It’s time for practicals! Reading about chemical compounds in a textbook is very different from dealing with them in the lab. If given an unknown substance, how can you tell what compound it is? If it’s a salt, what is the cation and what is the anion? You are required to do a series of test-tube reactions (chemical reactions with the compound in a test-tube) and note the observations in order to zero-in on the chemical composition of a substance.
In this article, we will focus on Inorganic Chemistry Analysis i.e., identification of inorganic compounds through test-tube reactions. If you want to learn about Organic Analysis, there is a separate article dedicated to just that.
- In this article, you will learn about identifying cations and anions in inorganic compounds by doing chemical reactions on them in a test-tube.
- First, you will learn how to do tests to identify cations in group 2 of the periodic table.
- For cations, you will also learn how to identify the ammonium ion (NH4+).
- Secondly, you will learn how to do tests to identify anions in group 7 of the periodic table (halide ions).
- For anions, you will also learn how to identify the hydroxide ion (OH-), carbonate ion (CO32-), and sulphate ions (SO42-).
- Throughout the article, you will read about safety precautions which are important to follow while handling chemicals.
Test-tube reactions: meaning
First of all, what type of chemical reactions are perform ed in a test-tube and why is there a need of such special equipment to perform chemical reactions?
Test tubes have the perfect shape and volume to perform chemical reactions. It allows us to observe reactions with very small amounts of chemicals. Test tubes are made with borosilicate glass which has a high melting point, so it allows us to heat the chemicals and simultaneously observe them from a safe distance. The chemical reactions to find out the chemical composition (for ex, the cation and anion in a salt) of an unknown substance are performed in a test tube.
Test-tube reactions: examples
Following are some examples of the types of test-tube reactions -
- Examples of test-tube reactions to identify the cation in a salt.
- Test-tube reactions examples to identify the anion in a salt.
- Examples of test-tube reactions to identify the functional group in an organic compound.
- Test-tube reactions examples to identify transition metal ions in a substance.
Inorganic test-tube reactions
Inorganic compounds are compounds which are not organic i.e. they lack the carbon-hydrogen bonds. The inroganic compounds we will be using for test-tube reactons are mostly salts. Salts are compounds which dissosiate into cations and anions when dissolved in water. Identifying which cation and anion the compound has can be useful in identifying the chemical composition of salts.
Consider a salt, CA. When dissolved in water, it undergoes a reaction like this:
In this reaction, C+ is the cation, and A- is the anion. For every salt, the cation and anion have to be identified by separate chemical reactions.
Test-tube reactions to identify cations and anions
We will first discuss the reactions that can be done to identify cations, then the test-tube reactions to identify anions.
Inorganic test-tube reactions to identify cations
Remember the periodic table? You will learn how to identify the following group 2 ions:
- Calcium (Ca2+)
- Magnesium (Mg2+)
- Strontium (Sr2+)
- Barium (Ba2+)
Besides these, you will also learn how to identify
You will be given salts of these cations such as magnesium chloride (MgCl2) or calcium bromide (CaBr2). These salts are in powder form, so first you will have to prepare a solution of them by dissolving them in distilled water in a test-tube. We will add sodium hydroxide (NaOH) and dilute sulphuric acid (H2SO4) (as separate tests) to these salt solutions, and observe the results of the chemical reactions.
Remember that group 2 elements are metals which have 2 electrons in their valence shell. These elements readily lose the 2 electrons to form a cation and acquire a positive charge of 2+.
Test 1 for Group 2 Cations: Addition of Sodium Hydroxide
Remember that sodium hydroxide is a strong base with the chemical formula NaOH. In solution, it dissociates, giving hydroxide ions (OH-).
When we add sodium hydroxide to our salt solution, the cation in the solution reacts with the hydroxide ions and forms the respective hydroxide. Take the example of magensium and calcium:
Similarly, Ca2+ will form Calcium Hydroxide (Ca(OH)2).
Some of these hydroxides are not soluble in water and are visible as precipitates in the test-tube. We can identify which cation is present in the compound (and which is not) by observing the reaction as it happens in the test-tube.
A precipitate (sometimes abbreviated as ppt.) is a substance that gets separated from a solution or suspension because it is insoluble. In reaction equations, they are represented by a downwards arrow.
