Hello I am a student that goes to High school and is looking towards a bright future. I've never asked for help before but here goes.

Lab - Percent Composition
Objective: To observe a chemical reaction, and to determine the percent
composition by mass of the compound formed.
Procedure:
1. Weigh the empty crucible.
2. Take a sample of the magnesium ribbon (6”). Record the mass of
the ribbon and crucible.
3. Position the crucible on the clay triangle. Cut the magnesium
ribbon into 1/2” segments and place them inside the crucible.
Light the Bunsen burner and cover the crucible. Heat the crucible
until all the magnesium pieces stop glowing. (Lift the cover
periodically to check the condition of the magnesium)
4. Turn off the Bunsen burner and allow the crucible to cool on the
5. When the crucible is completely cool (after about 10 minutes)
weigh the crucible and its contents again. Be careful not to
spill the contents. Record the mass.
6. Clean up your work area.
Results and Data Analysis: (Present your results in a data table)
(a) Mass of empty crucible = (b) Mass of magnesium =
(c) Mass of magnesium and crucible = (d) Mass of the product =
(e) Mass of the substance that reacted with the magnesium =
1. What evidence do you have that a chemical reaction took place?
2. Classify the type of chemical reaction in this experiment.
3. What substance did the magnesium react with?
4. According to atomic theory, who determined that atoms of elements
combine in fixed ratios to form new substances?
5. What is the name of the compound, and which class does it belong to?
6. Calculate the percent composition by mass (grams) of the compound
formed.

MY QUESTIONS: How do I find out what the reacted substance was?,
Can someone help me with the questions?

Here are my results:
Before
Mass of Empty Crucible without lid 22.96
Mass of Empty Crucible with lid 34.6
Mass of Magnesium 0.22
Mass of Crucible with Magnesium and lid 34.72
Mass of Crucible with Magnesium and without lid 23.17
Mass of Substance that reacted with Magnesium 0.08
After
Mass of Crucible with Magnesium and lid 34.79
Mass of Crucible with Magnesium and without lid 23.24