Page 55 - Science Class 07
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v Conversely, it can also be broken down into physical change can o en be reversed, a
elements hydrogen and oxygen by passing chemically change typically cannot be, except
electric current through it. through more chemical reac ons. When a
Water compound Hydrogen + Oxygen chemical change occurs, there is also a change
elements in the energy of the system. A chemical change
that gives off heat is called an exothermic
Till about 200 years ago, water was considered
reac on. One that absorbs heat is called an
to be an element. It was Henry Cavendish who
endothermic reac on.
proved that water is a compound of hydrogen
and oxygen. Similarly, lime (calcium oxide) was Chemical Formula
also believed to be an element. It was Sir In order to understand the chemical reac ons
Humphry Davy who proved that it is compound easily and clearly, we must first follow the
of calcium and oxygen. terms molecule and chemical formula.
This oxygen was not known ll 1774 A.D. when We know that:
J.B. Priestly obtained it from mercury oxide. v Atom is the smallest par cle of an element.
The proper es of mixtures and compounds can
v Atoms of elements combine together to
be expressed in a tabular form to bring out a
form compara vely bigger par cles called
comparison between the two.
molecules.
v Chemical formula is used to represent one
molecule of a substance (element or
compound) in the form of symbols.
Chemical Formula of an Element
1. What is a mixture?
v The elements having a single atom in their
2. The mixture of sugar and salt is an molecule are called monoatomic. Most of
example of homogenous or the elements are monoatomic. Therefore,
heterogeneous mixture? Why? while wri ng their chemical formula, we
3. We cannot separate the cons tuents of a just write their symbols. All metals and
compound by any physical means. Why? noble (insec ve) gases are monoatomic.
4. How compounds differ from mixture? For example, Chemical formula of Silver
5. Lime is an element or a compound. Why? (metal) is Ag. Chemical formula of Neon
(noble gas) is Ne.
CHEMICAL REACTIONS v One molecule of some non-metals contain
A chemical change, also known as a chemical two or more atoms. If one molecule of an
reac on, is a process where one or more element contains two atoms, they are
substances are altered into one or more new called diatomic. In this case, we indicate
and different substances. In other words, a the numbers of atoms by a subscript on the
chemical change is a chemical reac on right hand side of the symbol. Thus,
involving the rearrangement of atoms. While a chemical formula of Oxygen wri en as O .
2
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