The test should be conducted using the following steps:
- Add 10 drops of 0.6M NaOH to the salt solution in the test-tube.
- Mix properly by shaking and record any observations.
- Continue to add NaOH solution drop by drop and shaking continuously. Keep adding until NaOH is in excess (test-tube should not be more than half-full). Record any observations.
- Drain the contents in a sink and wash the test-tube.
The observations that you expect to see in the test-tube are summarised in this table:
| Test for Group 2 Cations: Sodium Hydroxide |
|---|
| Mg2+ | Ca2+ | Sr2+ | Ba2+ |
| Initial | Colourless solution | Colourless solution | Colourless solution | Colourless solution |
| 0.6M NaOH solution | Slightly white precipitate of Mg(OH)2 | Slightly white precipitate of Ca(OH)2 | Slightly white precipitate of Sr(OH)2 | Colourless solution |
| Excess NaOH | White precipitate of Mg(OH)2 | White precipitate of Ca(OH)2 | Slightly white precipitate of Sr(OH)2 | Colourless solution |
Thus, when you have added sodium hydroxide in excess to the solution:
- If the solution is colourless, the cation in the salt is Barium (Ba2+)
- If the solution has a slightly white precipitate, the cation in the salt is Strontium (Sr2+)
- If the solution has a white precipitate, the cation is the salt is either Magnesium (Mg2+) or Calcium (Ca2+).
Test 2 for Group 2 Cations: Addition of dilute Sulphuric Acid
Sulphuric acid is a strong acid. We are going to use a dilute solution of sulphuric acid for this test. It dissociates in solution to give sulphate ions as shown below.
The SO42- ion is the sulphate ion. Our salt solution contains group 2 ions. When dilute H2SO4 is added to the solution, the cation reacts with the sulphate ions to form the respective sulphate. For example, magnesium ions reacts with sulphate ions to give magnesium sulphate.
Similar to hydroxides, some of these sulphates are not soluble and are visible as precipitates in the test-tube.
The steps to be followed for this test are similar to those of the sodium hydroxide test -
- Add 10 drops of 1.0M sulphuric acid to the salt solution in the test-tube.
- Mix properly by shaking and record any observations.
- Continue to add H2SO4 solution drop by drop and shaking continuously. Keep adding until H2SO4 is in excess (test-tube should not be more than half-full). Record any observations.
- Drain the contents in a sink and wash the test-tube.
The observations that you should expect to see in the test tube are summarised in this table:
| Test for Group 2 Cations: Dilute Sulphuric Acid |
|---|
| Mg2+ | Ca2+ | Sr2+ | Ba2+ |
| Initial | Colourless solution | Colourless solution | Colourless solution | Colourless solution |
| 1.0M H2SO4 solution | Slight white precipitate of MgSO4 | Slight white precipitate of CaSO4 | White precipitate of SrSO4 | White precipitate of BaSO4 |
| Excess H2SO4 | Colourless solution | Slight white precipitate of CaSO4 | White precipitate of SrSO4 | White precipitate of BaSO4 |
Thus, when you add sulphuric acid to the solution:
- If there is a slight white precipitate in the beginning and then it disappears, the cation in the solution is that of Magnesium (Mg2+).
- If there is a slight white precipitate even when you have added Sulphuric acid in excess, the cation in the solution is that of Calcium (Ca2+).
- If there is white precipitate in the solution, the cation is either Strontium (Sr2+), or Barium (Ba2+)
Summary of Tests for Group 2 Cations |
|---|
| Mg2+ | Ca2+ | Sr2+ | Ba2+ |
| Excess NaOH | White ppt of Mg(OH)2 | White ppt of Ca(OH)2 | Slight white ppt | No change |
| Excess H2SO4 | Colourless solution | Slight white ppt | White ppt of SrSO4 | White ppt of BaSO4 |
The above 2 tests are for group 2 cations. Besides these, you will also learn how to test for Ammonium ions (NH4+).
Test for Ammonium Ions
To make a solution which contains ammonium ions (NH4+), mix ammonium chloride with distilled water. Follow these steps to test for ammonium ions:
- Add 10 drops of ammonium chloride solution to a test-tube.
- Add 10 drops of sodium hydroxide to the test-tube. Mix properly by shaking.
- Warm the mixture in the test-tube by placing it in a water bath.
