Video Transcript
In an experiment, four test tubes, each containing a different chemical compound, are heated. After heating for a while, a glowing splint is placed near the mouth of each test tube. Which test tube should result in the glowing splint reigniting? (A) Zinc hydroxide, (B) calcium carbonate, (C) lead sulfate, (D) sodium nitrate.
In this question, we have a selection of four test tubes, each of which contains a different solid. During the heating process, a thermal decomposition reaction takes place. During a thermal decomposition reaction, a substance breaks down when heated to form two or more other substances. Let’s take a look at the thermal decomposition reactions that take place for the solids in each test tube. In test tube (A), zinc hydroxide will thermally decompose into solid zinc oxide and water in the form of steam. In test tube (B), the calcium carbonate will also undergo thermal decomposition. In this case again, we have an oxide produced, which is known as calcium oxide, and the gas released is carbon dioxide.
In test tube (C), if lead(II) sulfate is heated to high temperatures over 1000 degrees Celsius, the compound decomposes into a solid oxide known as lead(II) oxide and the acidic gas sulfur trioxide. In the fourth and final test tube, we find that sodium nitrate thermally decomposes under heat to produce solid sodium nitrite and oxygen gas. We notice that in each of these reactions, gases are produced, either steam, carbon dioxide, sulfur trioxide, or oxygen gas. In chemistry, common gas tests include using a lit splint to test for hydrogen gas, using limewater to test for carbon dioxide gas, using damp litmus paper to test for sulfur trioxide gas, or using a glowing splint to test for oxygen gas.
When a glowing splint comes into contact with oxygen gas, the increased availability of oxygen reignites the splint, demonstrating a positive test for oxygen gas. Because oxygen gas was produced in the thermal decomposition of sodium nitrate, the correct answer is test tube (D).
