Chemistry Topics 6&7

Chemistry Topics 6&7

1. pH

   • The pH scale is a measure of the relative acidity or alkalinity of a solution
     • Allows you to see how acid/alkaline or neutral the solution
       • Smaller the pH = More Acidic
       • Greater the pH = more alkaline
     • Can be measured using indicator solution
       • Compare the colour using the standard chart
     • Also measured using pH paper
       • This is quite handy for testing soil mixed and shaken with water.
         • You can get special soil testing kits which use indicator solution and the colour of the indicator in the water is matched with a chart after the soil has settled out.
     • pH can be very accurately measured with a special instrument called a pH meter using a glass electrode probe

2. Acid-Base Theory

   • Water is a neutral liquid with a pH of 7
     • When a substance dissolves in water it forms an aqueous (aq) solution that may be acidic, neutral or alkaline.
     • Neutral solutions have a pH of 7. These are quite often solutions of salts, which are themselves formed from neutralising acids and bases.
     • The 'opposite' of an acid is called a base. Some bases are soluble in water to give alkaline solutions – these are known as alkalis.
     • Alkaline solutions have a pH of over 7
   • NEUTRALISATION usually involves mixing an acid (pH <7) with a base or alkali (pH > 7) which react to form a neutral SALT solution of pH ~7

3. Making insoluble salts

   • To make an insoluble salt, two soluble salts need to react together in a precipitation reaction.
   • We can see from the table that silver chloride is an insoluble salt. It can be made by reacting a soluble silver salt with a soluble chloride salt.

4. Making Soluble Salts

   • Soluble salts can be made by reacting acids with either soluble or insoluble bases.
   • If you are using an alkali - which is a soluble base - then you need to add just enough acid to make a neutral solution
     • Warm the salt solution to evaporate the water. You get larger crystals if you evaporate the water slowly.

5. Electrolysis

   • Ionic substances contain charged particles called ions.
   • Electrolysis is the process by which ionic substances are decomposed (broken down) into simpler substances when an electric current is passed through them.
   • the ions must be free to move
     • Ions are free to move when an ionic substance is dissolved in water or when melted.
   • During Electrolysis
     • Positively charged ions move to the negative electrode during electrolysis.
       • They receive electrons and are reduced.
         • Negatively charged ions move to the positive electrode during electrolysis.
           • They lose electrons and are oxidised.
             • The substance that is broken down is called the electrolyte.

6. Electroplating

   • Electrolysis is used to electroplate objects. This is useful for coating a cheaper metal with a more expensive one, such as copper or silver.
   • How it works
     • The negative electrode should be the object that is to be electroplated
       • The positive electrode should be the metal that you want to coat the object with
         • The electrolyte should be a solution of the coating metal, such as its metal nitrate or sulfate

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