10.1 Conservation of matter  (Page 2/3)

We will use two of our earlier examples of chemical reactions to demonstrate this:

1. The decomposition of hydrogen peroxide into water and oxygen

$$2{\mathrm{H}}_2{\mathrm{O}}_2\to 2\mathrm{H}{}_2\mathrm{O}+{\mathrm{O}}_2$$

Left hand side of the equation

$\mathrm{Total\; atomic\; mass}=(4\times 1)+(4\times 16)=68\mathrm{u}$

$\mathrm{Number\; of\; atoms\; of\; each\; element}=(4\times \mathrm{H})+(4\times \mathrm{O})$

Right hand side of the equation

$\mathrm{Total\; atomic\; mass}=(4\times 1)+(4\times 16)=68\mathrm{u}$

$\mathrm{Number\; of\; atoms\; of\; each\; element}=(4\times \mathrm{H})+(4\times \mathrm{O})$

Both the atomic mass and the number of atoms of each element are conserved in the reaction.

2. The synthesis of magnesium and oxygen to form magnesium oxide

Left hand side of the equation

$\mathrm{Total\; atomic\; mass}=(2\times 24,3)+(2\times 16)=80,6\mathrm{u}$

$\mathrm{Number\; of\; atoms\; of\; each\; element}=(2\times \mathrm{Mg})+(2\times \mathrm{O})$

Right hand side of the equation

$\mathrm{Total\; atomic\; mass}=(2\times 24,3)+(2\times 16)=80,6\mathrm{u}$

$\mathrm{Number\; of\; atoms\; of\; each\; element}=(2\times \mathrm{Mg})+(2\times \mathrm{O})$

Both the atomic mass and the number of atoms of each element are conserved in the reaction.

Activity : the conservation of atoms in chemical reactions

Materials:

1. Coloured marbles or small balls to represent atoms. Each colour will represent a different element.
2. Prestik

Method:

1. Choose a reaction from any that have been used in this chapter or any other balanced chemical reaction that you can think of. To help to explain this activity, we will use the decomposition reaction of calcium carbonate to produce carbon dioxide and calcium oxide. ${\mathrm{CaCO}}_3\to {\mathrm{CO}}_2+\mathrm{CaO}$
2. Stick marbles together to represent the reactants and put these on one side of your table. In this example you may for example join one red marble (calcium), one green marble (carbon) and three yellow marbles (oxygen) together to form the molecule calcium carbonate ( ${\mathrm{CaCO}}_3$ ).
3. Leaving your reactants on the table, use marbles to make the product molecules and place these on the other side of the table.
4. Now count the number of atoms on each side of the table. What do you notice?
5. Observe whether there is any difference between the molecules in the reactants and the molecules in the products.

Discussion

Experiment: conservation of matter

Aim:

Materials:

Method:

1. Carefully weigh out 5 g of lead (II) nitrate.
2. Dissolve the lead nitrate in 100 ml of water.
3. Weigh the lead nitrate solution.
4. Weigh out 4,5 g of sodium iodide and dissolve this in the lead (II) nitrate solution.
5. Weigh the beaker containing the lead nitrate and sodium iodide mixture.

1. Measure out 20 ml of sodium hydroxide.
2. Add a few drops of bromothymol blue to the sodium hydroxide.
3. Weigh the sodium hydroxide.
4. Weigh 5 ml of hydrochloric acid.
5. Add 5 ml of hydrochloric acid to the sodium hydroxide. Repeat this step until you observe a colour change (this should occur around 20 ml).
6. Weigh the final solution.

1. Measure out 100 ml of water into a beaker.
2. Weigh the beaker with water in it.
3. Place the Cal-C-Vita tablet into the plastic bag.
4. Weigh the Cal-C-Vita tablet and the plastic bag.
5. Place the plastic bag over the beaker, being careful to not let the tablet fall into the water
6. Seal the bag around the beaker using the rubber band. Drop the tablet into the water.
7. Observe what happens.
8. Weigh the bag and beaker containing the solution.