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Acids, Bases, Salts

Acids and Bases

Acid Base
Turns blue litmus red Turns red litmus blue
Turns purple cabbage indicator red Turns purple cabbage indicator green
pH: less than 7 pH: greater than 7
Corrosive Corrosive
Sour taste Flat taste
Proton donor Proton acceptor
Soluble in water Alkali: a base that is soluble in water


Acid + Base --> Salt + Water


Everyday Acids Everyday Bases
Vinegar: ethanoic acid Washing Soda: sodium carbonate
Vitamin C: ascorbic acid Baking soda: sodium hydrogen carbonate
Sour Milk:lactic acid Milk of Magnesia: magnesium hydroxide
Laboratory Acids Laboratory Bases
Hydrochloric: HCl Sodium Hydroxide: NaOH: Soda Lime
Sulphuric: H2SO4 Calcium Hydroxide: Ca(OH)2: Limewater


Acid + Metal --> Salt + Hydrogen
Acid + Carbonate --> Salt + Water + Carbon Dioxide


pH Scale
A scale from 0 to 14 is used to measure how acidic or how basic is the solution.
pH 7 Neutral solution - not acidic, not basic
pH < 7 Acidic - the lower the pH below 7, the more acidic the solution
pH > 7 Basic - the higher the pH above 7, teh more basic the solution

Indicator
An indicator is a substance that shows by a clear change in colour that a chemical reaction has been completed.

Some uses of indicators:

  • Show the presence of a substance e.g. iodine for starch.
  • Show if a solution is acidic, basic or neutral.
  • Show the pH of a solution.  

(i) Litmus
An acid-base indicator whose colour in acid is different to that in a base.
Acid: litmus is red. Base: litmus is blue.

Checking a solution to see if it is acidic or bases is usually done with litmus paper.
This is absorbent paper soaked in a solution of litmus and let to dry.

(ii) Universal Indicator
An indicator to measure the pH of a solution.
A mixture of different indicator dyes producing different colours at different pHs.
Used as a liquid or test paper.
The container has a pH-colour chart.

Place a drop of the solution onto the pH test paper.
Match the colour produced to a colour on the chart.
The number on the chart’s matching colour is the pH of the solution.


Making an acid-base indicator using ‘red’ cabbage
The leaves of this variety of cabbage have a strong purple colour.
Tear a few leaves up into small pieces.
Boil these leaves in a small quantity of water.
Decant the purple solution into a clean dry beaker.
This purple solution is an acid-base indicator.

Red in an acid; Green in a base; Purple if the solution is neutral.
 

Neutralisation
The reaction of an acid and a base in suitable quantities such that the salt solution formed is neutral, pH 7.

Everyday neutralisation reactions:

  • Indigestion, reflux of acid material from the stomach, is treated with a base.
  • Acid soils treated with lime.
  • Toothpaste counteracts the acid plaque on our teeth.


Laboratory Experiment: producing sodium chloride salt by neutralisation
Acid   +   Base    ->  Salt   +  Water
HCl    +   NaOH   ->  NaCl  +   H2O

  1. Set up a burette with hydrochloric acid.
  2. Using a pipette place 20 cm3 of sodium hydroxide solution in a beaker.
  3. Add a few drops of litmus indicator to the sodium hydroxide solution.
  4. The sodium hydroxide solution is now blue.
  5. While swirling the beaker slowly run the acid into NaOH solution.
  6. When the blue litmus indicator turns red stop adding the acid.
  7. Note the volume of acid added to neutralise the base.
  8. Repeat exactly without the indicator.
  9. A pure sodium chloride solution is in the beaker.
  10. Pure sodium chloride salt remains when the water is evaporated.


Salts

A salt is an ionic compound composed of a metal ionically bonded to a non-metal.

A salt is formed by the replacement of the hydrogen of an acid by a metal.

Metal  +  Acid          ->  Salt   +  Hydrogen
Acid   +   Base         -> Salt   +  Water
Acid   +  Carbonate  ->  Salt   +  Water  +  Carbon Dioxide  


Metal  +  Acid  ->  Salt   +  Hydrogen
1.  Zinc    +  Hydrochloric Acid  ->  Zinc Chloride  +  Hydrogen
2.  Zinc    +  Sulphuric Acid      ->  Zinc Sulphate +  Hydrogen
3.  Magnesium   +  Sulphuric Acid      ->  Magnesium Sulphate  +  Hydrogen


1. Zn  +  2HCl      ->  ZnCl2   +  H2
2. Zn  +  H2SO4   ->  ZnSO4  +  H2
2. Mg  +  H2SO4   ->  MgSO4  +  H2

Note: different salts are produced by changing the metal or the acid.


Acid   +   Base  ->  Salt   +  Water
1.  Hydrochloric Acid  +  Sodium Hydroxide  ->  Sodium Chloride  +  Water
2.  Sulphuric Acid  +  Sodium Hydroxide  ->  Sodium  Sulphate +  Water

1.  HCl  +   NaOH   ->  NaCl      +  H2O
2.  H2SO4 +  2NaOH  ->  Na2SO4  +  H2O

Note: different salts are produced by changing the acid (or the base).


Acid   +  Carbonate  ->  Salt   +  Water  +  Carbon Dioxide  
1.  Hydrochloric Acid  +  Calcium Carbonate  -> Calcium Chloride + Water + Carbon Dioxide
2.  Sulphuric Acid  +   Sodium Sulphate    -> Sodium Sulphate + Water + Carbon Dioxide

1.  2HCl   +  CaCO3   ->  CaCl2    +  H2O  +  CO2
2.  H2SO4  +  Na2CO3  ->  NasSO4 +  H2O  +  CO2

Note: different salts are produced by changing the acid or the carbonate.

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