Step-by-step explanation:
you need to show that the reaction is exothermic, that is, it's releasing heat to the surroundings. there are many measurements to show that
•if you touch the tube it should be hot.
•if the heat released is enough to produce a change, in the temperature of x amount of water, you can put the tube in a beaker and a thermometer in contact with the water and the thermometer should show that the temperature of water (surroundings) is rising
•since the reaction releases heat, putting the tube in a beaker with ice should accelerate the reaction. on the contrary, heating the tube should decrease the reaction rate or even stop it completely. use the chronometer app in your phone to measure how long it takes for the bubbles of h2 (product) to appear or how long it takes for the magnesium (reactant) to disappear. you should find out that it takes more time when you're heating the tube and less time when you're cooling the tube .
hope it was helpful!