Properties of Acids & Bases (Cambridge O Level Chemistry)

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Properties of Acids

  • Acids have pH values of below 7, have a sour taste (when edible) and are corrosive
  • Acids are substances that can neutralise a base, forming a salt and water
  • When acids are added to water, they form positively charged hydrogen ions (H+)
  • The presence of H+ ions is what makes a solution acidic

 Example: Hydrochloric Acid

HCl (aq)   →    H+ (aq)    +    Cl- (aq)

Typical reactions of acids

Acids and metals

  • Only metals above hydrogen in the reactivity series will react with dilute acids.
  • When acids react with metals they form a salt and hydrogen gas:

Acid    +    Metal   →    Salt    +    Hydrogen

  • The name of the salt is related to the name of the acid used, as it depends on the anion within the acid.
  • Examples of the names of salts from specific acids and metals are:

O_exjhH7_7-1-1-acid-and-metal-names

 

Acids with bases

  • Metal oxides and metal hydroxides (alkalis) can act as bases
  • When they react with acid, a neutralisation reaction occurs
  • In all acid-base neutralisation reactions, salt and water are produced

Acid    +    Base   →    Salt    +    Water

  • Examples of reactions between acids and bases:

7-1-1-acids-and-base-reactions

 Acids with metal carbonates

  • Acids will react with metal carbonates to form the corresponding metal salt, carbon dioxide and water:

Acid  +  Metal Carbonate → Salt  +  Carbon Dioxide  +  Water

  • Examples of reactions between acids and carbonates:

7-1-1-acids-and-base-reactions-1

Indicators

  • Two colour indicators are used to distinguish between acids and alkalis
  • Many plants contain substances that can act as indicators and the most common one is litmus which is extracted from lichens
  • Synthetic indicators are organic compounds that are sensitive to changes in acidity and appear different colours in acids and alkalis
  • Thymolphthalein and methyl orange are synthetic indicators frequently used in acid-alkali titrations

Two Colour Indicators Table

7-1-1-indicators

  • Synthetic indicators are used to show the endpoint in titrations as they have a very sharp change of colour when an acid has been neutralised by alkali 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
  • Litmus is very useful as an indicator paper and comes in red and blue versions, for dipping into solutions or testing gases

Properties of Bases & Alkalis

  • Bases have pH values of above 7
  • A base which is water-soluble is referred to as an alkali
  • In basic (alkaline) conditions red litmus paper turns blue, methyl orange indicator turns yellow and thymolphthalein indicator turns blue
  • Bases are substances which can neutralise an acid, forming a salt and water
  • Bases are usually oxides or hydroxides of metals
  • 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

Example: Sodium Hydroxide

NaOH (s)   →    Na+ (aq)    +    OH- (aq)

Typical reactions of bases

Bases and acids

  • When bases react with an acid, a neutralisation reaction occurs
  • Acids and bases react together in a neutralisation reaction and produce a salt and water:

Acid    +    Base   →    Salt    +    Water

Examples of reaction between bases and acids:

7-1-1-acids-and-base-reactions

 Alkalis and ammonium salts

  • Ammonium salts undergo decomposition when warmed with an alkali
  • Even though ammonia is itself a weak base, it is very volatile and can easily be displaced from the salt by another alkali
  • A salt, water and ammonia are produced

Example:

NH4Cl + NaOH →NaCl + H2O + NH3

  • This reaction is used as a chemical test to confirm the presence of the ammonium ion (NH4+)
  • Alkali is added to the substance with gentle warming followed by the test for ammonia gas using damp red litmus paper
  • The damp litmus paper will turn from red to blue if ammonia is present

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Alexandra Brennan

Author: Alexandra Brennan

Expertise: Chemistry

Alex studied Biochemistry at Newcastle University before embarking upon a career in teaching. With nearly 10 years of teaching experience, Alex has had several roles including Chemistry/Science Teacher, Head of Science and Examiner for AQA and Edexcel. Alex’s passion for creating engaging content that enables students to succeed in exams drove her to pursue a career outside of the classroom at SME.