LecturePLUS Timberlake1 Chapter 6.2 Dissolving and Solubility.

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Chapter 6.2

•Dissolving and Solubility

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Making process of dissolving FASTER!

• Many of same things that make chemical reactions go faster –

• But DISSOLVING IS A PHYSICAL CHANGE, NOT A CHEMICAL CHANGE.

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Faster dissolving

• When a chunk of something dissolves, you are just pulling it apart into smaller bits (even down to individual molecules or ions)

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Faster dissolving

• heat

• stirring

• shaking

• smaller chunks (larger surface area)

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In a solution, the solute moleculesare randomly distributed amongthe solvent molecules

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Figure 15.1: Dissolving of solid sodium chloride.

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• The solvent is the substance that dissolves the solute to make a solution.

• The solute is the substance that dissolves in a solution.

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Why is water such a great dissolver (solvent) for other

substances?

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Water is polar...

It has a slightly negative “side” by the oxygen atom, and slightly

positive “side” by the hydrogens.

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Water can dissolve ionic compounds, and covalent compounds that are polar (have “slightly negative”

and “slightly positive” parts).

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The interaction of polar water molecules with ions

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Representation of the polar hydrogen chloride molecule

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Chlorine hogs the electron blanket, leavinghydrogen partially, but positively, exposed

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Figure 15.3: The ethanol molecule contains a polar O—

H bond.

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Figure 15.3: The polar water molecule interacts strongly with the polar O—H bond in ethanol.

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Soap

• The nonpolar side of a soap molecule is “attached” (attracted to) nonpolar grease and oil, the polar side hangs on to the water, and the dirt slips off your hands and is pulled down the drain, trapped, along with the water molecules, by that pesky soap molecule!

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(a) non-polar gasoline and non-polar oil mix (b) polar vinegar and nonpolar oil do not mix (c) polar water and polar ethyl alcohol mix

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Figure 15.6: An oil layer floating on water.

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For a more detailed explanation of why polar and nonpolar materials can’t

dissolve each other:

• http://www.knoxnews.com/kns/science/article/0,1406,KNS_9116_1971484,00.html

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Saturated and Unsaturated

A saturated solution contains the maximum

amount of solute that can dissolve.

Undissolved solute remains.

An unsaturated solution does not contain all

the solute that could dissolve

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Supersaturated Solutions

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To see more of this supersaturation experiment:

• http://www.csudh.edu/oliver/demos/supersat/supersat.htm

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To see more exciting supersaturation stuff, go to:

• Crystallization from Supersaturated Solutions of Sodium Acetate

• Note, you may need to hit “refresh” to get demonstration started and restarted.

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Solubility

The maximum amount of solute that can dissolve in a specific amount of solvent usually 100 g.

g of solute

100 g water

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Molarity

• Concentration unit of a solution that expresses moles of solute dissolved per liter of solution.

• To make a 1 molar solution of a substance, put 1 mole of stuff into a container and add solvent (usually water) up to the “1 liter” mark.

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Not this type of Mole

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Or this type of Mole

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Mole• 6.02 x 1023 is a mole -

602,000,000,000,000,000,000• If you counted paper at the rate of one sheet

per second it would take you 19,089,294,774,226,281 years to count a mole of paper.

• It is a big number because atoms are small.

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• A mole of sugar weighs about one half pound but contains how many molecules of sugar?

• 602,000,000,000,000,000,000• 6.02 x 1023 molecules.

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Temperature and Solubility of Solids

Temperature Solubility (g/100 g H2O)

KCl(s) NaNO3(s) 0° 27.6 74 20°C 34.0 88 50°C 42.6 114100°C 57.6 182

Does the solubility of solids seem to increase or decrease with an increase in the temperature?

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A. Why would a bottle of carbonated drink

possibly burst (explode) when it is left out

in the hot sun ?

B. Why would fish die in water that gets too warm?

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Solutions

A. Gas in the bottle builds up as the gas becomes less soluble in water at high temperatures, which may cause the bottle to explode.

B. Because O2 gas is less soluble in warm

water, the fish may not obtain the needed

amount of O2 for their survival.