Chapter 10: Crystals, Ions, and Solutions

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Chapter 10 Crystals, Ions, and Solutions

• Alyssa Jean-Mary

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Solubility

• A solution is an intimate mixture of to or more
  substances
• A solution can involve any of the three states of
  matter.
• Air is a solution of several gases.
• Seawater is a solution of solids and gases in a
  liquid.
• Many alloys, such as brass, are solutions of two
  or more metals.
• A solution always contains two substances
• The solvent is the substance that is present in
  the larger amount.
• The solute is the other substance.
• When solids or gases are dissolved in a liquid,
  the liquid is always considered the solvent,
  while the solids or gases are considered the
  solute.
• Water is the most common and the most effective
  of all solvents.

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Concentration and Solubility

• The concentration of a solution is the amount of
  solute in a given amount of solvent.
• A solution is like a compound in that it is
  homogeneous, but it is unlike a compound since it
  doesnt have a fixed composition.
• For example, if you have the compound NaCl,
  sodium chloride, and you add some more sodium,
  nothing will happen to the sodium if no more
  chlorine is added. But, if you have a solution of
  NaCl in water, and you add some more NaCl,
  without adding some more water, or some more
  water, without adding some more NaCl, the
  concentration of the solution will be affected.
• Some pairs of liquids can form solutions in any
  and all proportions.
• For example, any amount of alcohol can be added
  to any amount of water to form a solution.
• Usually, however, a given liquid will only
  dissolve a limited amount of another substance.
• For example, at 20C, only 36g of NaCl can
  dissolve in 100g of water. No more than this
  amount of NaCl will dissolve, no matter how much
  the solution is stirred.
• The solubility of a substance is the maximum
  amount that can be dissolved in a given quantity
  of a particular solvent at a given temperature
  and pressure.

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Saturated Solutions Solids

• A saturated solution is a solution that contains
  the maximum amount of solute possible
• When the temperature is increased, the solubility
  of most solids will also increase.
• For example, hot tea can dissolve almost twice as
  much sugar as iced tea
• If a solution is saturated at a high temperature,
  when it is allowed to cool, some of the solute
  will crystallize out of solution.
• Sometimes, if a solution that was saturated at a
  high temperature is allowed to cool slowly and
  without disturbance, the solute will remain in
  solution without crystallizing out, even though
  the solubility of the solution has been exceeded.
  This solution is called a supersaturated
  solution. These solutions are quite unstable
  i.e. the solute could crystallize out suddenly if
  the solution is jarred or disturbed in any way.
• A solution usually has a higher boiling point and
  a lower freezing point than the pure solvent
  does.
• For example, seawater boils at 100.3C, which is
  higher than pure water, 100C, and freezes at
  -1.2C, which is lower than pure water, 0C.
• The more concentrated a solution is, the greater
  the boiling point and freezing point changes
  compared to those of the pure solvent.

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Saturated Solutions Gases

• Unlike most solids, whose solubility increases
  with increasing temperature, when the temperature
  is increased, the solubility of gases will
  decrease.
• For example, when you warm soda water, a solution
  of carbon dioxide in water, some of the gas
  escapes as bubbles.
• In addition to this dependence on temperature,
  the solubility of a gas also depends on pressure
  when the pressure is increased, the solubility of
  gases will also increase.
• For example, since soda water is bottled under
  high pressure, when it is opened, the decrease in
  pressure causes some of the gas to escape as
  bubbles from the solution.

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Polar and Nonpolar Liquids

• A substance dissolves better is some liquids than
  in other liquids. The reason for this depends on
  the electrical characters of the solvent and
  solute.
• A polar liquid is one whose molecules behave as
  if they are negatively charged on one end and
  positively charged on the other end. Water is an
  example of a polar liquid. A nonpolar liquid is
  one whose molecules have charges that are evenly
  distributed throughout the molecule. Gasoline is
  an example of a nonpolar liquid.
• Polar liquids consist of groups of molecules,
  instead of single, freely moving molecules. The
  molecules join together in clumps, with the
  positive charges and the negative charges near
  each other. The molecules of polar liquids can
  also join together with the molecules of other
  polar substances in a similar way, dissolving
  them with ease. These molecules cannot join
  together with molecules of a nonpolar substance
  in this way, however, so they dont dissolve
  them. The molecules of a nonpolar substance,
  however, can dissolve in a nonpolar liquid.
• Thus, polar substances dissolve better in polar
  solvents, and Nonpolar substances dissolve better
  in nonpolar solvents i.e. like dissolves like.

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Soaps and Detergents

• Some dirt is soluble in water, but not all of it
  is. A lot of dirt is greasy, and thus nonpolar,
  so it does not dissolve in water.
• Soap molecules are charged on one end and are
  nonpolar on the other end. In water, the soap
  molecules clump together to form clusters called
  micelles. These clusters have the nonpolar ends
  of the soap molecules inside. The dirt particles
  that are nonpolar are absorbed inside these
  micelles. Since the dirt particles are inside the
  micelles, and the outside of the micelles are
  charged (i.e. polar), they are free to move
  through water even though they are nonpolar and
  would normally not be able to move through water.
  Thus, dirt on the skin, clothing, and other
  surfaces that could not be removed by water
  alone, are removed by soapy water.
• In hard water, soap is not as effective because
  the minerals that are dissolved in hard water
  may interfere with the formation of the micelles.
• Synthetic detergents act as soap does, even in
  hard water.

