 1.0 L.6. Stopper the flask and swirl, mix thoroughly.

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Molality• Molal concentration is called molality, or m.

• m = moles of solute

mass of solvent in kg

• If 0.50 moles NaCl is dissolved in 1 kg of solution, its concentration is “zero point five molal” and is expressed as 0.50 m NaCl.

• 1000 g = 1 kg.

• For water, 1 g = 1 ml

1kg = 1000 ml = 1 L

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Molality Example

• Molality is used when studying properties of solutions related to vapor pressures and temperature changes. The expression for concentration using m is unaffected by changes in temperature.

• What is the molality of a solution of 3.73g KCl in 100 g water?

3.73 g KCl 1 mol KCl = 0.05 moles KCl

74.55 g KCl

m = 0.05 moles KCl = 0.5 m KCl solution

0.100 kg H2O

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Molarity Example

To produce 23.4 g of K2CrO4 from 6.0 M K2CrO4, what volume of the solution is needed?

Given: 23.4 g K2CrO4 needed

concentration of solution = 6 moles K2CrO4 per 1 L

Want: volume of K2CrO4 in solution

23.4 g K2CrO4 1 mol K2CrO4 = 0.120 mol K2CrO4

194.2 g K2CrO4

0.120 mol K2CrO4 1 L = 0.020 L K2CrO4 soln

6 mol K2CrO4

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Molality example

How much iodine (in grams) must be added to prepare a 0.480 m solution of iodine in CCl4 if 100.0 g of CCl4 is used?

Given: m = 0.480 m I2 Want: mass of solute in g

solvent = 100.0 g CCl4 = 0.100 kg

I2 = 253.8 g/mol

0.480 mol I2 = x mol I2

1 kg CCl4 0.100 kg CCl4

(0.480 mol I2)(0.100) = x = 0.0480 mol I2

0.0480 mol I2 253.8 g I2 = 12.2 g I2

1 mol I2

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Suspensions and Colloids

• Remember: A solution is a homogeneous mixture.

• Some mixtures are not solutions—It depends on the size of the particles.

• Large particles usually don’t make homogeneous mixtures.

• The names of 2 types of mixtures with large particles are suspensions and colloids.

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Suspensions• A mixture in which some particles

will settle out is a suspension.• Gravity will eventually make large

particles sink.• Particles are large enough to be filtered

out.• Examples: Muddy water and salad

dressing.• Particles are large enough to reflect

light, so a beam of light will be visible in a suspension.

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Comparing mixtures

• Solution = homogeneous, small particles

• Suspension = heterogeneous, large particles

• Colloid = in-between suspension and solution.– Is not completely homogeneous.– Examples: fog, homogenized milk,

toothpaste.

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Colloids

• How can you tell is a mixture is a colloid?

• A beam of light will be visible when shone through a colloid (like a suspension).

• Particles do not separate or settle out after standing.

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Colloids: 5 types• Sol

– Solid is dispersed in solid or liquid.– Examples: Clay, toothpaste, red glass

• Gel – Long particles that trap liquid inside.– Examples: Jam, jelly, gelatin dessert, hair gel

• Aerosol – Solid is dispersed in air.– Examples: Fog, some hair sprays

• Foam– Gas is dispersed in liquid.– Examples: Marshmallows, whipped cream

• Emulsion – Liquid is dispersed in liquid.– Examples: Milk, butter, some cosmetics

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Comparing mixtures

• Solution = homogeneous, small particles

• Suspension = heterogeneous, large particles

• Colloid = in-between suspension and solution.– Is not completely homogeneous.– Examples: fog, homogenized milk,

toothpaste.

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Solutes: Electrolytes• Electrolyte = Substance that dissolves in

water to give a solution that conducts electricity

• Mobile charged particles conduct electricity (ions)

• Soluble ionic compounds are electrolytes(Na+ and Cl-)

• Many acids are electrolytes

(HCl =>H3O+ and Cl-)

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Solute-Solvent InteractionsLike dissolves like

• Polar dissolves polar Water + NaCl

• Nonpolar dissolves nonpolar CCl4 + motor oil

• Immiscible liquids (oil and vinegar)– Do not mix

– Liquid solutes and solvents that are not soluble in each other

• Miscible liquids (gas and oil)– Mix

– Liquids that dissolve freely in one another in any proportion

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Henry’s LawSolubility of gas in liquid

• The solubility of a gas is directly proportional to the partial pressure of that gas on the surface of the liquid.

• Soda bottle: – High pressure at the surface while the bottle is

closed, so lots of CO2 in the liquid– Open bottle, pressure on surface lowers to room

atmosphere and CO2 leaves the liquid

• High pressure = High gas concentration• Low pressure = low gas concentration