Page 114 - ICSE Chemistry 8

A catalyst can be of two types: posi ve catalyst and nega ve catalyst. A catalyst which helps to increase
the rate of a reac on is called a posi ve catalyst. A catalyst which helps to decrease the rate of a reac on
is called a nega ve catalyst. The chemical equa on in such cases should indicate the use of a catalyst.
Reversibility

Some chemical reac ons proceed in both the direc ons and are called reversible reac ons. The chemical
equa on for such reac ons should indicate the double arrow ( ) between the reactants and
products.
Endothermic or Exothermic

A reac on in which energy is absorbed is called an endothermic reac on. A reac on in which energy is
evolved or given out is called an exothermic reac on. The absorp on of energy in a chemical reac on
can be indicated by adding heat on the reactant(s) side and the evolu on of energy can be indicated by
adding heat on the product(s) side.

Take a Break!

Write true (T) or false (F).

1. The law of conserva on of mass states that ma er is neither created nor destroyed.
2. A chemical equa on does not inform us about the name of the reactants.
3. Some chemical reac ons occur only when they are present in a solu on form.

4. All the chemical reac ons occur at high temperatures.
5. Photosynthesis is a chemical reac on that occurs in green plants in the presence of light.
6. A reac on in which energy is absorbed is called an exothermic reac on.

Terms to Know

Element: The simplest form of a pure substance which is made up of only one kind of
atom
Compound: A pure substance which is formed when two or more elements chemically
combine in a defi nite propor on by mass
Valency: The combining capacity of an atom of an element
Valence shell: The outermost shell of an atom
Valence electrons: The electrons present in the valence shell

Ion: A charged par cle which is formed when an atom or a group of atoms gains
or loses electrons
Ca on: An ion formed when an atom or a group of atoms loses electron(s) by
developing a posi ve charge
Anion: An ion formed when an atom or a group of atoms accepts electron(s) by
developing a nega ve charge
Radical: A group of diff erent atoms present as a single unit having a posi ve or
nega ve charge on it

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