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Transcript of Chapter 4
Understand what are elements,
Elements, Compounds and Mixtures
compounds and mixtures
elements, compounds and mixtures
Describe the differences between
Solid, liquid or gas
Metal, or non-metal
Molecules / Ions
be broken down into two or more simpler substances chemically
Video: 1:17 - 2:22
represented by chemical symbols (eg. Sodium - Na)
two or more elements chemically combined
in fixed ratio
different properties as the elements that it is made up of
Made up of two or more substances
homogeneous / heterogeneous
salt + water oil + water
Both are pure substances
One type of atom
Two or more elements
Both are made up of more than one type of elements
Mixed in fixed proportion
No fixed proportion
Different physical and chemical properties as constituent elements
Same chemical properties as its components
Chemical reaction takes place when formed - energy involved in its formation
No chemical reaction when formed - no energy involved in its formation
Can be separated into elements by chemical reactions eg electrolysis
Can be separated into its components by physical methods eg filtration
involves an energy change
similar chemical properties as its components
Usually high melting / boiling points
Usually low melting / boiling point
Good conductors of heat and electricity
Poor conductors of heat and electricity
Malleable and ductile
Brittle and non-ductile
Shiny and lustrous
Dull in appearance
(Will be covered after learning structure of atoms and bonding)
(Behave like both metals and non-metals)
Solid at r.t.p (except Hg)
Usually gases / liquids at r.t.p
An atom is the simplest substance of an element which can not be further broken down by chemical process.
A molecule is made up of two or more atoms chemically combined together.
Eg. Air (oxygen, carbon dioxide, nitrogen, other gases)
Mixture can exist in various ways
one element, one compund
Hydrogen & Neon
Hydrogen & Ammonia
The ratios of the components are
The components of a mixture can be separated by
methods, e.g. filtration, distillation or chromatography.
No chemical reaction takes place when a mixture is formed — usually there is
little or no energy change
An alloy is a mixture of metals with other elements (usually metals but sometimes non-metals such as carbon).
Alloys are widely used because they tend to be stronger than pure metals.
Examples of Alloy
: iron, carbon
: iron, chromium, nickel, carbon
: copper, zinc
: copper, tin
: aluminium, copper, magnesium
: lead, tin
Ions are electrically charged particles (either positive or negative)
Example: NaCl (Na and Cl )
Chemical Formula of a Compound
The chemical formula states
the types of atoms
the ratio of the different atoms
Number of Hydrogen atoms (two)
Thus, ratio of H:O is 2:1 and this is
A compound made up of two elements has a name that ends in -ide
• Sodium chloride — elements sodium and chlorine
2. A compound that contains hydroxide ions , OH– (a negatively charged ion made up of oxygen and hydrogen) is named a hydroxide.
• Potassium hydroxide — potassium ions and hydroxide ions
3. A compound that contains a negatively charged polyatomic ion containing oxygen usually has a name ending in –ate.
Carbon dioxide — elements carbon and oxygen
• Copper(II) sulphate — contains oxygen atoms in the sulphate ion
Writing Chemical Formula
1. For many compounds that contain both metallic and non-metallic elements, the symbol of the metallic element is written first.
2. The number of atoms is written as a subscript, to the right of the atom’s symbol.
• Water (H O, not H2O or 2HO or HO)
Calcium oxide (CaO), Magnesium carbonate (MgCO )
3. It is not necessary to write the subscript ‘1’.
Calcium oxide (CaO, not Ca O )
4. The oxygen atom is usually written at the end of the formula.
Carbon Monoxide (CO, not OC)
How many atoms of each element are there?
Name the compounds.
Heat can be used to break down compounds into elements or simpler compounds. Such a chemical reaction is called thermal decomposition.