chapter 7.  ELEMENTS OF GROUPS 16, 17 AND 18 class 12 chemistry textbook solution

3. Answer the following.

iii. How is dioxygen prepared in laboratory from KClO3 ?


Dioxygen (O2) can be prepared in the laboratory from potassium chlorate (KClO3) through a process known as thermal decomposition. Here are the steps involved in preparing dioxygen from potassium chlorate:

Materials Needed:

  1. Potassium chlorate (KClO3)
  2. Heat source (Bunsen burner or laboratory heating apparatus)
  3. Test tube or reaction vessel
  4. Delivery tube
  5. Water trough
  6. Collecting apparatus (such as an inverted graduated cylinder filled with water)


  1. Take a small quantity of potassium chlorate (KClO3) in a test tube or reaction vessel. You don’t need a large amount; a few grams will suffice for a simple demonstration.

  2. Attach a delivery tube to the mouth of the test tube or reaction vessel. Ensure that the delivery tube is securely attached.

  3. Set up a water trough filled with water. The water trough should be large enough to submerge the end of the delivery tube.

  4. Heat the test tube or reaction vessel containing potassium chlorate gently using a Bunsen burner or a laboratory heating apparatus. Apply heat to the bottom of the test tube.

  5. As the potassium chlorate (KClO3) is heated, it will undergo thermal decomposition, breaking down into potassium chloride (KCl) and oxygen gas (O2) according to the following chemical equation:

    2KClO3(s) → 2KCl(s) + 3O2(g)

  6. The oxygen gas (O2) generated during the decomposition will travel through the delivery tube and bubble up through the water in the trough.

  7. Collect the oxygen gas by displacement of water. Invert a graduated cylinder or a similar collecting apparatus filled with water into the water trough, making sure there are no air bubbles trapped inside. Position the open end of the delivery tube beneath the mouth of the collecting apparatus.

  8. As the oxygen gas is generated, it will displace the water in the collecting apparatus, causing the water level to drop. The oxygen gas will collect in the graduated cylinder.

  9. Once you have collected the desired amount of oxygen gas, remove the graduated cylinder from the water trough, and cover it with a rubber stopper or a glass plate to prevent gas from escaping.

  10. You now have a sample of laboratory-prepared dioxygen (O2) collected over water.

Remember to perform this experiment in a well-ventilated area and follow appropriate safety precautions, including wearing protective eyewear and lab attire, as well as working with a heat source with caution.

chapter 7.  ELEMENTS OF GROUPS 16, 17 AND 18 Textbook Solution page 164