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Solutions
This kind…
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Section 15.1
Forming Solutions
1. To understand the process of dissolving2. To learn why certain substances dissolve in water3. To learn qualitative terms describing the
concentration of a solution 4. To understand the factors that affect how fast a
solid dissolves
Objectives
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Section 15.1
Forming Solutions
What is a solution?
Solution – homogeneous mixture Solvent – substance present in largest amount Solutes – other substances in the solution Aqueous solution – solution with water as the
solvent
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Section 15.1
Forming Solutions
ExampleState of Solution
Original State of Solute
State of Solvent
Air, natural gas Gas Gas Gas
Antifreeze in water
Liquid Liquid Liquid
Brass Solid Solid Solid
Carbonated water (soda)
Liquid Gas Liquid
Seawater, sugar solution
Liquid Solid liquid
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Section 15.1
Forming Solutions
A. Solubility – Ionic Substances
Ionic substances breakup into individual cations and anions.
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Section 15.1
Forming Solutions
A. Solubility of Ionic Substances
Polar water molecules interact with the positive and negative ions of a salt.
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Section 15.1
Forming Solutions
Solubility of Polar Substances
Ethanol is soluble in water because of the polar OH bond.
Polar water
molecule
Polar ethanolMolecule
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Section 15.1
Forming Solutions
A. Solubility – Polar Substances
Why is solid sugar soluble in water?Polar substances are hydrophilic.
Polar OH bonds in
sugar molecule
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Section 15.1
Forming Solutions
A. Solubility – Substances Insoluble in Water
Nonpolar oil does not interact with polar water nonpolar substances are hydrophobic.
Water-water hydrogen bonds keep the water from mixing with the nonpolar molecules.
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Section 15.1
Forming Solutions
A. Solubility – How Substances Dissolve
A “hole” must be made in the water structure for each solute particle.
The lost water-water interactions must be replaced by water-solute interactions.
“like dissolves like” – polar solutes dissolve in polar solvents, nonpolar solutes dissolve in nonpolar solvents
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Section 15.1
Forming Solutions
What kind of bear is hydrophilic?
A Polar Bear!
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Section 15.1
Forming Solutions
Solution Composition: Concentration
• Amounts of substances can vary in different solutions. – Concentration - amount of solute in relation to solvent– Qualitative measures of concentration
• concentrated – relatively large amount of solute
• dilute – relatively small amount of solute
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Section 15.1
Forming Solutions
Solution Composition: Concentration
Which solution is more concentrated? B
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Section 15.1
Forming Solutions
Solution Composition: Concentration
Which solution is more concentrated? Neither
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Section 15.1
Forming Solutions
The solubility of a solute is limited. Saturated solution – contains as much solute as will dissolve at that temperature
Unsaturated solution – has not reached the limit of solute that will dissolve at that temperature
Solution Composition: How much solute can you dissolve in the solvent?
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Section 15.1
Forming Solutions
Interpreting Solubility Curves
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Section 15.1
Forming Solutions
Supersaturated solution – occurs when a solution is saturated at an elevated temperature and then allowed to cool but all of the solid remains dissolved
• Contains more dissolved solid than a saturated solution at that temperature
• Unstable – adding a crystal causes precipitation
B. Solution Composition: An Introduction
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Section 15.1
Forming Solutions
Making a Supersaturated Solution
At 20oC, a solution containing ~19g Na2SO4 dissolved in it would be
saturated. To dissolve more solute, the temperature has to
be increased.
If we dissolve more solute at a higher
temperature, then cool the solution slowly so the
extra solute stays dissolved in it, the
solution will be supersaturated.
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Section 15.1
Forming Solutions
Supersaturated SolutionYouTube Video
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Section 15.1
Forming Solutions
Factors Affecting the Rate of Dissolving Increasing the number of collisions between solute and
solvent particles increases the rate (speed) at which a solid dissolves.
Three factors that speed up dissolving:– Increasing Surface area – dissolving occurs at the
surface the greater the surface area exposed to solvent, the faster the dissolving processEx.: A sugar cube will dissolve faster if it is ground to small crystals, increasing the total surface area.
– Stirring – removes newly dissolved particles from the surface of the solid and continuously exposes the surface to fresh solvent.
– Increasing Temperature – higher temperatures cause molecules to move more rapidly; in addition, most solids are more soluble at higher temperatures*.Ex.: More sugar will dissolve in hot water than in cold water.
*For gases dissolved in water, the opposite is true: the solubility of gases in water typically decreases as the temperature increases.
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Got problems?Don’t Worry…
Chemistry has SOLUTIONS!