**Solids – Liquids – Gases:**

**Solids:**

The molecules in a solid
are packed closely together in regular structure. They do not have enough
energy to break free of the forces of attraction which bind them to their
neighboring molecules. They can only vibrate. This is why solids have a fixed
shape and affixed volume and do not flow like liquids.

**Liquids:**

The molecules in a liquid have
just enough energy to break free of the forces which bind them to their
neighbors. This is why liquids are able to flow and do not have a fixed shape.
However, the forces are strong enough to hold the molecules close together,
giving liquids a fixed volume.

**Gases:**

The molecules in a gas
have so much energy that the force of attraction between them is negligible.
They can move freely and at great speed. The molecules in a gas are much
further a part than those in a liquid or a solid. This is why gases can be
compressed easily.

**Brownian Motion:**

The molecules in liquids
and gases are continually moving in a completely random fashion. This is known
as Brownian motion, after the botanist Sir Robert Brown who first studied the
nature of their movement. He demonstrated that pollen grains placed in water
move erratically. This motion is due to the pollen grains unseen impact with
water molecules. The tiny water molecules are able to move the much larger
pollen grains because there is large number of water molecules and they are
moving very fast.

**Diffusion:**

Diffusion is the gradual
mixing of two or more different gases or liquids. Diffusion happens when the
molecule of the substances collides and intermingles. For example, the scent of
flowers spreads through a room because its molecules diffuse through the air.
The process of diffusion supports the idea of moving molecules, since the
particle must be moving in order to mix.

**Atoms and Helium Atoms:**

The number of protons in a nucleus is called the proton number (Z). The total number of protons and neutrons in a nucleus is called the nucleon number (A).

The nucleon and the proton number of an atom are written next to the symbol for the element to which the atom belongs. For example, Helium is written as:

_{2}

^{4}He

**Measuring Mass:**

The mass of an object is
the measure of how much matter it contains. Mass is measured in kilograms (Kg).
To find the mass of an object, simple balancing scales like the ones shown are
used to compare the unknown mass with a known mass.

These quantities are used
in the equation:

Mass (M) = Density (D) X
Volume (V)

Volume (V) = Mass (M) /
Density (D)

Density (D) = Mass
(M)/Volume (V)

**Measuring Volume:**

The volume of an object is
the measurement of the amount of space it occupies. It is measured in cubic
meters (m3) or cubic centimeters (cm3). The volume of regular shaped solids is
found using a ruler and mathematical formula.

For example, The volume of
a rectangular block is found using the equation: length X breadth X height.

The volume of liquid can
be found by pouring it into a measuring cylinder. The volume of an irregular shaped
solid is measured by displacement as in the diagram.

**Measuring Density:**

To find the density of a solid or a liquid its mass and volume must be measured using the methods described above.

These quantities are used in the equation:

Density (D) = Mass (M) / Volume (V)

Density is measured in kilograms per cubic meter (kg/m

^{3}) or grams per cubic centimeter (g/cm

^{3})