Year 9 Chemistry – Worksheet 12
3.1 Decomposition and synthesis
Demonstration experiment: Decomposition of iodine pentoxide M
A spatula tip of iodine pentoxide is placed in a test tube, which is fitted with an adsorption tube stopper (to stop harmful gases from escaping).
The following steps are performed:
1. The iodine pentoxide is heated with a Bunsen burner.
2. The test tube is left to cool.
3. The test tube is opened and a glowing splint (piece of wood) is held into it.
4. Ethanol (alcohol) is filled into the test tube and several drops of the resulting solution are mixed with a starch suspension.
O
1. The test tube is filled with a purple gas, the iodine pentoxide is no longer visible.
2. After a while the purple gas is no longer visible. Dark crystals form on the test tube wall.
3. The glowing splint catches fire when held into the test tube.
4. When the brown ethanol solution is dripped into the starch suspension it turns a dark blue/purple.
Task: Please work together with your neighbor to answer the questions on the back!
C
1. Iodine pentoxide decomposed into two substances: iodine (which forms a purple gas) and oxygen, a colorless gas.
2. When cooled the iodine desublimates (deposits) to form a crystalline solid on the test tube wall. The oxygen remains gaseous.
3. The glowing splint ignites because of the amount of oxygen in the test tube.
4. The iodine that dissolved in the ethanol colors the starch solution dark blue/purple
Word equation: Iodine pentoxide Iodine + Oxygen
adsorption tube(filled withactivated carbon)
rubber stopper syringe
test tube iodine pentoxide
Year 9 Chemistry – Worksheet 12
Questions to understand the experiment
a) In step two the purple gas that was created in step one disappears while dark crystals form on the test tube wall. State whether this is a chemical or physical change and give reasons.
It is a physical change. The purple gas is iodine, the crystals are also iodine but in solid form. So no substance has changed through the reaction.
b) State the number of substances (apart from the air that filled the test tube to begin with) in the test tube before and after step one. Give reasons.
Before: one substance – the iodine pentoxide
After: two substances – the iodine and a clear gas that causes the splint to light up.
BONUS: Explain how the experiment performed proves that iodine pentoxide is a compound.
Because we form two different substances out of one pure substance. Hence at least two atom types have to be contained in the iodine pentoxide, making it a compound.
Test reactions
To find out which substances are created in a chemical reaction, chemists have developed tests for many common substances. The experiment made use of two of these tests:
Test for oxygen – “glowing splint test”
If a gas contains high amounts of oxygen, it will ignite a glowing splint.
Test for iodine/starch
If a solution containing iodine is put in contact with starch (solid or in suspension), a dark blue/purple coloring will be visible. This can be used to test for either substance (using the other to perform the test).
