Language of Chemistry
ICSE Class 8 · Chemistry · Chapter 5
What this chapter covers
- Why chemistry needs its own language
- Symbols of the first 20 elements
- Radicals and their valencies
- Formulae of compounds
- From word equations to chemical equations
- The law of conservation of mass
- Balancing simple equations
- State symbols
- Information from a chemical equation
- Limitations of a chemical equation
Key points
- The language of chemistry is made of symbols, formulae and equations.
- A symbol stands for an element and for one atom of it; the first letter is a capital and any second letter is small.
- Na (natrium) and K (kalium) come from Latin names.
- Valency is the combining capacity of an element or radical, measured by the hydrogen atoms it combines with or replaces.
- A radical is an atom or group of atoms that acts as one charged unit; compound radicals take brackets when more than one is needed.
- Formulae are written from valencies by the criss-cross method, then simplified to the smallest whole-number ratio.
- A word equation uses names; a chemical equation uses symbols and formulae.
- The law of conservation of mass says the mass of products equals the mass of reactants.
- An equation is balanced by changing coefficients, never subscripts.
- State symbols (s), (l) and (g) show solid, liquid and gas.
- A balanced equation shows the reactants, products, number of molecules and atoms, and their ratio.
- An equation does not show the catalyst, the conditions, the speed or heat change, or whether the reaction happens at all.
Questions answered
What are the symbols of sodium, potassium and chlorine? Why do sodium and potassium have symbols that do not match their English names?
Na, K and Cl. The symbols of sodium and potassium come from their Latin names, natrium and kalium.
A student writes the symbol of magnesium as "MG". What is wrong?
The second letter of a symbol must be small, so the correct symbol is Mg.
Write the valency of nitrogen, aluminium, sulphur and argon.
Nitrogen 3, aluminium 3, sulphur 2, argon 0.
Use valencies to write the formulae of (a) sodium chloride, (b) magnesium hydroxide, (c) calcium phosphate.
(a) NaCl. (b) Mg(OH)2, since Mg has valency 2 and OH has valency 1. (c) Ca3(PO4)2, since Ca has valency 2 and PO4 has valency 3.
Count the atoms of each element in Ca(OH)2.
1 calcium, 2 oxygen and 2 hydrogen atoms, making 5 atoms.
In the compound AlCl3, what is the valency of aluminium? Give your reason.
Chlorine has a valency of 1, and one aluminium atom holds three chlorine atoms, so aluminium has a valency of 3.
Change the word equation "zinc + sulphuric acid → zinc sulphate + hydrogen" into a chemical equation. Is it balanced?
Zn + H2SO4 → ZnSO4 + H2. It is already balanced, with 1 Zn, 2 H, 1 S and 4 O on each side.
Balance N2 + H2 → NH3.
N2 + 3H2 → 2NH3. There are 2 N and 6 H on each side.