Tests for Chemistry

• To test for sulfate ions, add a few drops HCl to a test tube, to remove carbonate impurities.

  Add a few drops of $BaCl_2$ to the solution. If $SO_4^{2-}$ is present, a white ppt. of $BaSO_4$is formed
• $Mg^{2+}$ is a bright white flame.


  $Ca^{2+}$is a brick red/orange flame

  $Sr^{2+}$is a red/crimson flame

  $Ba^{2+}$is a pale green/yellow green flame
• Testing for halides is done by this method:

  Add a few drops of $AgNO_3$ solution, then dilute $NH_3$:

  $F^-$: A dissolved ppt. of AgF is formed

  $Cl^-$: A white AgCl ppt is formed. High solubility in NH3

  $Br^-$: A cream AgBr ppt is formed. Medium solubility in NH3

  $I^-$: A yellow AgI ppt is formed. Low solubility in NH3
• The test for carbonate ions:

  
  

  Add dilute HCl.

  If CO3- is present, effervescense of Co2 is observed.
• The test for $NH_4^{+}$ions is:

  Add NaOH solution, if $NH_4^{+}$is present a pungent odour is formed as $NH_3$gas is produced.

  Holding a damp red litmus paper to the test tube will turn blue if $NH_3$is present
• The test for $OH^-$ions can be done in two ways:

  Method 1: Use a pH meter probe, if OH- is present then pH is above 7

  Method 2: Add drops of dilute HCl: if OH- present effervescence is observed as a salt and water are formed
• Test for alkenes: add drops of bromine water, and the solution will change colour from orange brown to colourless if a C=C bond is present.
• Tollens reagent:

  • Add a small amount of $NaOH_{aq}$to $AgNO_{3(aq)}$
  • This forms a brown ppt. of $Ag_2O$. $NH_3$is added until the ppt. dissolves to form $[Ag(NH_3)_2]^+$$_{(aq)}$
  • The test substance is added to this test tube. If an aldehyde is present, a silver mirror is formed of metallic silver. Ketones don't react with tollens reagent.
• Fehlings solution:

  • A blue, alkaline solution of Cu(II)ions.
  • This is added to the test sample, and warmed in a water bath
  • Ketones provide no colour change, but aldehydes will change the solution from blue to brick-red
  • $Cu^{2+}$ is reduced to $Cu^+$, forming a ppt of $Cu_2O$
• Test for alcohols:

  • Add sodium metal
  • If an alcohol is present, effervescense occurs and $H_2$gas is produced.

  $\textbf{OR}$

  • Add acidified potassium dichromate
  • $1^o, 2^o$alcohols will turn orange to green
• Test for carboxylic acids:

  Add test sample to solid $Na_2CO_3$ (sodium carbonate)

  If carboxylic acid present, CO2 gas is formed which can be tested
• The test for $CO_2$ gas:

  Bubble the gas through limewater. If the gas is CO2, the limewater turns cloudy (a white ppt of $CaCO_3$)