Chemistry, Third edition

2 · ELEMENTS, COMPOUNDS AND REACTIONS

Revision questions

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2.1.Classify each of the following as an element, compound or

mixture:

(i) milk (vi) brass

(ii) white sand, or silicon dioxide (vii)nickel

(iii)rubidium (viii)ink

(iv)soil (ix) barium

(v) carbon dioxide (x) chalk.

2.2.The element lithium has a melting point of 181 °C and a

boiling point of 1342 °C. It is a soft metal that corrodes rapidly in air.

Lithium also conducts electricity and reacts violently with water.

Classify all of these properties of lithium as physical or chemical.

2.3.Ethanol (common alcohol) is a compound with a melting

point of 114°C and a boiling point of 78 °C. What is its

physical state at room temperature (room temperature is about

20 °C)?

2.4.Butane is the chemical name for the compound used in

lighter fuel. It melts at 138 °C and boils at 0 °C. What is its

physical state at room temperature?

2.5.Use your library to look up the words from which the

symbols for the following elements are derived:

(i)copper(ii)antimony(iii)mercury.

2.6.Write the correct symbols or formulae for each of the

following:

(i) one atom of oxygen

(ii)two atoms of oxygen

(iii)two molecules of oxygen gas

(iv)three molecules of sulfur trioxide

(v)four atoms of neon.

2.7.How many atoms of each element are present in the

following?

(i)Li 2 CO 3 (ii)Ca(NO 3 ) 2 (iii)(NH 4 ) 3 PO 4.

2.8.Write formulae for the following compounds:

(i) barium fluoride (vi) potassium hydroxide

(ii)tin(II) bromide (vii)sodium bromate

(iii)tin(IV) chloride (viii)potassium dichromate

(iv)sodium ethanoate (ix) ammonium phosphate

(v)aluminium sulfide (x) lead(II) cyanide.

2.9.Find out a common use for hydrogen peroxide (H 2 O 2 ).

2.10.Liquid hydrogen peroxide slowly decomposes into water

and oxygen gas. Write a balanced equation for the reaction.

2.11.Balance the following equations:

(i) KClO 4 KClO 2

(ii) S 8 AsF 5 S 16 (AsF 6 ) 2 AsF 3

(iii) HgNH 4 IHgI 2 H 2 NH 3

(iv) Al 4 C 3 H 2 OAl(OH) 3 CH 4

(v) ZnH 3 PO 4 Zn 3 (PO 4 ) 2 H 2

(vi) Fe 2 O 3 Na 2 CO 3 NaFeO 2 CO 2.

2.12.Construct balanced equations (including state symbols)

to describe the following reactions:

(i) Zinc metal burns in oxygen gas (dioxygen) to form solid

zinc oxide.

(ii) Potassium metal reacts with water to form potassium

hydroxide solution and hydrogen gas (dihydrogen).

(iii)Carbon dioxide gas reacts with magnesium metal to give

solid magnesium oxide and carbon.

DID YOU KNOW?

Alloys

An alloy is NOT a Compound. Alloys are mixturesof a metal and at least one other substance – that sub-

stance might be another metal, a non-metal or a compound. Alloying a metal with other substances

modifies its physical properties such as: strength, appearance and conductivity.

Some examples are:

(a) Pure iron corrodes easily and is soft but when alloyed with carbon, and various other metals, steels

with different properties are produced. Stainless steel, for example, contains chromium and nickel.

(b) Amalgamsare alloys in which one metal is mercury.