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162 Science and Technology

pH 14

Caustic soda

(oven cleaner)

Caustic soda reacts with

fat and grease marks, and

destroys skin and flesh.

pH 4

Oranges

Oranges are mildly acidic,

which is why they taste

slightly sharp.

pH 1

Hydrochloric acid

This is a strong, corrosive

acid. It eats through most

metals in a fizzing reaction.

pH 2

Lemon juice

Lemons taste sour because

they contain citric acid.

Acid means “sour” in Latin.

pH 11

Milk of magnesia

This alkali is used to cure

indigestion – it neutralizes

the acid in your stomach.

pH 9

Liquid soap

All cleaning products are

bases. Like most bases,

soap feels slippery.

pH 6–7

Pure water

Pure water is neutral –

it is neither an acid nor

a base.

pH 3

Vinegar

The acid in vinegar kills

bacteria, so it is used in

pickling to preserve food.

pH 7.5

Blood

Medical injections contain

chemicals called buffers to

match the pH of blood.

WHAT ARE ACIDS AND BASES?

Acids are chemicals that react with metals.

Strong acids, such as sulphuric, nitric, and

hydrochloric acid, are very corrosive. A base is

a chemical that neutralizes an acid. When acids

and bases react, they form chemicals called salts.

Alkalis are bases that dissolve in water.

Chemistry is the science of matter. Scientists study

chemicals, their properties, and. REACTIONS.

. BIOCHEMISTRY is the chemistry of living things.

WHAT IS A CHEMICAL?

Chemicals are the basic types of matter. The elements,

such as hydrogen and oxygen, are chemicals. So are

the compounds, such as water, that are made when

atoms of different elements join. Our bodies, food, and

the clothes we wear are all made from chemicals.

WHY IS LIFE BASED ON CARBON?

Carbon atoms can link to each other (and to other

elements) in many, many ways, including rings

and chains of any length. This enormous variety

means carbon is the perfect main building block

for the complicated molecules of life.

WHY ARE SOME CHEMICALS DANGEROUS?

Our bodies contain tens of thousands of chemicals

that mix and react to give us energy and keep us

alive. Toxic (poisonous) chemicals interfere with these

reactions. Some chemicals are also corrosive – they

eat through solid material.

WHAT ARE CHEMICAL PROPERTIES?

The properties of a chemical describe what it does to

other chemicals. Oxygen, for example, makes iron

rust, or oxidize, so oxygen is an oxidizing agent.

A chemical reaction happens when two

or more chemicals combine to make new

chemical compounds. Many reactions

release heat, for example burning.

Biochemistry studies the molecules in

living things, including plants, animals,

and the human body. These molecules are

all compounds of the element carbon.

DIAMOND 3

The carbon atoms in diamond are

bonded in a strong 3-D network.

This makes diamond the hardest

material on Earth.

4 GRAPHITE

Graphite is a form of pure carbon. The carbon

atoms in graphite are bonded in sheets that

slide over each other easily. This is why a

graphite pencil leaves black marks.

STRONG REACTION 3

Chemists use experiments

to find out how chemicals

react when they are mixed

in different ways. Some

chemicals react strongly,

bubbling, burning, or even

exploding. Other chemicals

do not react at all.

4 THE pH SCALE

The pH scale is a measure of acidity. Very strong acids have a pH of 1,

neutral chemicals are pH 7, and strong bases, or alkalis, are pH 14.

Chemistry

CHEMICAL REACTIONS

Two liquids

react in a flask

Vapour

escapes

into the

atmosphere

BIOCHEMISTRY

FIND OUT MORE. Elements 160–161 • Materials 170 • Matter 156

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