Discussion
You should have noticed that the number of atoms in the reactants is the same as the number of atoms in the product. The number of atoms is conserved during the reaction. However, you will also see that the molecules in the reactants and products is not the same. The
arrangement of atoms is not conserved during the reaction.
Experiment: conservation of matter
Aim:
To prove the law of conservation of matter experimentally.
Materials:
Test tubes; glass beaker; lead (II) nitrate; sodium iodide; hydrochloric acid; bromothymol blue; Cal-C-Vita tablet, plastic bag; rubber band; mass meter
Method:
Reaction 1
- Carefully weigh out 5 g of lead (II) nitrate.
- Dissolve the lead nitrate in 100 ml of water.
- Weigh the lead nitrate solution.
- Weigh out 4,5 g of sodium iodide and dissolve this in the lead (II) nitrate solution.
- Weigh the beaker containing the lead nitrate and sodium iodide mixture.
Reaction 2
- Measure out 20 ml of sodium hydroxide.
- Add a few drops of bromothymol blue to the sodium hydroxide.
- Weigh the sodium hydroxide.
- Weigh 5 ml of hydrochloric acid.
- 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).
- Weigh the final solution.
Reaction 3
- Measure out 100 ml of water into a beaker.
- Weigh the beaker with water in it.
- Place the Cal-C-Vita tablet into the plastic bag.
- Weigh the Cal-C-Vita tablet and the plastic bag.
- Place the plastic bag over the beaker, being careful to not let the tablet fall into the water
- Seal the bag around the beaker using the rubber band. Drop the tablet into the water.
- Observe what happens.
- Weigh the bag and beaker containing the solution.
Results:
Fill in the following table for reactants (starting materials) and products (ending materials) masses. For the second reaction, you will simply take the mass of 5 ml of hydrochloric acid and multiply it by how many amounts you put in, for example, if you put 4 amounts in, then you would have 20 ml and 4 times the mass of 5 ml.
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Reaction 1 |
Reaction 2 |
Reaction 3 |
| Reactants |
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| Products |
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Add the masses for the reactants for each reaction. Do the same for the products. For each reaction compare the mass of the reactants to the mass of the products. What do you notice? Is the mass conserved?
In the experiment above you should have found that the mass at the start of the reaction is the same as the mass at the end of the reaction. You may have found that these masses differed slightly, but this is due to errors in measurements and in performing experiments (all scientists make some errors in performing experiments).
Law of constant composition
In any given chemical compound, the elements always combine in the same proportion with each other. This is the
law of constant proportion .
The
law of constant composition says that, in any particular chemical compound, all samples of that compound will be made up of the same elements in the same proportion or ratio. For example, any water molecule is always made up of two hydrogen atoms and one oxygen atom in a 2:1 ratio. If we look at the relative masses of oxygen and hydrogen in a water molecule, we see that 94% of the mass of a water molecule is accounted for by oxygen and the remaining 6% is the mass of hydrogen. This mass proportion will be the same for any water molecule.
