Identifying Washing Soda and Baking Soda

This experiment aims to identify washing soda among the given samples of chemicals.

Baking soda (NaHCO­₃) and washing soda (Na₂CO₃.10H₂O) are white sodium salts of hydrogen carbonate and carbonate, respectively. Their aqueous solutions are alkaline and turn red litmus blue.

• CO₃^2- (aq) + H₂O (l) → H₂CO₃(aq) + 2OH^– (aq)
• HCO₃^– (aq) + H₂O (l) → H₂CO₃ (aq) + OH^– (aq)

Salts of hydrogen carbonates and carbonates react with dilute acids to liberate carbon dioxide that reacts with lime water to form calcium carbonate. Insoluble calcium carbonate gives a milky appearance to lime water.

• Na₂CO₃ (s) + 2HCl (aq) → 2NaCl (aq) + H₂O (l) + CO₂ (g)
• NaHCO₃ (s) + HCl (aq) → NaCl (aq) + H₂O (l) + CO₂ (g)
• Ca(OH)₂ (aq) + CO₂ (g) → CaCO₃ (s) + H₂O (l)

Clear Lime Water                Milky 

On passing excess CO2 through lime water, the formed calcium carbonate is converted into calcium hydrogen carbonate. Calcium hydrogen carbonate is soluble in water, and hence, the milkiness disappears.

• CaCO₃ (s) + H₂O (l) + CO₂ (g) → Ca(HCO₃)₂ (aq)

Apparatus and Materials Required

Apparatus:

• Thistle funnels
• Delivery tubes
• Test tubes containing lime water
• Boiling tubes
• Retort stands
• Test tube holder
• Watch glasses
• Glass rods
• Two-holed rubber stoppers
• Red litmus paper
• Spatulas

Chemicals

• Unknown samples (copper sulphate, sodium acetate, sodium chloride, washing soda)
• Dilute hydrochloric acid
• Lime water

Procedure

The following steps to follow for the procedure:

You are given four unknown samples, ‘A’, ‘B’, ‘C’ and ‘D’. You have to identify the sample that is washing soda. (Note that baking soda is not one of the samples.)

First, perform the preliminary tests to identify the samples that could be washing soda.

Colour Test: 

Record the colour of the four unknown samples given to you.

Perform the next steps only on samples that are white in colour, as washing soda is white in colour.

Prepare aqueous solutions for the remaining samples to perform the remaining tests. Transfer 1 g of each of the samples into their respective boiling tubes using a different spatula for each sample. Pour 5 mL of water into each of the boiling tubes. Stir the solutions with different glass rods to dissolve the salts.

Litmus Test:

Put a drop of every salt solution on separate red litmus paper strips using the glass rods. Record if the blue litmus turns red or not. Perform the next test only on samples that turned red litmus blue as an aqueous solution of washing soda is alkaline and turns red litmus blue.

The action of dilute hydrochloric acid (dil. HCl):

Install a two-bored rubber cork fitted with a thistle funnel and delivery tube into the boiling tube of the sample to be tested.

1.  Add dil. HCl into the thistle funnel and turn the pinchcock to add HCl dropwise into the boiling tube.
2.  Record if you observe any effervescence or not. 
3. Repeat for other samples. 
4. Perform the confirmatory test, the lime water test, on samples that produced effervescence on the addition of dilute HCl. Washing soda reacts with dil. HCl to liberate CO₂, which appears as effervescence.

Lime Water Test:

1. Pass the gas liberated on reaction with dilute HCl through lime water with the help of a delivery tube.
2. Record if the lime water turned milky or not.
3. Continue passing the liberated gas through lime water to check if the milkiness disappears.
4. If the milkiness disappears, it reconfirms that the gas liberated is CO₂ and that particular sample is washing soda.
5. Repeat the lime water test for the other samples that reacted with dil. HCl to produce effervescence.

In this experiment, we learned that:

Sample ‘A’ is washing soda. Washing soda is a white sodium salt of carbonate (Na₂CO₃.10H₂O). An aqueous solution of it turns red litmus blue. It reacts with dil. HCl produces CO₂ that turns lime water milky.