4a) Acids and Alkalis

Indicators:

1. Universal indicator is a very useful combination of dyes which give one of the colours on the pH scale
2. Litmus paper tests whether a solution is acidic or alkaline because it changes colour at pH 7.  
1. Red in acidic solutions 
2. Blue in alkaline solutions 
3. Purple in neutral solutions 
3. Phenolphthalein will change from colourless in acidic solutions to bright pink in alkaline solutions 
4. Methyl Orange changes from red in acidic solutions to yellow in alkaline solutions. 

pH Scale:

+----+---------------------------+---------+--------------------------------+
|    |           Acids           | Neutral |            Alkaline            |
+----+---------------------------+---------+--------------------------------+
| pH | 0 | 1 | 2 | 3 | 4 | 5 | 6 |    7    | 8 | 9 | 10 | 11 | 12 | 13 | 14 |
+----+---+---+---+---+---+---+---+---------+---+---+----+----+----+----+----+

Acids;

• below pH scale 
• produce hydrogen ions, H+ when dissolved in water 
• acids + metal ----> salt and hydrogen 
• acids + metal oxides ----> salt and water 
• acids + metal carbonates ----> salt and water and carbon dioxide 

Common acids and salts produced

-hydrochloric acid to chloride

-nitric acid to nitrates 

-ethanoic acid to ethanoates 

Alkalis:

• above pH 7 
• produce OH- (hydroxide) ion when dissolved into water
• alkalis + acids ----> salt and water 

Common alkalis 

-sodium hydroxide 

-ammonia solution 

General rules for predicting the solubility of salt in water:

• All common sodium, potassium and ammonium salts are soluble 
• All nitrates are soluble 
• Common chlorides are soluble, except silver chloride 
• Common sulfates are soluble, except those of barium and calcium
• Common carbonates are insoluble, except those of sodium, potassium and ammonium 

Preparing soluble salts from acids and insoluble bases:

• Add excess solid to ensure that all the acid has reacted 
• Filter it 
• Evaporate half water 
• Leave to crystallise 

Insoluble salts using precipitation reactions:

• Mix the two solutions which contain the ions you need 
• filter precipitate
• wash 
• dry

Acid-Alkais titration:

• Using a pipette and pipette filler, add some alkali to a conical flask, along with two or three drops of indicator 
• Fill a burette with acid 
• Using the burette, add the acid to the alkali a bit at a time- giving the conical flask a regular swirl. Go especially slowly when you reach the end point of the colour change of the indiactor
• The indicator changes colour when all the alkali has been neutralise 
• Record the volume of acid used to neutralise the alkali, it's best to repeat the process several times to make sure you have reliable results.