Chemistry, Third edition

2 · ELEMENTS, COMPOUNDS AND REACTIONS

the mass of charcoal used up, you would find that the sum was exactly equal to the

mass of (invisible) carbon dioxide produced.

When a substance undergoes a chemical change, it changes into another type of

matter, often with quite different chemical properties. For example, heated sodium

reacts violently with chlorine. Sodium is a highly reactive metal, whereas chlorine is

an extremely poisonous gas. After the reaction sodium chloride (salt) is formed; this

is not a particularly reactive substance and it is essential to life!

Physical and chemical properties

The physical properties of a particular type of matter are those properties of the sub-

stance that do not involve a chemical reaction. Examples are state (solid,liquid,gas

orin solution), melting point, boiling point, colour and electrical conductivity.

The chemical properties of a substance describe the chemical reactions that a

substance will undergo. For example sodium reacts violently with water to form a

solution of sodium hydroxide and hydrogen gas. The reactivity shown by sodium

towards water is a chemical property of sodium.

Extensive and intensive properties

Any property of matter that can be measured can be classified as extensiveorintensive.

Extensive properties depend on the amount of substance present; intensive

properties do not.

Volume is an extensive property. If you have 100 cm^3 of water and add 50 cm^3

more, the total volume is 150 cm^3 – the volumes can be added to give a total.

Temperature is an intensive property. If you have water at a temperature of 50 °C

and add more water at 50 °C the temperature of the whole is still 50 °C and not100 °C.

Chemical formulae

Symbols

Chemists use a kind of shorthand to describe elements and compounts. All

elements have characteristic symbols. These symbols sometimes consist of the first

letter(s) of the name of the element; for example, hydrogen is given the symbol H,

whereas the chemical symbol for bromine is Br. Some symbols, however, are not

derived from the modern or English name of the element: gold has the chemical

symbol Au (from the Latin Aurum), whereas tungsten is denoted by W (from Wol-

fram, a German word). Note that the first letter only of a chemical symbol is written as a

capital letter. Box 2.4 gives the symbols of some elements.

2.3

20

Extensive and

intensive

properties

Classify the following

properties as extensive or

intensive:

(i)length(ii)colour

(iii)mass(iv)density.

Exercise 2C

Physical and

chemical properties

Classify the following as

physical or chemical

properties:

(i) thehardnessof

diamond

(ii)the low densityof

aluminium

(iii)woodburnsin air

(iv)arsenic is poisonous

(v)milk goessour.

Exercise 2B

Using symbols and formulae

1. Use Information Box 2.4 to name the following elements:
   (i)Ba(ii)Si(iii)Ar(iv)F(v)Li(vi)Hg(vii)Sn(viii)Ag(ix)B(x)Ti.


2. Name the elements in the following compounds and state how many atoms of each element
   are present:
   (i)CsBr(ii)CuCl 2 (iii)Fe 3 O 4 (iv)CH 4 (v)PbSO 4 (vi)K 2 Cr 2 O 7 (vii)Na 3 PO 4 (viii)MnO 2.

Exercise 2D