Samacheer Class 7 Science - Matter Around Us

Chemical fromula
Elements present
a.
Sodium chloride
NaCl
Sodium (Na), Chlorine (Cl)
b.
Potassium hydroxide
KOH
Potassium (K), Oxygen (O), Hydrogen (H)
c.
Carbon-di-oxide
CO 2
Carbon (C), Oxygen (O 2 )
d.
Calcium oxide
CaO
Calcium (C), Oxygen (O 2 )
e.
Sulphur dioxide
SO 2
Sulphur (S), Oxygen (O 2 )

A chemical formula is a symbolic representation of one molecule of an element or a compound.
It provides information about the elements present in the molecule and the number of atoms of each element. The chemical formula tells us the types of atoms and the number of each type of atom in one molecule of substance.

(a) Element: It is a substance that cannot be broken down into simpler substance by chemical means Ex. : Oxygen, Hydrogen, Gold & Helium.
(b) Compound A compound is a pure substance that is formed when the atoms of two or more elements combine chemically in definite proportions. Ex : H?0, NaCl.
(c) Metal A chemical element that is an effective conductor of electricity and heat can be defined as a metal. Ex.: Copper, Iron, Silver, etc.
(d) Non-Metal Non-metal is an element that doesn’t have the characteristics of metal including, (i.e.) ability to conduct heat or electricity luster or flexibility. Ex. Carbon Iodine, Sulphur.
(e) Metalloid : Metalloid is a chemical element that exhibits some properties of metals and some of non-metals. Metalloids are generally semi-conductors. Ex. : Silicon. Arsenic, Antimony and Boron.

Elements are chemically the simplest substances and hence cannot be broken down using chemical reactions.
It is made of entirely from one type of atom.
Example : Hydrogen, Oxygen is made from atoms containing a single proton and a single electron.

When an atom combines with another atom (or atoms) and forms a compound it is called as molecule.
A molecule is made up of two or more atoms chemically combined.

A compound is a pure substance that is formed when the atoms of two or more elements combine chemically in definite proportions.
Example : H 2 O, NaCl,

Metals
Non-Metals
Metals are lustrous. They have a shiny
Non metals are non lustrous. They
surface
have non- shiny surface
Metals are generally hard
Non-metals are generally soft
Most metals are bendable
Non-metals are non bendable
Most metals can be bent, beaten into sheets and they can drawn into wires
Non-metals are non ductile
Most metals are good conductors of electricity
Non-metals are bad conductors of electricity
Most metals are good conductors of heat
Non-metals are bad conductors of heat
Most metals are making ringing sound when struck. Hence, they are used to make objects like bells
Non-metals does not make any sound when they struck
Ex. : Copper, Lead, Tin, Nickel
Ex. : Carbon, Iodine, Sulphur

A compound is formed only when the constituent elements combine in a fixed proportion.
The properties of a compound are different from those of its constituent elements
A compound cannot be broken down by physical methods.
This is because a compound is made up of different elements that are chemically combined. Sodium chloride cannot be separated by physical methods such as filtration.
A compound can be separated into its constituent elements by chemical methods only.

The following rules are followed while assigning symbol to an elements:
Chemical symbols usually consist of one or two letters.
The symbols of most elements correspond to the first letter (which is capitalized) of their English name. For example, the symbol for oxygen is “O” and that for hydrogen is “H”.
When there is more than one element that begins with the same letter, their symbols take two letters.
The first letter is capitalised while the second letter has a lower case.
For example, the names of both hydrogen and helium begin with H. So, hydrogen is represented by the symbol H and Helium by He.
Example:

Elements
Compounds
An element is the simplest substance
A compound is a chemical substance formed by the combination of two or more elements
Elements combine to form compounds
Compounds can be split into elements
Atoms are the fundamental particle of an element
Molecules are the fundamental particles of a compound

A compound is formed only when the constituent elements combine in a fixed proportion.
The properties of a compound are different from those of its constituent elements.
A compound cannot be broken down by physical methods.
Compound is made up of different elements that are chemically combined.
A compound can be separated into its constituent elements by chemical methods only.

Metals
Non-Metals
Metals are lustrous. They have a shiny
Non metals are non lustrous. They
surface
have non- shiny surface
Metals are generally hard
Non-metals are generally soft
Most metals are bendable
Non-metals are non bendable
Most metals can be bent, beaten into sheets and they can drawn into wires
Non-metals are non ductile

Properties of metalloids :
Metalloids usually look like metals but behave largely like non-metals.
Physically they are shiny, brittle solids with intermediate to relatively good electrical conductivity.
They can from alloys with metals.
Most of their physical and chemical properties are intermediate in nature, (v) They are semiconductors.
All are solids at room temperature.
Example : Germanium, Boron, Antimony, Silicon.

Metals and their uses in our houses & schools :
Magnesium is used in the laboratory as magnesium ribbon.
Copper is used cooking utensils.
Zinc is mainly used as a protective coat for iron is our school and house gates.
Aluminum is used as cooking utensils and electrical cables which are used in our schools and houses.
Iron and steel are widely used is construction of house and school.
Sodium is used as a table salt and for flavoring, preserving food.
Coins are made up of nickel.
Lead is used in car batteries. Lead based alloys find extensive use in printing
Non-metals and their uses in houses & school:
Oxygen is essential for the breathing. It is used as oxidising agent in laboratory. Chlorine is used for purifying water.
Graphite is used in pencil leads.
Carbon is used as a fuel.
Bromine is used in dyes.
Iodine is used in laboratory to test for starch.
Metalloids and their uses in houses & schools :
Silicon is used in glass items.
Antimony is used in optical discs.
Tellurium is used in solar cells.
Boron is used in washing powders.
Germanium is used to make transistors which is used in laboratory in electrical devices.
Comparsion of properties of metals, non-metals & metalloids
Metals
Non-metals
Metalloids
They have lustre
They are not lustrous
Intermediate
Conduct heat and electricity
Poor conductors of electricity except graphite
Intermediate (semi­conductors)
They are malleable and ductile
They are neither malleable nor ductile.
Intermediate

(a) Solids (Iron) expand on heating and contract on cooling. This causes the volume to matter of increase during heating or expansion, the mass of matter does not change.
(b) During heating the distance between the particles of the iron latch change. In an iron latch the distance between the iron particles increases when they gain enough heat. So during hot days the latch on the gate was difficult to open. At night, because of cooling the distance between the particles decreases. So at night it was not difficult to open the latch on the gate.

When a substance is heated, its internal energy increases.
The movement of its particles increases.
Bonds between particles break when a substance melts evaporates. This causes the volume of matter to increase.
The closeness, arrangement and motion of the particles in a substance change when it changes state. But the size of the particles remain in same size, (i.e.) the mass of matter does not change.