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Test Tube Reactions

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Chemistry

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 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 the identification of inorganic compounds, or Inorganic Analysis. 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.

Inorganic Analysis: Identifying Cations and Anions

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.

Identification of 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

  • Ammonium Ions (NH4+).

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.

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:

  1. Add 10 drops of 0.6M NaOH to the salt solution in the test-tube.
  2. Mix properly by shaking and record any observations.
  3. 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.
  4. 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+
InitialColourless solutionColourless solutionColourless solutionColourless solution
0.6M NaOH solutionSlightly white precipitate of Mg(OH)2Slightly white precipitate of Ca(OH)2Slightly white precipitate of Sr(OH)2Colourless solution
Excess NaOHWhite precipitate of Mg(OH)2White precipitate of Ca(OH)2Slightly white precipitate of Sr(OH)2Colourless 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.

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 -

  1. Add 10 drops of 1.0M sulphuric acid to the salt solution in the test-tube.
  2. Mix properly by shaking and record any observations.
  3. 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.
  4. 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+
InitialColourless solutionColourless solutionColourless solutionColourless solution
1.0M H2SO4 solutionSlight white precipitate of MgSO4Slight white precipitate of CaSO4White precipitate of SrSO4White precipitate of BaSO4
Excess H2SO4 Colourless solutionSlight white precipitate of CaSO4White precipitate of SrSO4White 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 NaOHWhite ppt of Mg(OH)2White ppt of Ca(OH)2Slight white pptNo change
Excess H2SO4Colourless solutionSlight white pptWhite ppt of SrSO4White 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:

  1. Add 10 drops of ammonium chloride solution to a test-tube.
  2. Add 10 drops of sodium hydroxide to the test-tube. Mix properly by shaking.
  3. Warm the mixture in the test-tube by placing it in a water bath.
  4. 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.
  5. Record any observations. You should expect to see the red litmus paper turning blue. This is because ammonia is basic in nature.
  6. 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!

Identification of 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:

  1. Add sodium hydroxide (NaOH) solution to a test-tube until it is 1cm high in the test-tube.
  2. Drop a damp piece of red litmus paper in the test-tube.
  3. Record your observations. You know it - the red litmus paper should turn blue!
  4. 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:

  1. Place a few drops of 1.0M ammonia solution on filter paper.
  2. Place this filter paper in a petri dish.
  3. Dampen a red litmus paper with distilled water.
  4. Place this litmus paper on the other side of the petri dish.
  5. Cover the petri dish with a lid.
  6. 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:

  1. Add a small volume of sodium carbonate (Na2CO3) to a test-tube.
  2. Add an equal quantity of dilute hydrochloric acid to the test-tube.
  3. A gas will be produced. Use a delivery tube to transfer the gas to another test-tube containing calcium hydroxide (Ca(OH)2) (limewater).
  4. Put a stopper on this test-tube with the limewater and shake. The limewater should turn cloudy.

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 -

  1. Add a small volume of dilute hydrochloric acid to a test tube.
  2. Add 10 drops of sodium sulphate (Na2SO4) to the test tube.
  3. Add 10 drops of barium chloride (BaCl2) to the test tube.
  4. A white precipitate of barium sulphate (BaSO4) will be formed.

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 such as 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 -

  1. Make a solution of the salt to get the ions in aqueous form.
  2. Add a small volume of nitric acid to the test-tube.
  3. Add a small amount of silver nitrate to the test-tube and record any observations. A precipitate should form.
  4. Shake the test-tube and divide the contents into another two test-tubes. Shaking is necessary to ensure the precipitate also gets equally divided.
  5. To one test-tube, add excess dilute ammonia solution. Record your observations
  6. 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 solutionColourless solution
Br-Cream precipitate (AgBr)Cream precipitateColourless solution
I-Yellow precipitate (AgI)Yellow precipitateYellow 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 -

  1. Dilute nitric acid is an irritant. Wear gloves and a lab coat.
  2. 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

  1. Add a small amount of dry salt of potassium chloride (KCl) in a dry test-tube.
  2. Using a fume cupboard, add conc. sulphuric acid to the test-tube.
  3. Record any observations. A gas will be evolved.
  4. Test the gas with blue litmus paper.
  5. Repeat the experiment with KBr and KI.

The observations you should expect are given below:

Test for Halide Ions: Concentrated Sulphuric Acid
Conc. Sulphuric AcidBlue Litmus Paper
KClWhite steamy fumesTurns red
KBrOrange fumesTurns red
KIPurple fumes with purple/black solidTurns red

Safety Precautions -

  1. Make sure to use eye protection and nitrile gloves. The gases evolved are corrosive and toxic.
  2. Perform the test in a fume cupboard
  3. 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.

Test Tube Reactions

The types of chemical reactions that can occur in a test-tube are:

Synthesis

Decomposition

Single replacement

Double replacement

Acid-base.

Test-tube reactions to identify the cation in a salt.

Test-tube reactions to identify the anion in a salt.

Test-tube reactions to identify the functional group in an organic compound.

Test-tube reactions to identify transition metal ions in a substance.

Chemical reactions done on organic or inorganic substances to identify their chemical composition are done in a test-tube.

Performing test-tube reactions in the chemistry lab is important to have a better understanding of the subject by getting some practical experience apart from textbook knowledge.

Final Test Tube Reactions Quiz

Question

How are precipitates represented in a chemical equation? 

Show answer

Answer

In a chemical equation, precipitates are represented with a downwards arrow in front of the precipitate compound. (↓) 

Show question

Question

Which gas is evolved when testing for carbonate ions with diluted hydrochloric acid? 

Show answer

Answer

Carbon dioxide (CO2)  

Show question

Question

During the test for NH4+ ions, why is it advised to heat the solution in a water bath and not over a flame?

Show answer

Answer

Because water bath is more gentle than a direct flame. If heated over a flame, Ammonia fumes could escape the test tube before letting us test them.

Show question

Question

Is barium chloride harmful?

Show answer

Answer

Yes. Barium chloride is toxic.

Show question

Question

Is barium sulphate soluble in water?

Show answer

Answer

No. barium sulphate forms a white precipitate and hence, it is insoluble.

Show question

Question

Why should you dampen a litmus paper when testing for ammonium ions?

Show answer

Answer

Ammonia reacts with water to give OHions. If there is no water present, OH- ions will not be released and detection of ammonia will not be successful.

Show question

Question

What is the chemical formula for ammonium chloride?

Show answer

Answer

NH4Cl

Show question

Question

Which cations can be present in the solution if there is a white precipitate in the test-tube after adding excess conc. H2SO4 in excess?

Show answer

Answer

Either Strontium ions (Sr2+), or Barium ions (Ba2+)

Show question

Question

What are the safety precautions to be followed while in the chemistry lab?

Show answer

Answer

  • Wear nitrile gloves
  • Wear a lab coat
  • Wear splash-proof eye protection gear
  • Use a fume cupboard whenever required
  • Handle toxic and corrosive chemicals with utmost care.

Show question

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