Some more on solutions

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Some more on solutions

• Concept map
• please use the following words to create a
  concept map relating to solutions...

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words

• Solvent
• Solute
• Solutions
• Dissolving
• Dynamic equilibrium
• Crystalizing
• Temperature
• Pressure
• volume

• Be creative, you have 8 circles and a square to
  work with!

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Aqueous Solutions

• since water dissolves many substances easily,
  pure water never exists in nature
• water can dissolve other polar molecules and
  ions
• water containing dissolved substances are called
  aqueous solutions
• the dissolving medium is the solvent (what does
  the dissolving) usually H20

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Cont'd

• the dissolved particles are solute (what
  dissolves)?
• examples
• sodium chloride (salt) ? NaCl(s)?
• sodium chloride dissolved in water ? NaCl (aq)?
• solutions are homogeneous mixtures and are stable

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Cont'd

• solute particles may be ionic or molecular. They
  are usually polar, because non-polar molecules
  will not dissolve in water. However, solute
  particles will dissolve in other solvents. Ex.
  Grease in gasoline

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Solvation

• Water molecules, in continuous motion, collide
  with the solute particles placed in it. As a
  result, solvent molecules attract the solute
  particles causing the individual particles to
  break away from each other. This causes the
  crystal structure to break down and dissolving
  occurs.

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more

• particles become solvated ie. Surrounded by
  solvent molecules, due to attractions between the
  solute and solvent particles
• if the attractive forces between the particles in
  the crystal are greater than those with water the
  particles cannot be solvated these compounds
  are insoluble in water eg. Barium sulphate or
  calcium carbonate

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continued

• nonpolar solvents such as gasoline will dissolve
  in non-polar compounds not because of attractive
  forces, but rather due to the lack of repulsion
  between the solvent and solute molecules
• General rule Like Dissolves Like!

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Solution Formation

• Four factors affect the rate at which a solute
  dissolves
• 1. Temperature (heat)?
• 2. Agitation (stirring)?
• 3. Particle Size
• 4. Nature of the particle (solubility)?

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• Temperature affects the amount of solute that can
  dissolve
• Stirring, particle size, and temp affect the rate
  at which the solute will dissolve

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example

• When 36 g of sodium chloride are added to 100g of
  water at 25º C, all the sodium chloride will
  dissolve. However, if one more gram of salt is
  added it will not dissolve no matter how
  vigorously it is stirred. Since water particles
  are in continuous motion they must collide with,
  and solvate, the solute particles

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explained..

• The explanation
• Particles are bombarded by water molecules. The
  water will release one particle as it collides
  with another. In turn, the water molecules pick
  up the new particle.

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• There is no net change in the overall system as
  particles move from solution to solid and back
  again.
• A state of dynamic equilibrium exists between the
  solution and the undissolved solute.
• It's like stuffing people in the gymnasium,
  people out people in... this is dynamic
  equilibrium

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• The sodium chloride solution is said to be
  saturated
• A saturated solution contains
• The maximum amount of solute that dissolves in a
  given quantity of a solvent at a given temp
• An unsaturated solution contains
• Less solute than a saturated solution

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Solubility

• This is the maximum amount of solute that can
  dissolve in a particular volume of solvent at a
  particular pressure and temperature
• solute is expressed in grams of solute/100 mL of
  solvent
• every pure substance has its unique solubility

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Solubility ranking

• Solubility Table a reference for how ionic
  compounds will behave when added to water
• - based on the cut off points between very
  soluble (0.1gtmol/L ) to slightly soluble (lt 0.1
  mol/L)?

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• Comparing the Solubility of Different Phases of
  Matter (in an aqueous environment)?

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Solids as a rule..

• Solubility of Solids
• Have higher solubility in water at higher
  temperatures

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Gases as a rule...

• Solubility of Gases
• Have higher solubility in water at lower
  temperatures (opposite that of solids)?
• Have higher solubility in water at higher
  pressures

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Liquids as a rule...

• Solubility of Liquids
• For polar liquids, the solubility usually
  increases with temperature
• Mostly non-polar liquids, do not dissolve in
  water to any appreciable extent but form a
  separate layer. Liquids that behave in this way
  are said to be immiscible
• Examples
• Olive oil and water

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sometimes...

• Some liquids (such as those containing small
  polar molecules with hydrogen bonding (methanol
  for example), dissolve completely in water (never
  become saturated) and are said to be miscible
• Examples
• Ethanol and soda pop

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Elements as a rule...

• Elements
• Generally have low solubility in water
• Halogens and oxygen dissolve in water to only a
  very tiny extent but they are so reactive that
  even in tiny concentrations, they are often very
  important in solution reactions

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check

• Based on our knowledge of solution formation and
  solubility,
• What happens when the temperature of a saturated
  solution is raised? Can we add more solute or
  will excess solute become undissolved?

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answer

• We can add excess solute to the solution and it
  will dissolve

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From last week...

• What is a supersaturated solution?

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answer...

• A solution which contains more solute than it can
  theoretically hold
• Ex Perrier or beer (CO2 in water)?

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crystallization..

• Crystallization can be initiated in a
  supersaturated solution
• A supersaturated solution has more solute than a
  saturated solution, and yet the solute is able to
  remain in solution. It is UNSTABLE. If a few
  crystals of solute are added to serve as
  particles on which crystallization can occur, the
  excess solute crystallizes quickly.

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Practice Questions

• Give examples of two liquids that are immiscible
  and two that are miscible with water.
• Can more oxygen dissolve in a litre of water in a
  cold stream or a litre of water in a warm lake?
  Include your reasoning.
• State why you think clothes might be easier to
  clean in hot water.
• Why do carbonated beverages go flat when opened
  and left at room temperature and pressure?

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• a) The solubility of oxygen in blood is much
  greater than its solubility in pure water.
  Suggest a reason for this observation.
• b) If the solubility of oxygen in blood were the
  same as in pure water, how would your life be
  different?

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• c) Is there an advantage for animals that are
  cold blooded? Explain briefly.
• For any solute, what important condition must be
  stated in order to report the solubility

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problem...

• 1. In a chemical analysis experiment, a student
  notices that a precipitate has formed, and
  separates this precipitate by filtration. The
  collected liquid filtrate, which contains aqueous
  sodium bromide, is set aside in an open beaker.
  Several days later, some white solid is visible
  along the top edges of the liquid and at the
  bottom of the beaker.
• What does the presence of the solid indicate
  about the nature of the solution?

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equlibrium

• Write a brief theoretical explanation for this
  equilibrium mixture.
• State two different ways to convert the mixture
  of the solid and solution into a homogeneous
  mixture.

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Fishy thinking...

• Different species of fish are adapted to live in
  different habitats. Some, such as carp, can
  survive perfectly well in relatively warm, still
  water. Others, such as brook trout, need cold,
  fast-flowing streams, and will die if moved to he
  carps habitat.
• Describe and explain the oxygen conditions in the
  two habitats.

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More fish

• Hypothesize about the oxygen requirements of the
  two species of fish.