Acids, Bases & Indicators

Acids & Bases

• 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 bases are added to water, they form negative hydroxide ions (OH-)

  • The presence of the OH- ions is what makes the aqueous solution a base (alkali)

• The pH scale is a numerical scale which is used to show how acidic or basic (alkaline) a solution is

  • It is a measure of the amount of the hydrogen ions present in solution

Neutralization

• A neutralization reaction occurs when an acid reacts with a base (alkali)

• When these substances react together in a neutralization reaction, the H+ ions react with the OH- ions to produce water

• For example, when hydrochloric acid is neutralized sodium chloride and water are produced

• Not all reactions of acids are neutralizations

  • 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 neutralization

• The net ionic equation of all acid-base neutralizations and is what leads to a neutral solution, since water has a pH of 7:

H+(aq) + OH- (aq)—> H2O (l)

• Neutralization 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

  
  

Indicators

• Indicators are used to distinguish between acids and bases (alkalis)

• Litmus is very useful as an indicator paper and comes in red and blue papers, for dipping into solutions or testing gases

• Litmus is a naturally occurring indicator and is extracted from lichens

• Synthetic indicators are organic compounds that are sensitive to changes in acidity and appear different colours in acids and bases (alkalis)

• Phenolphthalein and methyl orange are synthetic indicators frequently used in acid-bases titrations

• Synthetic indicators are used to show the endpoint in titrations as they have a very sharp change of colour when an acid has been neutralized by an alkali (base) and vice-versa

• Litmus is not suitable for titrations as the colour change is not sharp and it goes through a purple transition colour in neutral solutions making it difficult to determine an endpoint

Indicator	Colour in acid	Colour in base
Litmus	red	blue
Phenolpthalein	colourless	pink
Methyl orange	red	yellow

The pH scale

• The pH scale is a numerical scale which is used to show how acidic or basic (alkaline) a solution is

  • It is a measure of the amount of the hydrogen ions present in solution

• The pH scale goes from 0 - 14

• All acids have pH values of below 7, all bases (alkalis) have pH values of above 7

• The lower the pH then the more acidic the solution is

  • pH 0-3 => 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 8-10 => weak alkali

  • pH 11-14 => strong alkali

• A solution of pH 7 is described as being neutral

Universal indicator/pH paper

• Universal indicator is prepared by mixing a number of indicators. It shows a different but characteristic colour at each pH.

• Universal indicator is a wide range indicator and can give only an approximate value for pH

• Universal indicator is useful for estimating the pH of an unknown solution

• A few drops are added to the solution and the colour is matched with a colour chart which indicates the pH which matches with specific colours

Please leave a comment below if you want to know more, you have any questions or suggestions.