- Fumes will come out of the test-tube. These are fumes of ammonia gas (NH3). Carefully hold a damp piece of red litmus paper to the mouth of the test-tube.
- Record any observations. You should expect to see the red litmus paper turning blue. This is because ammonia is basic in nature.
- Dispose of the contents by placing them in a test-tube with boiling water.
We tested for the presence of ammonium ions in the solution by using the fact that ammonia is slightly basic. The reaction of ammonium chloride with sodium hydroxide is shown below:
When the solution is heated, ammonia gas (NH3) is released as fumes. Since ammonia is basic, it turns red litmus paper blue.
You must be wondering why we heat the test-tube in a water bath and not simply over a flame. This is because heating in a water bath is more gentle than heating over a flame. We don't want the fumes to float out of the test-tube before we can test them!
Inorganic test-tube reactions to identify anions
Recall the periodic table again. For anions, you will learn how to identify group 7 ions, also called halide ions:
- Chloride ion (Cl-)
- Bromide ion (Br-)
- Iodide ion (I-)
Besides halide ions, you will also learn how to identify:
- Hydroxide ions (OH-)
- Carbonate ions (CO32-)
- Sulphate ions (SO42-)
Test for Hydroxide Ions: Aqueous Solution
You are very familiar with hydroxide ions by now. It is the ion that gives a solution its basic nature! So naturally, we will test for hydroxide ions with the help of a litmus paper. Follow these steps to test for OH- ions:
- Add sodium hydroxide (NaOH) solution to a test-tube until it is 1cm high in the test-tube.
- Drop a damp piece of red litmus paper in the test-tube.
- Record your observations. You know it - the red litmus paper should turn blue!
- Dispose of the contents in the sink and wash the test-tube.
That was easy. There is also another way to test for hydroxide ions.
Test for Hydroxide Ions: Ammonia
When ammonia comes into contact with water, hydroxide ions are formed. The reaction is shown below:
We can use this reaction to test for hydroxide ions by following these steps:
- Place a few drops of 1.0M ammonia solution on filter paper.
- Place this filter paper in a petri dish.
- Dampen a red litmus paper with distilled water.
- Place this litmus paper on the other side of the petri dish.
- Cover the petri dish with a lid.
- After a few minutes, record your observations. The red litmus paper should turn blue.
Ammonia fumes reach the damp litmus paper and react with the water soaked by the litmus paper to release hydroxide ions, which turns it blue.
Test for Carbonate Ions: Aqueous Solution
Follow these steps to test for carbonate ions:
- Add a small volume of sodium carbonate (Na2CO3) to a test-tube.
- Add an equal quantity of dilute hydrochloric acid to the test-tube.
- A gas will be produced. Use a delivery tube to transfer the gas to another test-tube containing calcium hydroxide (Ca(OH)2) (limewater).
- Put a stopper on this test-tube with the limewater and shake. The limewater should turn cloudy.
We have taken the example of sodium carbonate as the source of carbonate ions. When dilute hydrochloric acid is added to sodium carbonate, the following reaction takes place:
The reaction releases carbon dioxide gas. When carbon dioxide reacts with limewater, a white precipitate of calcium carbonate is formed, which makes the solution appear cloudy.
Test for Sulphate Ions: Aqueous Solution
Follow these steps to test for sulphate ions -
- Add a small volume of dilute hydrochloric acid to a test tube.
- Add 10 drops of sodium sulphate (Na2SO4) to the test tube.
- Add 10 drops of barium chloride (BaCl2) to the test tube.
- A white precipitate of barium sulphate (BaSO4) will be formed.
Above, we have taken the example of sodium sulphate as the source of sulphate ions. The reaction of barium chloride with hydrochloric acid goes like this:
You must be wondering why we added HCl in the beginning if it had nothing to do with he reaction. Adding hydrochloric acid removes any carbonate ions that might be already present in the test-tube. This would give a false positive test as they would also produce a white precipitate.
Barium Chloride (BaCl2 ) is harmful to skin, so make sure you're wearing a lab coat and gloves.
Test for Halide Ions: Aqueous Solution
Halide ions are ions of Group 7 elements (chlorine (Cl), bromine (Br) and iodine (I)).They have 7 electrons in their outermost shell, so they readily accept 1 electron to stabilize their electron orbit resulting in a fully-filled outermost electron shell. This gives them a net charge of -1.
