StudySmarter - The all-in-one study app.
4.8 • +11k Ratings
More than 3 Million Downloads
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.
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.
Remember the periodic table? You will learn how to identify the following group 2 ions -
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+.
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:
The observations that you expect to see in the test-tube are summarised in this table:
|Test for Group 2 Cations: Sodium Hydroxide|
|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:
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 -
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|
|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:
|Summary of Tests for Group 2 Cations|
|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+).
To make a solution which contains ammonium ions (NH4+), mix ammonium chloride with distilled water. Follow these steps to test for ammonium ions:
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!
Recall the periodic table again. For anions, you will learn how to identify group 7 ions, also called halide ions:
Besides halide ions, you will also learn how to identify:
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:
That was easy. There is also another way to test for hydroxide ions.
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:
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.
Follow these steps to test for 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.
Follow these steps to test for 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.
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 -
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 -
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 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 -
The types of chemical reactions that can occur in a test-tube are:
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.
How are precipitates represented in a chemical equation?
In a chemical equation, precipitates are represented with a downwards arrow in front of the precipitate compound. (↓)
Which gas is evolved when testing for carbonate ions with diluted hydrochloric acid?
Carbon dioxide (CO2)
During the test for NH4+ ions, why is it advised to heat the solution in a water bath and not over a flame?
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.
Is barium chloride harmful?
Yes. Barium chloride is toxic.
Is barium sulphate soluble in water?
No. barium sulphate forms a white precipitate and hence, it is insoluble.
Why should you dampen a litmus paper when testing for ammonium ions?
Ammonia reacts with water to give OH- ions. If there is no water present, OH- ions will not be released and detection of ammonia will not be successful.
What is the chemical formula for ammonium chloride?
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?
Either Strontium ions (Sr2+), or Barium ions (Ba2+)
What are the safety precautions to be followed while in the chemistry lab?
Be perfectly prepared on time with an individual plan.
Test your knowledge with gamified quizzes.
Create and find flashcards in record time.
Create beautiful notes faster than ever before.
Have all your study materials in one place.
Upload unlimited documents and save them online.
Identify your study strength and weaknesses.
Set individual study goals and earn points reaching them.
Stop procrastinating with our study reminders.
Earn points, unlock badges and level up while studying.
Create flashcards in notes completely automatically.
Create the most beautiful study materials using our templates.
Sign up to highlight and take notes. It’s 100% free.