The pH scale & neutralisation

• • When acids are added to water, they form positively charged hydrogen ions (H+)
  • The presence of H+ ions is what makes a solution acidic
• • When alkalis are added to water, they form negative hydroxide ions (OH–)
  • The presence of the OH– ions is what makes the aqueous solution an alkali
• • The pH scale is a numerical scale which is used to show how acidic or alkaline a solution is
  • It is a measure of the amount of the hydrogen ions present in solution
• • The pH scale goes from 1 – 14 
  • All acids have pH values of below 7
  • All alkalis have pH values of above 7
• • The lower the pH then the more acidic the solution is
  • pH 1-3 = strong acid
  • Extremely acidic substances can have values of below 1
  • pH 4-6 = weak acid
• • The higher the pH then the more alkaline the solution is
  • pH 8-11 = weak alkali
  • pH 12-14 = strong alkali
  • A solution of pH 7 is described as being neutral
• • An indicator is a substance which changes colour depending on the pH of the solution to which it is added
• • Universal indicator is a dye and when it is added to a solution, it changes colour depending on the solution's pH level. it is a wider range indicator which means it changes colour across a wider range of pH
  • A few drops are added to the solution and the colour change of the solution is matched with a colour chart which indicates the pH which matches with specific colours
• A common error is to suggest using universal indicator as a suitable indicator for an acid-base titration.

  This is incorrect as a sharp colour change is required to identify the end-point ( when you know the titration is complete),which cannot be achieved with Universal Indicator.
• • A neutralisation reaction occurs when an acid reacts with an alkali
  • Acids are a source of hydrogen ions, H+
  • Bases (or alkalis) are a sources of  hydroxide ions, OH– 
  • When they react together in a neutralisation reaction, the H+ ions react with the OH– ions to produce water
• • This is the net ionic equation of all acid-base neutralisations and is what leads to a neutral solution, since water has a pH of 7:

  H+  (aq) + OH– (aq)⟶ H2O (l)
• • Not all reactions of acids are neutralisations
  • For example when a metal reacts with an acid, although a salt is produced there is no water formed so it does not fit the definition of neutralisation
• • Neutralisation is very important in the treatment of soils to raise the pH as some crops cannot tolerate pH levels below 7
  • This is achieved by adding bases to the soil such as limestone and quicklime
• an acid is any substance that forms an aqueous solution with a pH of less than 7
• bases are any substance with a pH more than 7. Alkalis are a base that dissolve in water to form a solution with a pH greater than 7
• a pH probe can be used to measure pH of a substance by connecting the probe to a pH metre. By dipping the probe into a solution, we can electronically measure the PH by getting an accurate numerical reading on the metre. A probe is much more accurate and precise compared to a universal indicator because it dosen't involve humans guessing shades or colours.
• Bases like metal oxides and carbonates can neutralise acids without dissolving. Just because they can’t dissolve in water doesn’t mean they cant form a neutralisation reaction
• A base that doesn’t dissolve in water ( not alkali) Will not produce hydroxide ions OH minus