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Dissociation

• Ionic compounds consist of positive and negative
  compounds. Because of this, they only dissolve in
  highly polar liquids.
• Sodium chloride, NaCl, an ionic compound,
  dissolves in water, a highly polar liquid. At the
  surface of a crystal of NaCl, the water molecules
  are attracted to the sodium ions and the chloride
  ions, with the negative side of water (i.e. the
  oxygen) attracting the positive sodium ions and
  the positive side of water (i.e. the hydrogen)
  attracting the negative chloride ions. The pull
  of several molecules of water is enough to
  overcome the electric forces that hold an ion to
  the crystal. The ion then moves into the
  solution, where it is surrounded by water
  molecules. As each layer of ions in a crystal are
  removed, the next layer is attacked. This is done
  until either the salt is completely dissolved, or
  until the solution is saturated. This process,
  the separation of a compound into its ions when
  it is dissolved, is called dissociation. The ions
  that are released into the solution when an ionic
  compound dissolves are the same ions that are in
  the crystal structure. This is true for simple
  compounds as well as for more complex compounds.
  For example, NaCl breaks up into Na and Cl- ions.

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Electrolytes

• Electrolytes are substances that separate into
  ions when they dissolve in water. Electrolytes
  include all ionic compounds that are soluble in
  water as well as some covalent compounds
  containing hydrogen that form ions by reaction
  with water.
• Covalent compounds that are soluble in water, but
  dont dissociate when dissolved in water are
  called nonelectrolytes.
• Electrolytes have the ability to conduct an
  electric current when they are in solution.
  Conduction is possible because since the ions are
  free to move about, the positive ions will move
  towards the negative terminal and the negative
  ions will move towards the positive terminal.

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Ions in Solution 1

• An early objection to the idea of ionic solutions
  was that if sodium chloride breaks down into
  separate particles of sodium and chlorine, then
  the solution should have the greenish-yellow
  color of chlorine, not the colorless solution
  that actually forms. The reason that the solution
  is colorless is because the chloride ion has
  completely different properties that the chlorine
  gas. It has a different color, a different taste,
  different chemical reactions, and many other
  different properties.
• A solution of sodium chloride is not a solution
  of NaCl. It is not a solution of Na atoms and Cl
  atoms. It is a solution of sodium ions, Na, and
  chloride ions, Cl-. Each of these ions in
  solution has its own properties. These properties
  are quite unlike the properties of NaCl crystals.
  They are also quite unlike the properties of the
  active metal sodium and the poisonous gas
  chlorine. Thus, since they have their own
  properties, each ion in an electrolytic solution
  is a new and separate substance.

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Ions in Solution 2

• The properties of an ion are really the
  properties of the solutions in which the ion
  occurs. A solution with only one kind of ion
  cannot be prepared. Thus, a solution always
  contains positive ions and negative ions, so that
  the total number of charges of each sign will be
  the same (i.e. so that the solution is neutral).
  Each ion gives its own characteristic properties
  to all the solutions that contain it. These
  properties can be recognized whenever they are
  not masked by other ions.
• Some examples
• The copper ion, Cu2, has a blue color, and all
  of its solutions are blue, unless, of course,
  another ion is present that has a stronger color.
• The hydrogen ion, H, has a sour taste, and all
  of its solutions are sour (specifically acids).

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Ions in Solution 3

• The differences between the properties of a
  chloride ion and the properties of the neutral
  chlorine atom
• The chloride ion is colorless, has a mild,
  pleasant taste, does not react with metals or
  hydrogen, and forms AgCl with the silver ion,
  Ag.
• On the other hand, the neutral chlorine atom has
  a greenish-yellow color, has a strong, irritating
  taste and odor, combines with all metals and
  readily with hydrogen, and does not react with
  the silver ion, Ag.
• The properties of a solution of an electrolyte
  are the sum of the properties of the ions that
  are contained within that solution. Thus, the
  properties of a solution of sodium chloride are
  the properties of Na plus the properties of Cl-.
  So, instead of learning the individual properties
  of hundreds of different electrolyte solutions,
  we only need to learn the properties of a few
  ions to be able to predict the behavior of any
  electrolytic solution that contains these ions.

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Acids

• Acids produce hydronium ions (H3O) or hydrogen
  ions (H) when they are dissolved in aqueous
  solutions
• They turn blue litmus paper red
• Litmus paper is used as an indicator to identify
  acidic and basic substances
• They are electrolytes
• They neutralize bases by reacting with them to
  produce a salt and water (neutralization
  reaction)
• They have a sour taste

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Bases

• Bases produce the hydroxide ion (OH-) when
  dissolved in aqueous solutions
• They turn red litmus paper blue
• Like acids, they are electrolytes
• They react with acids in neutralization reactions
• They have a bitter taste
• They have a slippery or soapy feel

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General Formulas for Acids and Bases

• Acids
• Hydrogen is present, and is written first
• The second ion is a non-metal ion or a polyatomic
  ion
• HX

• Bases
• The hydroxide ion is present, and is written
  second
• The first ion is a metal ion or the ammonium ion
• MOH

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Strong and Weak Acids and Bases

• Strong acids and strong bases are strong
  electrolytes, meaning that they completely
  dissociate into ions when dissolved in water
• Some strong acids HNO3 (nitric acid), HCl
  (hydrochloric acid), H2SO4 (sulfuric acid)
• Some strong bases Group 1A hydroxides (LiOH,
  NaOH, KOH, RbOH, CsOH, FrOH), Ba(OH)2
• Weak acids and weak bases are weak electrolytes,
  meaning that they dissociate into ions less than
  50 when dissolved in water
• If an acid or a base is not considered a strong
  acid or base, then it is considered a weak acid
  or base

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pH

• The pH scale expresses acid and base
  concentrations
• It ranges from 0 to 14
• pH 0 to 7 are acids
• pH 7 are neutral solutions
• pH 7 to 14 are bases