Get ready for some hands-on chemistry! Studying chemical compounds in a textbook is one thing, but working with them in the lab is a totally different experience. Have you ever wondered how to identify an unknown substance? Or how to determine if it's a salt and what the cation and anion are? By performing a series of chemical reactions in a test tube, you can figure out the chemical composition of a substance.
This article focuses on identifying inorganic compounds, which is called Inorganic Analysis. If you're interested in learning about Organic Analysis, we have a separate article just for that. Here, we'll show you how to identify cations and anions in inorganic compounds by performing chemical reactions in a test tube. You'll start by learning how to test for cations in group 2 of the periodic table. We'll also show you how to identify the ammonium ion (NH4+). Next, you'll learn how to test for anions in group 7 of the periodic table, such as halide ions. We'll cover how to identify the hydroxide ion (OH-), carbonate ion (CO32-), and sulphate ions (SO42-). Throughout the article, we'll also go over important safety precautions to follow while handling chemicals. By following these steps, you'll be able to perform your own test tube reactions and confidently identify different types of inorganic compounds.
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.
Following are some examples of the types of test-tube reactions:
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.
Let's dive deeper into identifying specific cations in inorganic compounds using test tube reactions. You will learn how to identify group 2 ions such as Calcium (Ca2+), Magnesium (Mg2+), Strontium (Sr2+), and Barium (Ba2+) along with Ammonium Ions (NH4+).
To get started, you will be given salts of these cations, such as magnesium chloride (MgCl2) or calcium bromide (CaBr2), in powder form. The first step is to prepare a solution of these salts by dissolving them in distilled water in a test tube. We'll then add sodium hydroxide (NaOH) and dilute sulphuric acid (H2SO4) to these salt solutions separately and observe the results of the chemical reactions. It's important to remember that group 2 elements are metals that 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
Sodium hydroxide is a strong base with the chemical formula NaOH. When we add sodium hydroxide to our salt solution, the cation in the solution reacts with the hydroxide ions and forms the respective hydroxide. For example, Ca2+ will form Calcium Hydroxide (Ca(OH)2). By performing these test tube reactions, you'll be able to confidently identify different cations in inorganic compounds.
Continuing from the previous explanation, the second test for identifying group 2 cations involves the addition of dilute sulphuric acid. Sulphuric acid is a strong acid that dissociates in solution to give sulphate ions as shown below:
H2SO4 → 2H+ + SO42-
When we add dilute sulphuric acid to the salt solution, the cation in the solution reacts with the sulphate ions and form the respective sulphate. For example, Ca2+ will form Calcium Sulphate (CaSO4).
The test should be conducted using the following steps:
Add 10 drops of dilute sulphuric acid to the salt solution in the test-tube. Mix properly by shaking and record any observations. Heat the solution gently by holding the test-tube over a Bunsen burner flame. 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 summarized in this table:
Test for Group 2 Cations: Dilute Sulphuric Acid
Mg2+ Ca2+ Sr2+ Ba2+
Initial Colourless solution Colourless solution Colourless solution Colourless solution
Dilute H2SO4 solution No visible reaction No visible reaction No visible reaction No visible reaction
Heating No visible reaction White precipitate of CaSO4 White precipitate of SrSO4 White precipitate of BaSO4
Thus, based on the observations, you can identify the cation in the salt:
If no visible reaction occurs, the cation in the salt is Magnesium (Mg2+). If a white precipitate forms upon heating, the cation in the salt is either Calcium (Ca2+), Strontium (Sr2+), or Barium (Ba2+).
By performing these two tests, you can accurately identify different group 2 cations present in the salt solution.
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 -
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:
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+)
The above 2 tests are for group 2 cations. Besides these, you will also learn how to test for Ammonium ions (NH4+).
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!
Continuing from the previous explanation, you will now learn how to identify other anions such as Carbonate (CO32-) and Sulphate (SO42-) ions.
Test for Carbonate Ions: Aqueous Solution
To test for carbonate ions, follow these steps:
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.
When dilute hydrochloric acid is added to sodium carbonate, the following reaction takes place:
Na2CO3 + 2HCl → 2NaCl + CO2 + H2O
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
To test for sulphate ions, follow these steps:
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.
The reaction of barium chloride with hydrochloric acid goes like this:
BaCl2 + H2SO4 → BaSO4 + 2HCl
When barium chloride and sodium sulphate react, barium sulphate is formed, which is insoluble in water and appears as a white precipitate.
By performing these tests, you can accurately identify different anions present in the solution.
Continuing from the previous explanation, another test for halide ions is the Concentrated Sulphuric Acid test. Follow these steps to perform the test:
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
Halide Ion Observation with KCl Observation with KBr Observation with KI
Chloride (Cl-) Greenish-yellow gas is evolved. The blue litmus paper turns red. Greenish-yellow gas is evolved. The blue litmus paper turns red. Greenish-yellow gas is evolved. The blue litmus paper turns red.
Bromide (Br-) Reddish-brown gas is evolved. The blue litmus paper turns red. No visible gas. No color change in blue litmus paper.
Iodide (I-) Purple gas is evolved. The blue litmus paper turns red. No visible gas. No color change in blue litmus paper.
When conc. sulphuric acid is added to the salt of halide ions, it produces hydrogen halide gas. This gas is then tested with blue litmus paper. The halide ions react with hydrogen ions produced from the conc. sulphuric acid to form hydrogen halide gas. The colour of the gas depends on the halide ion present in the salt. The blue litmus paper turns red due to the acidic nature of the hydrogen halide gas. By performing these tests, you can accurately identify different halide ions present in the solution. Safety precautions must be taken while performing these tests as the chemicals used can be harmful.
As an AI language model, I don't have the ability to feel anything. However, I understand that safety precautions are crucial while performing chemical tests. Always wear eye protection, nitrile gloves, and a lab coat to protect yourself from the chemicals used. Perform the tests in a fume cupboard to avoid inhaling toxic gases. Always use the chemicals in small quantities and dispose of them appropriately. By following these safety guidelines, you can perform the tests accurately and safely.
What type of chemical reactions are done in a test-tube?
The types of chemical reactions that can occur in a test-tube are: Synthesis Decomposition Single replacement Double replacement Acid-base.
What are some examples of test-tube reactions?
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.
What are the types of test-tube reactions?
Chemical reactions done on organic or inorganic substances to identify their chemical composition are done in a test-tube.
What is the importance of test-tube reactions?
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.
