All
the substances you know can be sorted into groups. One way of grouping substances is into solids, liquids or gases. Can you match up the state of matter with the correct
description? Using the mouse, drag the term to the text box beside its description.
Solid, liquid and gas are the 3 states of matter. Each state has different properties.
These different properties are due to the way in which the particles are arranged.
Can you match up the state of matter with the correct description? Using the
mouse, drag the term to the text box beside its description.
Some substances exist naturally in all three states - solid, liquid, and gas.
Under normal conditions, whether something appears as a solid, liquid, or gas
depends on its temperature. For example, water freezes when its temperature
falls to, or below, 0 degrees Celsius. This temperature is the melting point
of water. Above 100 degrees Celsius, water exists as a gas, steam. This is its
boiling point. Between these two temperatures water is a liquid.
When a solid is heated, the particles gain energy and vibrate faster, and the
temperature rises. The more the solid is heated, the more violently the particles
vibrate. The temperature continues to rise. At the melting point, the temperature
remains the same. This is because the energy being supplied is being used to
overcome the forces holding the particles together, freeing them from their
solid structure. If you continue to heat the liquid, the particles move around
faster and faster, and the temperature rises. Some particles gain enough energy
to overcome attractive forces and escape from the liquid to become a gas. This
is evaporation. When the temperature is higher, more particles have enough energy
to escape so evaporation is faster. When the temperature is high enough, the
liquid will boil. The temperature at which a liquid boils is called its boiling
point. When the liquid is boiling the temperature stays the same even though
heat energy is still being supplied. This energy is being used to separate the
liquid particles from each other.
Melting and evaporating are endothermic processes because in both cases energy
is required. Changes such as melting and boiling are called physical changes.
The particles stay the same, but the way they are arranged changes. That is
why physical changes are reversible. If you cool steam back down to 100 degrees
Celsius, it will condense to form liquid water again, as the particles lose
energy and clump back together. If you continue to cool the water, it will freeze
back to solid ice again at 0 degrees Celsius. Condensing and solidifying are
exothermic processes because in both cases energy is transferred from the substance
to the surroundings.
There are three main types of particle - atoms, molecules and ions. Atoms are
the smallest particles that can exist on their own. Everything is made up of
atoms. There are 92 kinds of atom which occur naturally on Earth. These 92 atoms
combine in millions of different ways to form millions of substances. Atoms
of different elements react together to form compounds. The properties of the
compound are usually very different from the properties of the elements from
which it is made. For example, sodium is a very reactive metal. When it reacts
with chlorine, a poisonous gas, the compound sodium chloride or common salt
is produced. This chemical reaction can be represented by a word or symbol equation.
Sodium and chlorine are the reactants and sodium chloride is the product of
the reaction. Reactants and products can be solids, liquids or gases, or can
be dissolved in water. We show this by using the state symbols s, l, g, or aq.
Elements react because of the electron arrangement in their atoms. The electrons
whizz around the nucleus of an atom in shells or energy levels. The first energy
level can only hold two electrons. The second and third energy levels can hold
up to eight electrons each. Atoms have no overall charge because there are equal
numbers of negative electrons and positive protons. Scientists noticed that
the noble gases in Group 8 are different from other elements, because they don’t
usually form compounds. Their atoms are described as unreactive or stable. They
are stable because they have full outer electron energy levels. When elements
react to form compounds they do so in a way that enables them to obtain full
electron energy levels, so they form stable compounds. This idea is the basis
of chemical bonding. The word bond is used to describe the forces that hold
atoms, molecules, and ions together.
Ionic bonds are usually found in the compounds formed when metals, such as sodium,
react with non-metals, such as chlorine. Sodium chloride is an ionic compound.
A sodium atom, atomic number 11, has just one electron in its outer energy level.
The easiest way for it to become stable is to lose this single electron. The
next energy level is completely full. The sodium ion now has 11 protons in the
nucleus surrounded by 10 electrons, so the overall charge is +1. It is written
as Na+, and is a positive ion. A chlorine atom, atomic number 17, has seven
electrons in its outer energy level. The easiest way for it to become stable
is to gain one electron to fill this energy level. The chloride ion now has 17
protons in the nucleus surrounded by 18 electrons, so the overall charge is
-1. It is written as Cl- and is a negative ion. So, when a sodium atom reacts
with a chlorine atom, the sodium atom loses its electron to the chlorine atom,
and two ions are formed. The two ions have opposite charges, so they attract
each other. The electrostatic force of attraction between them is strong. It
is called an ionic bond. In this reaction sodium electrons are shown as dots
and chlorine electrons as crosses, but remember that all electrons are exactly
the same. This type of diagram is called a ‘dot and cross’ diagram.
When sodium reacts with chlorine, billions of sodium and chloride ions form,
and are attracted to each other. The ions do not stay in pairs, but cluster
together so that each ion is surrounded by six ions of opposite charge. They
are held together by strong ionic bonds. The pattern grows until a giant 3D
structure of ions is formed. This giant structure is the compound sodium chloride
or common salt. Because the sodium chloride is made of ions it is called an
ionic compound. Its formula is NaCl. The structure has no charge because it
contains equal numbers of sodium and chloride ions. Compounds with giant structures
have high melting and boiling points because of the strong electrostatic forces
or bonds between the ions.
Other metals also react with non-metals to form ionic compounds. For example,
magnesium burns strongly in oxygen to form magnesium oxide. A magnesium atom
has 2 electrons in its outer energy level, and an oxygen atom has 6 in its outer
energy level. During the reaction, each magnesium atom loses its 2 outer electrons
to an oxygen atom. Magnesium ions and oxide ions are formed. The ions attract
each other because of their opposite charges, and they group together into a
giant ionic structure, called magnesium oxide. Magnesium oxide contains one
magnesium ion for each oxide ion, so its formula is MgO. The compound has no
overall charge, because there are equal numbers of positive and negative ions.
