The Language of Chemistry
ICSE Class 9 · Chemistry · Chapter 1
What this chapter covers
- Symbols of elements
- Valency
- Radicals
- Compounds: names and formulae
- Chemical equations
- Relative atomic mass and relative molecular mass
- Percentage composition of a compound
Key points
- A symbol stands for an element and for one atom of it; some symbols come from Latin names (Na, K, Fe, Cu, Ag, Au, Pb, Hg, Sn).
- Valency is the combining capacity of an element, measured against hydrogen; for metals it equals the number of valence electrons, for non-metals it is 8 minus that number.
- Elements may be monovalent, divalent, trivalent or tetravalent; some metals show variable valency (-ous for lower, -ic for higher).
- A radical is a charged atom or group that acts as one unit; basic radicals are positive, acid radicals negative.
- Free radicals in the air (OH, NO, Cl) are highly reactive; harmful ones are reduced by cutting CFCs, vehicle exhaust and burning.
- Formulae are written by crossing over valencies, simplifying, and bracketing compound radicals with subscripts.
- A chemical equation shows reactants and products by formulae; it must be balanced because mass is conserved.
- Balance by hit and trial, changing only coefficients, never subscripts.
- Relative atomic mass and relative molecular mass compare masses with one-twelfth of a carbon-12 atom (earlier, with a hydrogen atom) and have no unit.
- Percentage of an element = (mass of the element in one molecule ÷ relative molecular mass) × 100; all percentages add to 100.
Questions answered
Define a symbol of an element. What two things does the symbol O represent?
Write the symbols of sodium, potassium, lead and mercury, and name the language they come from.
Define valency. Why is the valency of oxygen 2?
Classify these as monovalent, divalent, trivalent or tetravalent: Na, Ca, Al, C, Cl, Mg.
What is the difference between a basic radical and an acid radical? Give two examples of each.
Name one free radical present in the atmosphere, one harmful effect it has, and one way of reducing it.
Write the formulae of: magnesium nitrate, ammonium carbonate, iron(II) sulphate and calcium phosphate.
Name the compounds: K2Cr2O7, NaHSO4, Cu2O, N2O5.
Past board exam questions
ICSE 2019 sample paper · 5 marks
(i) Balance each of the chemical equations given below: 1. FeCl₃ + NH₄OH → NH₄Cl + Fe(OH)₃ 2. KI + Cl₂ → KCl + I₂ (ii) Identify the substance which matches the description given below: 1. White crystalline substance which sublimes on heating. 2. The gas which turns lime water milky but has no effect on potassium dichromate paper. 3. The metal that cannot displace hydrogen from dilute hydrochloric acid.
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ICSE 2019 sample paper · 3 marks
The formula of the chloride of a metal ‘M’ is MCl₂. State the formula of its: (i) Carbonate (ii) Nitrate (iii) Hydroxide
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ICSE 2019 sample paper · 3 marks
Write balanced chemical equations for each of the following: (i) Reaction of iron with chlorine. (ii) Addition of silver nitrate solution to sodium chloride solution. (iii) Addition of zinc to sodium hydroxide solution.
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ICSE 2019 sample paper · 4 marks
Calculate the relative molecular mass of each of the following: (i) Ammonium dichromate [(NH₄)₂Cr₂O₇] (ii) Hydrated Copper Sulphate [CuSO₄.5H₂O] [H = 1, N = 14, O = 16, S = 32, Cr = 52, Cu = 64]