Page 31 - Chemistry ICSE Class IX

The Language of Chemistry 19

EXAMPLE 1.6 Write the balanced chemical equations for the following reactions:
(a) Calcium hydroxide + Carbon dioxide o Calcium carbonate + Water
(b) Zinc + Silver nitrate o Zinc nitrate + Silver
(c) Lead + Copper chloride o Lead chloride + Copper
(d) Barium chloride + Sodium sulphate o Barium sulphate + Sodium chloride

Solution:
(a) Ca(OH) 2 + CO o CaCO 3 + H O
2
2
calcium hydroxide carbon dioxide calcium carbonate water
(b) Zn + 2AgNO o Zn(NO ) + 2Ag
3
3 2
zinc silver nitrate zinc nitrate silver
(c) Pb + CuCl o PbCl 2 + Cu
2
lead copper chloride lead chloride copper
(d) BaCl 2 + Na SO o BaSO 4 + 2NaCl
4
2
barium chloride sodium sulphate barium sulphate sodium chloride
EXAMPLE 1.7 Write the balanced chemical equations with the state symbols for
the following reactions:
(a) Iron filings react with steam to produce solid iron(III) oxide and
hydrogen gas.
(b) Sodium hydroxide solution (in water) reacts with hydrochloric
acid solution (in water) to produce sodium chloride and water.
Solution:
(a) 4Fe(U) + 6H O(I) o 2Fe O 3 + 6H (I)
2
2
2
iron lings steam iron(III) oxide hydrogen
(b) NaOH(CS) + HCl(CS) o NaCl(CS) + H O(N)
2
sodium hydroxide hydrochloric acid sodium chloride water
EXAMPLE 1.8 Write a chemical equation for each of the following reactions:
(a) Zinc metal reacts with aqueous hydrochloric acid to produce a
solution of zinc chloride and hydrogen gas.
(b) When solid mercury(II) oxide is heated, liquid mercury and
oxygen gas are produced.
Solution:
(a) Zn(U) + 2HCl(CS) o ZnCl (CS) + H (I)
2
2
heat
(b) 2HgO(U) o 2Hg(N) + O (I)
2
Atomic Mass and Relative Atomic Mass
Atom is the smallest particle which takes part in a chemical reaction. A
molecule is the smallest unit of any substance which can exist independently,
and shows all the properties of that substance. Each molecule is a group of
chemically bonded atoms.
Each atom or a molecule has its own characteristic mass. So, the atomic
and molecular masses may be expressed in any unit of mass, i.e., in gram,
kilograms, etc. But the actual mass of a single atom is very small. An atom dŚĞ ĂƚŽŵŝĐ ŵĂƐƐ ƵŶŝƚ ŝƐ ĂďďƌĞǀŝĂƚĞĚ
ĂƐ ĂŵƵ͕ ĂŶĚ ĚĞŶŽƚĞĚ ďLJ ƚŚĞ ƐLJŵďŽů
of hydrogen (the lightest atom) is 0.000 000 000 000 000 000 000 000 001 Ƶ Žƌ ŵ ͘ tĞ ĂƌĞ ƵƐŝŶŐ ƚŚĞ ƐLJŵďŽů Ƶ
Ƶ
673 kg (1.673 × 10 –27 kg). It is not possible to measure such small masses ĨŽƌ ĂƚŽŵŝĐ ŵĂƐƐ ƵŶŝƚ ŝŶ ƚŚŝƐ Ŭ͘
even with the help of the most sensitive balance. Such small numbers are
not very convenient to write also.
IUPAC accepted in 1961 the atomic mass unit scale to be used for
expressing atomic and molecular masses. In this scale of atomic masses,
12
the C isotope has been assigned an atomic mass of 12.000000 atomic mass
6
units.