UNIT 2.5

The definitions of mass and matter are

inter-related

Matter- anything that has MASS and takes

up space

Mass- a measure of how much MATTER

is in an object

Do we destroy mass or

matter when we lose

weight?

• No we convert it into a

different form (fat is just

stored energy)

How do we release that

extra mass?

• Most of it is converted

into thermal and kinetic

energy to maintain and

move our bodies

Law of Conservation of Mass – during a chemical

reaction, mass is not created or destroyed

All of the atoms that are present at the beginning

of the reaction (reactants), are present at the end

of the reaction (products).

When a chemical reaction occurs, mass is never

gained or lost, it is just transferred into other

forms of the original atoms used.

massreactants= massproducts

Law of Conservation of Matter

• Law of Conservation of Matter – during a

chemical reaction, matter is not created or

destroyed

• All of the atoms that are present at the beginning

of the reaction (reactants), are present at the end

of the reaction (products).

• The mass of the matter does not change even if

the substance changes to a new phase. Ex.

Liquid water changing to gaseous water.

Saved

Keep

What goes in must come out

Reactants equal products

Changes

remains

massreactants= massproducts

“Yields”

means

equals

The reactants are Fe and S; the product is FeS

Fe weighs 14 g and S weighs 8 g.

By the conservation of mass/matter, the product, FeS, will weigh 14 g + 8g = 22 g

Consider the following reaction. Mg+ HCl MgCl2 + H2

1) What are the reactants for the reaction?

2)What are the products for the reaction

Mg and HCl

MgCl2 and H2

Mg + HCl MgCl2+ H2

10 g of Mg reacts with 14 g of HCl.

After the reaction, we have yielded 8 g of MgCl2. By the law of conservation of mass/matter, how many grams of H2 had to be formed?

Mg + HCl MgCl2+ H2

10 g + 14 g = 8g + ?

24 g = 8 g + ___ g

What number plus 8 will equal 24 g for the products side to equal the reactants side in terms of grams?

8 g + 16 g = 24 g

So H2 is 16 g

Another way to say mass is conserved is to

say that the mass of the reactants equals the

mass of the prodcuts

50 grams of sodium reacts with chlorine to form 126 grams of salt. How many grams of chlorine reacted?

Na + Cl2 NaCl 50 g + ?? = 126 g

We know the reactants have to equal the products. If

the products weigh 126 g, then the reactants must weigh 126 g.

So far, we know we have 50 g of the 126 g.

So 126 g – 50 g = 76 g of Cl2

Open System - matter can enter from or

escape to the surroundings • This is when you can have sources of matter coming

from different places

Closed System – matter is not allowed to

enter or leave

First, a tree is part of an

open system: sky, air,

and ground

When a tree is born, it

grows from changing

CO2 into sugars to add

mass. The CO2 comes

from the air.

When a tree dies, the

elements and molecules

that made up the tree do

not disappear.

The tree’s nutrients go back

into the ground and are

recycled.

There is a conservation of

mass (and matter).