an experiement to test for anions and cations

 

Safety assessment

 

Method

Testing for cations

(Substance I)

 

1. A small spatula of substance I was added to a test tube, along with distilled water filling half of the test tube. 
2. Using a Bunsen burner the test tube was placed into a blue flame to help it dissolve, and was then allowed to cool. 
3. A clean test tube was taken and 1/3 of the solution was added to it, along with 5 drops of dilute sodium hydroxide. 
4. No precipitate was formed, so the test tube was placed back into the Bunsen flame, with a piece of moist litmus paper placed over the top of the tube (ensuring that it didn’t touch) 
5. The paper turned from red to blue, indicating that substance I was ammonium.

 

 

Testing for cations

(Substance J)

 

1.  A small spatula of substance I was added to a test tube, along with distilled water filling half of the test tube. 
2. Using a Bunsen burner the test tube was placed into a blue flame to help it dissolve, and was then allowed to cool. 
3. A clean test tube was taken and 1/3 of the solution was added to it, along with 5 drops of dilute sodium hydroxide. 
4. No precipitate was formed, so the test tube was placed back into the Bunsen flame, with a piece of moist litmus paper placed over the top of the tube (ensuring that it didn’t touch) 
5. The paper had no change and remained red. 
6. A flame test was then done using nichrome wire and a small sample of the solid. 
7. The flame turned a yellow/orange colour and was identified as sodium.

 

Testing for anions

(Substance J)

 

1. A small spatula of substance J was added to a test tube, along with 2 dropfulls of dilute hydrochloric acid. 
2. Using a Bunsen burner the test tube was placed into a blue flame and warmed gently. 
3. No gas was given off, so three drops of barium chloride was added to the test tube. 
4. A white precipitate had formed, indicating that it was a sulphate.

 

 Testing for anions

(Substance I)

 

1.  A small spatula of substance I was added to a test tube, along with 2 dropfulls of dilute hydrochloric acid. 
2. Using a Bunsen burner the test tube was placed into a blue flame and warmed gently. 
3. No gas was given off, so three drops of barium chloride was added to the test tube.
4. No white precipitate had formed, and the solution was discarded with lots of water down the sink. 
5. A fresh solution of substance I was made, with a small amount of water. 
6. An equal amount of dilute nitric acid was then added to the test tube, along with 3 drops of silver nitrate solution. 
7. No precipitate was formed and the solution was again discarded with lots of water down the sink. 
8. A fresh solution of substance I was made, with a small amount of water. 
9. Using gloves, a few crystals of iron (II) sulphate were added to the test tube and concentrated sulphuric was added slowly and in drops down the side of the test tube, in a fume cupboard. 
10. A brown ring had formed between the two liquids and the substance was identified as a nitrate.

 

 

Results

 

Interpretation

From the results of the experiment, it is proven that the hypothesis stated at the beginning is correct.  When testing for anions, “substance J” was identified as a sulphate (SO42-).  The observations recorded were a white precipitate that was not able to dissolve in dilute hydrochloric acid.  “Substance I” was identified as a nitrate (NO3).  The observation recorded was a brown ring had formed between the two liquids.

 

When testing for cations, “substance J” was identified as ammonium (NH+4), this was proven by the red litmus paper turning blue and a slight ammonia gas being given off.  “Substance I” was identified as sodium (Na+).  The observation which was recorded was a yellow/orange flame during the flame test.

 

Evaluation

 If the experiment was to be repeated again I would carry out further practise tests when testing for nitrate.  As when I previously tested for this substance, I didn’t have the technique right and didn’t know what the end result would look like.  When using the flame test for testing cations, it was difficult to note any colour change in the flame, to identify the unknown substance; this was because there wasn’t enough of the unknown substance on the nichrome wire.  A change that I would implement if the experiment was to be carried out again, would be for a spatula type/small spoon, with a wooden or glass handle (to prevent any burns) to be used with a small amount of the unknown substance on the end, that way, it would be much clearer to detect a colour change when using the flame test method.  When testing for anions, it was difficult to identify whether the unknown substance was giving off a gas or was dissolving.  Again, this technique would need to be practised several times, in order to be clear on the end result, and to ensure that the experiment ran smoothly.  

 

 

 

 

 Reference/research:

 (viewed 26/01/09)

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