The kids took 25 ml samples of bottled and tap water in Erlenmeyer flasks. Then, they added five drops of phenolphthalein and checked whether they could notice some pink color, which would mean that there were carbonates in water. To find out the miligrams per litre of carbonate they would have to pour drops of 0,1 M HCl_(aq) from a burette in the flasks until the pink color disappeared. We can calculate the mg/L of carbonate after the spent volume of HCl_(aq), as the moles of carbonate are the same as the reacted moles of HCl.

The test went on by adding five drops of methyl orange and titrating with HCl_(aq) until the color of the dissolution changed from orange to red. The moles of bicarbonate measured this way include those that previously were in the form of carbonate, so they have to be substracted in order to find out the moles of bicarbonate in the sample of water.

 

Average: 122 mg/L

In the label of the bottle can be read 108 mg/L

Kids took 5 ml samples of tap and bottled water and poured them in test tubes. They added three drops of a 10 % BaCl₂ aqueous dissolution to each sample and some white turbidity was noticed, more visible in tap water. When heated to boiling for around one minute at the flame of a Bunsen burner the white solid was observed at the bottom of the test tubes, even more noticeable in tap water. The white precipitate could be either barium sulfate or carbonate. While barium carbonate is soluble in 0,1 M HCl (aq) sulfate is not. Adding 10 drops of hydrochloric acid dissolution to each sample white solid and turbidity disappeared, so we concluded there were no sulfates in our water samples.

This test is based on the lower solubility in water of the barium carbonate and sulfate than that of chloride. Chemical reactions can be written as:

 

 

 

Carbonate and bicarbonate are in equillibrium in aqueous dissolutions. At pH = 7 the equilibrium is largely shifted in favor of bicarbonate.