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In this video, we examine the historical and present-day significance of the law of conservation of mass.
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In the 18th century, scientists thought that when things burn, a substance called phlogiston came out of that.
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Then experiments enclosed vessels where substances could be accurately weighed, began to help early scientists,
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such as Lavoisier, understand that when things burn oxygen is added.
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He realized that matter could be changed but not destroyed.
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In 1789, he established the law of conservation of mass.
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in chemical reactions no matter is lost or gained.
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These experiments on combining weights also showed that elements always combined in fixed ratios.
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The law of definite proportions and constant composition.
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And they led Dalton to publish his atomic theory in 1808.
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Modern chemistry had started.
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By mid 19th century, only half the elements had been discovered,
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but enough to enable Mendeleev to produce his famous periodic table.
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That was in 1869.
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Chemical equations began to be written, as we now know them, from about this time.
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For example, the burning of magnesium.
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Ah, this needs balancing!
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When we balance an equation, we're simply obeying the law of conservation of mass.
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All the atoms in the reactants must be accounted for in the products.
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It's easy to see this if we draw out the atoms, rearranging themselves to form the products.
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You can see clearly that no atoms are gained or lost during a reaction,
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so the mass is conserved.
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Here is another example, a reaction you'll see every time you use a gas cooker which uses natural gas.
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But is this reaction balanced?
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Pause the video and count the carbon, hydrogen and oxygen atoms on each side.
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It is easier to count if we draw out the molecules as models.
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Carbon, one each side, oxygen - two to start with, but now three,
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hydrogen four to start with, but now only two.
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To balance the hydrogens we need to add two more hydrogens on the right - to do this, we add another water molecule.
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Pause and count. Is it balanced now?
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Well the carbon and the hydrogen balance.
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But we have four oxygens in the product, but only two in the reactant, so we add another oxygen molecule to the reactant.
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Now we have conserved mass.
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In summary, the law of conservation of mass is simply saying that during chemical change there is no loss or gain of atoms.
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It is for this reason that we always balance chemical equations.
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Until you are really familiar with using formulae, it's easier to draw out the molecules as models to enable you to account for all the atoms.