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-All biological reactions take place in water (aqueous solution).
-Almost all reactions that occur in aqueous solution involve ions.

-3 types of reactions that occur in aqueous solution.

Precipitation reactions

Acid-base reactions

Redox reactions

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Reactions in Aqueous Solution - ppt video online download

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The reaction of aqueous solutions of silver nitrate with sodium chloride to form solid silver chloride and aqueous sodium nitrate is a double-replacement reaction As it is written, this equation represents ionic compounds by their formula units. However, it is more realistically written if you recognize that most ionic compounds dissociate, or separate, into cations and anions when they dissolve in water.

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-Ions are present in water = water is able to conduct electricity

Electrolyte
An electrolyte is a substance that contains free ions and behaves as an electrically conductive medium.

-Ionic solutions = ions in solution (electrolytes)

-Strong electrolytes has many ions present.
-Weak electrolytes has few ions present

-Strong electrolytes are good at conducting electricity, weak are not and non-electrolytes do not conduct electricity at all.

-Conductivity in a aqueous solution is a measure of the ability of water to conduct electricity.

Factors that affect conductivity of electrolytes:

Concentration of ions in solution

Type of substance that dissolves in water

temperature

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-Structure of water particle allows substances to easily dissolve in it.
-Ex. Oxygen dissolves in water so organisms are able to respire.

Dissociation in water
-Water is a polar molecule
-Oxygen side of the molecule has higher electron density than the hydrogen side (more negative).
-Both sides have a slightly negative side and a slightly positive side. Have negative (poles) and positives (poles) chargers.
-Molecule is Dipole

Dissociation of sodium chloride in water
-Polar nature of water allows ionic compounds to dissolve in it.
-Positive (Na +) ions attracted to negative pole of water molecule.
-Negative (Cl-) ions attracted to the positive pole of the water molecule
-When sodium chloride is dissolved in water, polar molecules able to work their way in between the individual ions in the lattice.
-Water molecules surround the negative chloride ions and positive sodium ions and pull them away from the solution.
THIS IS CALLED DISSOCIATION

https://i.stack.imgur.com/JAQvZ.gif

-Dissolution of a substance is when a substance dissociates or dissolves

Dissociation
The general process in which ionic compounds separate into smaller ions, usually in a reversible manner
Dissolution
Dissolution or dissolving is the process where ionic crystals break up into ions in water
Hydration
Hydration is the process where the ions become surrounded with water molecules

CATIONS ARE +vely charged
ANIONS ARE -vely charged

(s) represents Solid Phase
(aq) represents ions in aqueous solution

-Molecular substances may dissolve, but most will not form ions. (Covalent compounds)

-Exceptions form ions when they dissolve (Hydrogen Chloride can ionise in water to from hydronium ions and chloride ions)

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Solutions with free moving ions can conduct electricity.

For electricity to flow ions need to flow through.

Ions in solids are held together very tightly within the crystal lattice.

Covalent compounds do not contain ions and therefore can’t conduct electricity.

Salts dissociate into their ions so that they are free to move in the solution.

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Test for a chloride
- Prepare solution of unknown salt using distilled water and add silver nitrate solution
- If white precipitate forms then the salt is either a chloride or a carbonate
- The precipitate must then be treated with some concentrated nitric acid
- If this precipitate remains unchanged then the salt is a chloride while if CO2 forms and the precipitate disappears then the salt is a carbonate

Test for bromides and iodides
- Bromides and iodides also form precipitates when reacted with silver nitrate
- Silver chloride is a white precipitate but the silver bromide and silver iodide precipitates are both pale yellow
- Chlorine water and carbon tetrachloride are used to determine whether the precipitate is a bromide or iodide
- Chlorine water produces bromine gas from the bromide ions and colours the carbon tetrachloride a reddish brown colour
- Chlorine water produces iodine gas from the  bromide ions and colours the carbon tetrachloride purple

Test for Sulphate
- Barium chloride solution is added to a solution of the test salt and if a white precipitate forms then the salt is either a sulphate or a carbonate
- This precipitate is treated with nitric acid which then allows you to determine whether the precipitate is a sulphate or carbonate

Test for Carbonate
-  If a sample of dry salt is treated with some acid and the production is CO2 - then this is a positive test for carbonate

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- Redox reactions and ion-exchange reactions can also occur in aqueous solutions
- Ion exchange reactions include precipitation reactions, gas forming reactions and acid-base reactions
- Redox reactions are electron transfer reactions

Ion Exchange reactions
- These can be represented by :
AB(aq) + CD(aq) --> AD + CB
- AD or CB can be solid or gas
-  If a solid forms it is a precipitation reaction
- If a gas forms then it is a gas forming reaction
- The formation of a precipitate or a gas helps the reaction happen and it can therefore be said that the reaction is driven by the formation of the fate or precipitate
Definition: An Ion Exchange reaction is a type of reaction where the positive ions exchange their respective negative ions due to a driving force

Gas Forming Reactions
- These are similar to precipitation reactions with the exception that a gas is formed instead of a precipitate
- An example of this is the reaction between sodium carbonate and hydrochloric acid

Acid-Base Reactions
- Acid + Base -- > Salt + Water
- This is a special case of an ion exchange reaction since the sodium in the sodium hydroxide swaps places with the hydrogen in the hydrogen chloride forming sodium chloride
- At the same time the hydroxide and hydrogen combine to form water

Redox Reactions
- Involve exchange of electrons
- The charge of the atoms, ions and molecules can be looked at to determine if a redox reaction has taken place
- IF one of them has become more positive and the other one has become more negative then a redox reaction has taken place

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Sometimes ions react to form a new substance called a precipitate.

Definition: A precipitate is the solid that forms in a solution during a chemical reaction.

Precipitates form when combinations of cations and anions become solids.

Rules for solubility of salts:

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Dissociation - a general process in which ionic compounds separate into smaller ions, usually in a reversible manner.

Dissolution/Dissolving - the process where ionic crystals break up into ions in water.

Hydration/Solvation - the process where ions become surrounded with water molecules, via electrostatic forces.

Electrolyte - a substance that contains free ions, behaving as an electrically conductive medium.

Precipitate - an insoluble solid that forms in a solution during a chemical reaction.

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Dissociation - a general process in which ionic compounds separate into smaller ions, usually in a reversible manner.

Dissolution/Dissolving - the process where ionic crystals break up into ions in water.

Hydration/Solvation - the process where ions become surrounded with water molecules, via electrostatic forces.

Electrolyte - a substance that contains free ions, behaving as an electrically conductive medium.

Precipitate - an insoluble solid that forms in a solution during a chemical reaction.

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Electrolytes are chemicals that break into ions (ionize) when they are dissolved in water. The positively-charged ions are cations; the negatively charged ions are anions.

Substances can be categorized as: s trong electrolytes, weak electrolytes, or non-electrolytes.