Lesson 31 notes - Solids, Liquids and Gases
Objectives
Be able to describe solids, liquids and gases in terms of the spacing, ordering and
motion of atoms or molecules.
Be able to describe a simple kinetic model for solids, liquids and gases.
Outcomes
Be able to describe solids, liquids and gases in terms of the spacing, ordering and
motion of atoms or molecules.
Be able to describe a simple kinetic model for solids, liquids and gases.
3 States of Matter
Matter can exist is three states,
as a solid, a liquid or a gas.
Water shows us these 3 states
brilliantly. The solid form, ice is
hard and rigid, a piece of ice has
a shape of its own. The liquid
form, water is "runny", has no
shape of its own and takes up the
shape of its container. The
density of liquid water is just
slightly higher than that of ice (both at 0oC). Steam is what we think of as water in its
gaseous form, this has a very much larger volume (1600 times) than the water that
produced it, a low density and no shape of its own. (NB Steam as we see it is not a
true gas, the gas produced when water boils is invisible, steam is a cloud of fine
droplets of water vapour).
Phase Changes
The diagram shows phase changes
between solid, liquid and gas.
Between each phase change there
is no temperature change as the
bonds are breaking or forming. It
also shows Plasma which is an
ionized gas normally formed by
heating it up.
The enthalpy of the system is the
heat energy of an object.
As molecules inside an object have
more enthalpy their vibrations and
therefore their kinetic energy goes
up.
The Kinetic Molecular Theory
Particles are in constant motion. In solids the particles are close together and have
limited motion. In a liquid some of the attraction between particles is overcome which
allows the particles more freedom of movement. In a gas particles attraction between
particles is minimized and the particles move freely throughout the container.
Standard temperature and pressure (STP on NTP)
This set of conditions which is usually applied to gases is defined as a temperature
of 273 K and a pressure of 760 mm of mercury (1.013x105 Pa). At STP 1 mole of
any gas has a volume of 22.4x10-3 m3.
Extension
Burning Candles
Another example of solids, liquids and gases is a burning candle. Carefully light a
candle and watch the surface of the candle and the flame. The flame melts the solid
wax, creating a pool of liquid wax. This creeps up the wick inside the flame and
becomes a gas, which then burns in the flame. The flame melts more wax and the
cycle continues.
Sublimation
A few compounds go from solid to gas without becoming liquid in between. Solid
carbon dioxide (dry ice) becomes carbon dioxide gas without becoming a liquid, as
does iodine.
In some circumstances, snow and ice can sublime – sometimes, wet washing hung
outside freezes solid and then dries without melting. This is called freeze-drying and
is used in food preserving and for making instant foods and drinks.
Intermolecular Forces
In a solid the molecules are held together by intermolecular forces, the exact type of
force depending on the type of solid.
These forces are divide into four types:
(a) ionic bonds - sodium and chloride ions in sodium chloride;
(b) covalent bonds - shared electrons between atoms;
(c) metallic bonds - free electrons wandering through a metal;
(d) Van der Waal's bonds - electric dipole forces
Structure of Solids
Crystalline solids have specific molecular structures – here are a few examples: