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Conservation of mass

17/05/2016 ©cgrahamphysics.com 2016

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Starter: Element, mixture or compound?

• Hydrogen gas is an example of a(n)….

• Air is an example of a(n)…..

• Carbon dioxide is an example of a(n)….


Quick review Element

• Contain only one type of atom

• Cannot be broken down into anything smaller

• There are just over 100 different types

• Hydrogen gas is an example

• A mixture contains two or more substances that are not chemically combined.

• Air is an example

Compounds

• Contain more than one type of atoms that are chemically joined

• Can be broken down into separate elements

• There are millions of different types

• Carbon dioxide is an example Mixture


Aim

• Explain the law of ‘conservation of mass’

Key words

• Conservation of mass

• Element

• Compound

• Mixture


2H


2H + O


2H + O

H2O


9H Burning magnesium

Conservation of Mass



Items on top-pan balance Mass Calculation

a. Crucible

b. Crucible and magnesium

c. Crucible and product

after heating

d. Magnesium b – a

e. Magnesium oxide c – a

f. Oxygen e – d

1. Weigh the crucible.

2. Take a piece of magnesium ribbon and coil it so that it will fit into the crucible.

3. Weigh the crucible and magnesium.

4. Place the crucible (with magnesium and lid) over the Bunsen burner

5. Heat gently at first, then strongly.

6. Every few minutes use tongs to lift the crucible lid a little. If the magnesium flares up, put the lid back down.

7. When no flaring is seen turn off the heat and leave the crucible to cool.

8. Re-weigh the crucible, lid and contents.

Instructions



Weigh a crucible, put in the

magnesium and weigh it again.

Heat the crucible with the lid on, lifting

the lid occasionally to let air in.

When the crucible is cool

weigh it again.



Questions Stick your table into your book and then answer these questions

1. What did the magnesium look like before it was burned?

2. What did the product look like after the magnesium had been heated?

3. Where did this extra mass come from?

4. Write out the word equation for this reaction. 17/05/2016 ©cgrahamphysics.com 2016

magnesium + oxygen magnesium oxide

9H Explaining the change in mass

Mass of magnesium at start = 4.8 g

Mass of magnesium oxide = 8.0 g

Mass change = +3.2 g

Think about the burning reaction:

We have measured the mass of the magnesium and the

mass of the magnesium oxide.

So where did the 3.2 g come from?

You’ve guessed it – oxygen from the air.






Mass change = 3.2 g

We have just measured

a change in mass.

So how can mass be

conserved?

Weigh a crucible, put in the

magnesium and weigh it again.

Heat the crucible with the lid on, lifting

the lid occasionally to let air in.

When the crucible is cool

weigh it again.



magnesium + oxygen magnesium oxide

9H Explaining the change in mass



Mass change = 3.2 g

Think about the burning reaction:

We have measured the mass of the magnesium and the

mass of the magnesium oxide.

So where did the 3.2 g come from?

Where did the 3.2 g

come from?

You’ve guessed it – oxygen from the air.



In any reaction the total mass of products is the same as the total mass of the reactants

Magnesium and oxygen

Mg O Mg O




Magnesium oxide and hydrochloric acid

Mg O

H Cl

Mg Cl Cl

H H O H Cl

1 x magnesium, 1 x oxygen, 2 x hydrogen and 2 x chlorine atoms

Also 1 x magnesium, 1 x oxygen, 2 x hydrogen and 2 x chlorine atoms



Conservation of mass in reactions


Burning methane


C

H

H

H

H O

O

O O

O H H

O H H

C O

O


9H Quiz

1. During a chemical reaction, the total mass…

a) always increases

b) always decreases

c) stays the same.

2. When a heavy piece of wood burns, only a little light ash is left. This is because…

a) mass is not conserved in this reaction

b) most of the products are gases

c) wood is a renewable fuel.

Plenary: 8 quick questions


9H Quiz

3. When you burn magnesium, the mass seems to increase. This is because…

a) carbon dioxide is given off into the air

b) oxygen is used from the air

c) some extra solid magically appeared in the crucible.

4. You lift the crucible lid to…

a) let in oxygen

b) let in carbon dioxide

c) let out extra ash.



9H A little less conservation?

Mass is conserved.



Key words • Conservation of mass - a principle stating that mass

cannot be created or destroyed.

• Element - Contains only one type of atom and cannot be broken down into anything smaller

• Compound - Contain more than one type of atoms that are chemically joined

• Mixture - A mixture contains two or more substances that are not chemically combined


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