For the formation of magnesium chloride, a magnesium atom has 2 electrons in
its outer energy level, and a chlorine atom has 7 in its outer energy level.
To obtain full outer shells, a magnesium atom must lose its 2 outer electrons,
and a chlorine atom must gain 1 electron. So when magnesium burns in chlorine,
each magnesium atom reacts with two chlorine atoms, to form magnesium chloride.
The ions attract each other because of their opposite charges, and they group
together into a giant ionic structure. Because magnesium chloride contains two
chloride ions for each magnesium ion, its formula is MgCl2. The compound has
no overall charge.
Ionic compounds form regular structures or giant ionic lattices. In all ionic
solids, the ions are packed in a regular pattern, and held together by strong
ionic bonds. This means that all ionic solids are crystalline. An ionic lattice
has very strong electrostatic forces between oppositely charged ions, which
result in these compounds having high melting and boiling points. A lot of energy
is needed to break these bonds and prise the ions apart. Ionic compounds do
not conduct electricity when solid, because the ions are strongly held in the
giant lattice and cannot move. When they are melted or dissolved in water, ionic
compounds conduct electricity because the ions are now free to move. Positive
ions like Na+ move to the negative cathode, and negative ions like Cl- move
to the positive anode. Sodium chloride is the electrolyte and is decomposed
during the electrolysis to sodium and chlorine. Ionic solids have high melting
and boiling points. They conduct electricity when molten or dissolved, but not
when solid.
When two non-metal atoms react together, both of them need to gain electrons
to reach full energy levels. They can manage this only by sharing electrons
in their outer energy levels. The simplest example of this is seen in the hydrogen
molecule. Each hydrogen atom has only 1 electron. To obtain a full outer energy
level and gain the structure of the nearest inert gas, helium, each of the hydrogen
atoms must have two electrons. To do this, two hydrogen atoms move close to
each other, and allow their outer energy levels to overlap. A molecule of hydrogen
is therefore formed with two hydrogen atoms sharing a pair of electrons. This
shared pair of electrons is a single covalent bond. It can be represented as
a single line.
A whole range of non-metal atoms can be held together to form molecules by sharing
electrons and forming covalent bonds. These simple molecules only have weak
forces between the molecules, so they have very low melting and boiling points.
They are usually gases or liquids at room temperature. Examples are chlorine
and oxygen. When oxygen atoms bond together to form oxygen molecules, they have
to share two pairs of electrons to gain full outer energy levels. The bond between
them is called a double covalent bond, or double bond. It can be represented
by two lines.
Many non-metal elements exist as molecules, for example hydrogen, oxygen, and
chlorine. A huge number of compounds also exist as molecules. In a molecular
compound, atoms of different elements share electrons. These compounds are covalent
compounds, because of the covalent bonds in them. Hydrogen chloride, water,
methane, ammonia and carbon dioxide are all covalent compounds.
The bonds within covalent molecules are very strong, but as the particles have
no electric charge, there are only very weak forces between the molecules. Very
little energy is needed to separate the molecules. Because of this, molecular
substances have low melting and boiling points. Small molecules like hydrogen
and oxygen are therefore gases at room temperature, while larger molecules may
be liquids, like water or soft solids, like iodine. Simple molecular compounds
do not conduct electricity, even when melted. This is because: the molecules
do not carry an overall electric charge.
Atoms that share electrons can also form giant molecular structures or macromolecules.
These have hundreds of thousands of atoms joined by strong covalent bonds. Sand,
diamond and graphite all have giant molecular structures. Their different structures
also help to explain why they have such varied properties. Diamond and graphite
are made up of the same element - carbon, and exist in the same physical state.
However, the difference In their structure helps explain why diamond is the
hardest naturally occurring material known, whereas graphite, is one of the
softest materials. In the diamond molecule, each carbon atom is held by four
others in a regular tetrahedral arrangement. This makes the structure very difficult
to break - a lot of energy would be needed to separate the atoms. For the same
reason, diamond has a very high melting point. Diamond cannot conduct electricity
because each atom is covalently bonded to 4 others, so there are no free electrons
or ions to carry charge.
The molecules in graphite are organised in layers. Within these layers, each
carbon atom is joined to three others by strong covalent bonds, forming a pattern
of hexagonal rings. Because each atom has 4 outer electrons, but forms only
3 bonds, the fourth electron is free to move through the graphite. For this
reason, graphite is a good conductor of electricity. The layers then lie on
top of each other, held together by weak forces. The layers can therefore slide
across one another easily, rather like sheets of paper. The graphite is therefore
soft and slippery. When you draw layers of graphite are removed from the pencil
lead and stick to the paper.
Metals are also giant structures. The atoms are packed tightly together in a
regular pattern. The electrons from the outer energy levels of metal atoms are
free to move through the whole structure. When the metal atoms lose these electrons,
they become positive ions. Metals therefore consist of positive ions surrounded
by a sea or cloud of moving electrons. The negatively charged electrons attract
the positive metal ions and bond them together with strong electrostatic forces
of attraction. This is the metallic bond. It gives metals their special properties.
They are good conductors of heat and electricity due to the mobile electrons,
which carry both heat energy and charge. They are malleable and ductile, they
can be easily bent or stretched because the rows of atoms can slide over one
another. They have high densities because the atoms are very closely packed
in a regular manner.