Test for Hydrogen & Carbon Dioxide

Specification Point 3.12:
  • Describe the chemical test for:
    a) hydrogen
    b) carbon dioxide (using limewater)
  • Many chemical processes produce gases which then need to be tested.
  • Two common gases are hydrogen and carbon dioxide.

Test for Hydrogen

  • This test involves holding a lighted splint over a test tube or boiling tube of the gas.
  • If it pops with a loud “squeaky pop” then the gas is hydrogen.
  • The noise is produced by the combustion of hydrogen gas with oxygen from the air.

Test for H2

Hydrogen gas in a tube produces a loud “squeaky pop” when in contact with a lighted splint

Test for Carbon Dioxide

  • This test involves bubbling the gas through a solution of limewater, a solution of Ca(OH)2.
  • If the gas is CO2 then the limewater will turn from colourless to cloudy due to the production of CaCO3 which is insoluble in water.

Test for CO2, Edexcel GCSE Chemistry

Limewater turns milky in the presence of CO2

Neutralisation Reactions

Specification Point 3.13:
  • Describe a neutralisation reaction as a reaction between an acid and a base.
  • When acids and bases react together a neutralisation reaction occurs.
  • They are called neutralisation reactions since the resulting solution is pH neutral while the reactants had high (base) and low (acid) pH values.
  • Neutrality occurs at pH 7 and is when the concentration of H+ ions in solutions is equal to the concentration of OH
  • All neutralisation reactions produce a salt and water.
  • Examples:

2HCl +  2NaOH → 2NaCl + 2H2O

H2SO4 + 2KOH → K2SO4 + 2H2O

Explaining Acid-Base Neutralisation

Specification Point 3.14:
  • Explain an acid-alkali neutralisation as a reaction in which hydrogen ions (H+) from the acid react with hydroxide ions (OH–) from the alkali to form water.
  • The chemistry of neutralisation reactions can be explained using ionic equations.
  • Ionic equations are used to show only the particles that chemically participate in a reaction.
  • The other ions present are not involved and are called spectator ions.
  • For example the neutralisation reaction between hydrochloric acid and sodium hydroxide:

HCl(aq) + NaOH(aq) → NaCl(aq) + H2O(l)

  • If we write out all of the ions present in the equation and include the state symbols, we get:

H+(aq) + Cl (aq)+ Na+(aq) + OH(aq) → Na+ (aq)+ Cl(aq) + H2O(l)

  • The spectator ions are thus Na+ and Cl. Removing these from the previous equation leaves the overall net ionic equation:

H+(aq) + OH(aq) →H2O(l)

  • The H+ ions come from the acid and the OH ions come from the base, both combine to form the product water molecules.
  • This ionic equation is the same for all acid-base neutralisation.

Edexcel GCSE Chemistry Notes

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Morgan Curtin Chemistry

Author: Morgan

Morgan’s passion for the Periodic Table begun on his 10th birthday when he received his first Chemistry set. After studying the subject at university he went on to become a fully fledged Chemistry teacher, and now works in an international school in Madrid! In his spare time he helps create our fantastic resources to help you ace your exams.