© KCL. WCED. PENTECH Reacting metals 1 Melanie and Zena are investigating the reactions of metals. They have been asked by their teacher to look at how much metal oxide is formed when a metal burns. For their experiment they have been given some magnesium ribbon. Magnesium is a metal with a low density and a greyish colour that comes in thin strips about 5mm wide and less than 1mm thick. The long strip is wound into a roll like a roll of sticky tape for parcels and letters. Here is their experimental design. 1. Cut 15cm of magnesium ribbon. Weigh it. 2. Burn the magnesium and collect the ash 3. Weigh the ash.. Because they have learned that scientists repeat experiments, they repeat their experiment. But each time they get completely different results for the weight of the ash. Help Melanie and Zena by DECIDING what is wrong with their experiment. DESIGN a better experiment that overcomes the deficiencies of the girls’ design.

© KCL. WCED. PENTECH Reacting metals 2 One of Melanie’s sisters finished high school last year. She tells Melanie that she should have used a crucible to stop the ash escaping. Melanie has never seen a crucible. Here is her sister’s description. It is made of porcelain (that is a special type of china) and has a cup and lid. The cup is only 300mm high. The base is narrower than the rim. The base is about 150mm diameter while the rim is about 200mm diameter. The lid has a diameter that is slightly bigger than the rim of the cup. (This means it fist snugly over the rim of the cup with a tight fit.) The lid is just slightly domed and rises 50mm from its own rim. In the centre of the top there is a small handle so that you can use tongs to put the lid on and off. DRAW what a crucible looks like so Melanie will recognise it.

© KCL. WCED. PENTECH Reacting metals 3 on tare g on tare g steel wool magnesium ribbon on tare g clean magnesium ribbon DESCRIBE the steps in this experiment in your own words. This is what Melanie and Zena then did.

© KCL. WCED. PENTECH Reacting metals By the time the girls had carried out their new experiment several of their classmates had also got results. The class results are displayed in the table below. Mass of crucible in gms. Mass of crucible + magnesium in gms. Mass of crucible + burnt magnesium GROUP AB M + Z’s results D COMPLETE the table so the data is ready for a graphical display Mass of magnesium in gms Mass of magnesium oxide in gms Mass of oxygen in gms EXPLAIN how Melanie and Zena calculated the mass of magnesium. EXPLAIN how they calculated the mass of oxygen 4

© KCL. WCED. PENTECH Reacting metals Mass of magnesium in gms 0.20 Mass of oxygen in gms Here is the completed set of class calculations for the combining masses of magnesium and oxygen. PLOT the results on the graph paper below. LABEL the axes carefully. 5

© KCL. WCED. PENTECH Reacting metals The class results have been plotted onto the graph shown below. Three late results from other groups - shown with (o) - were added. Mass of oxygen in grams Mass of magnesium in grams DESCRIBE in your own words what the graph shows in terms of the mass of magnesium and oxygen. x x x x o o o EXPLAIN what you think this could mean in terms of atoms of oxygen and magnesium 6

© KCL. WCED. PENTECH Reacting metals 7 Here is a diagram from a chemistry textbook Magnesium oxide plus Vibrating magnesium atoms Very fast moving oxygen atoms Powder of molecules of magnesium oxide Writing equations 2Mg + O 2  2MgO changes to Looking at substances Thinking of atoms Use the diagram to EXPLAIN the chemical combination in terms of atoms. Represents two atoms of magnesium Each magnesium atom has a mass of 24.3 units Represents a molecule of Oxygen with two atoms Represents two molecules of magnesium oxide Oxygen Each oxygen atom has a mass of 16 units Each magnesium oxide Molecule has a mass of 40.3 units plus Energy

© KCL. WCED. PENTECH Reacting metals Mass of magnesium oxide produced in grams Length of magnesium ribbon used for burning in air in centimetres DESCRIBE in your own words what this graph shows. EXPLAIN the pattern in the graph in terms of atoms of magnesium and oxygen. Here is a graph found in a science text book. Zena tells Melanie that they should be able to work out what the graph shows and means.