To test for halide ions (Cl-, Br-, I-), we take salts of these ions. For example - potassium chloride (KCl), potassium bromide (KBr), and potassium iodide (KI). These salts are generally in powdered form so we will make a solution.
Follow these steps with the salt solution to test for halide ions -
- Make a solution of the salt to get the ions in aqueous form.
- Add a small volume of nitric acid to the test-tube.
- Add a small amount of silver nitrate to the test-tube and record any observations. A precipitate should form.
- Shake the test-tube and divide the contents into another two test-tubes. Shaking is necessary to ensure the precipitate also gets equally divided.
- To one test-tube, add excess dilute ammonia solution. Record your observations
- To the other test-tube, add excess concentrated ammonia. Record your observations.
The observations you should expect are given in the following table:
| Test for Halide Ions: Aqueous Solution |
|---|
| Silver Nitrate (AgNO3) | Excess dilute Ammonia (NH3) | Excess conc. Ammonia (NH3) |
| Cl- | White precipitate (AgCl) | Colourless solution | Colourless solution |
| Br- | Cream precipitate (AgBr) | Cream precipitate | Colourless solution |
| I- | Yellow precipitate (AgI) | Yellow precipitate | Yellow precipitate |
Addition of nitric acid removes any other ions that may be present in the test-tube which may form a different precipitate, giving false results.
Safety Precautions -
- Dilute nitric acid is an irritant. Wear gloves and a lab coat.
- Conc. ammonia is corrosive so eye protection gear and nitrile gloves should be worn, and the test should be performed in a fume cupboard.
When you add silver nitrate to the initial solutions, it forms silver halide (AgCl, AgBr, or AgI), which is a precipitate. You can tell which halide is in the solution by the colour of this ppt.
The solubility of this ppt in ammonia solution is the most for AgCl, less for AgBr, and the least for AgI. (Decreases as you go down group 7). This is why we confirm which halide is present in the solution by trying to dissolve the ppt in dil. and conc. ammonia solution.
- the precipitate of AgCl gets dissolved in dilute ammonia solution
- the precipitate of AgBr requires concentrated ammonia to dissolve
- the precipitate of AgI doesn't even dissolve in concentrated ammonia solution
Test for Halide Ions: Concentrated Sulphuric Acid
- Add a small amount of dry salt of potassium chloride (KCl) in a dry test-tube.
- Using a fume cupboard, add conc. sulphuric acid to the test-tube.
- Record any observations. A gas will be evolved.
- Test the gas with blue litmus paper.
- Repeat the experiment with KBr and KI.
The observations you should expect are given below:
| Test for Halide Ions: Concentrated Sulphuric Acid |
|---|
| Conc. Sulphuric Acid | Blue Litmus Paper |
| KCl | White steamy fumes | Turns red |
| KBr | Orange fumes | Turns red |
| KI | Purple fumes with purple/black solid | Turns red |
Safety Precautions -
- Make sure to use eye protection and nitrile gloves. The gases evolved are corrosive and toxic.
- Perform the test in a fume cupboard
- Conc. sulphuric acid is highly corrosive, so use in small quantities.
Test-Tube Reactions - Key takeaways
- Inorganic Analysis involves the identification of cations and anions.
- For cations, you learnt how to identify group 2 elements - Mg2+, Ca2+, Sr2+, and Ba2+ . Besides these, you also learnt how to identify the Ammonium ion (NH4+).
- For group 2 cations, the tests include NaOH and dil. H2SO4. For NH4+ , the test is addition of NaOH and heating.
- For anions, you learnt how to identify OH-, SO42-, CO32-, and the group 7 ions (also called Halides) Cl-, Br-, and I-.
- When testing for OH-, you need ammonia solution, and a damp red litmus paper.
- When testing for SO42-, you need to add BaCl2, and observe a white precipitate forming.
- When testing for CO32-, you need to add HCl, transfer the evolved CO2 to another test- tube containing Ca(OH)2, and shake. You will observe the solution turning cloudy.
- For Halide ions, you need to add AgNO3. and observe a precipitate forming. You can tell which halide it is by the colour of the precipitate. You need to confirm it by trying to dissolve it in dilute and concentrated ammonia solution.
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