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FIND OUT MORE. Atoms 157 • Chemistry 162 • Electricity 182 • Heat 168–169 • Matter 156 • Mixtures 172 • Micro-organisms 85

HOW DOES CONCRETE CHANGE FROM

A LIQUID TO A SOLID?

Concrete is a mixture of sand, gravel, cement, and

water. Cement powder contains calcium oxide (lime)

and silica or similar chemical compounds (substances

that are two or more elements). When cement is

mixed with water, the compounds react and set into a

solid. The setting cement glues the sand and gravel

particles together to make a permanent solid structure.

HOW DO CHEMICAL REACTIONS

CHANGE MATERIALS?

Chemical reactions take place when existing bonds

between atoms are broken and new bonds form. When

the gas ethene is heated at high pressure, its molecules

join together in long chains to make the plastic

polythene. Polythene is used to make washing up

bowls, squeezy bottles, and plastic bags.

CAN STONE MELT?

Candle wax melts at 60°C (140°F), lead melts at

327.5°C (621.5°F), iron at 1,540°C (2,804°F). Even

stone can melt. The material with the highest

known melting temperature is the metal tungsten,

which melts at 3,387°C (6,129°F). Tungsten wire is

used to make the filaments of electric light bulbs

and television tubes.

We can use chemical reactions and heat to change

materials and their properties to our needs. Some

changes are. PERMANENT , others are. REVERSIBLE.

HOW DOES HEAT CHANGE MATERIALS?

Heat makes many solids, especially metals, softer and

easier to shape. As the temperature rises, most solids

eventually melt to the liquid state, but some materials

react differently to heat. Heat can trigger chemical

reactions between mixtures. In an oven heat changes

a cake mix from a sticky liquid into a fluffy solid.

WHAT MAKES GLUE STICK?

A good glue is a substance that starts as a liquid, but

transforms into a solid when exposed to air. As a

liquid, the glue can flow into every nook and cranny

of the surfaces where it is spread. The glue molecules

form bonds with the molecules in the surfaces. As the

glue sets, the surfaces are held firmly together.

Melting and boiling are reversible changes produced by heat.

Steam from a boiling kettle condenses back into drops of water

when it comes into contact with a cold surface, such as a window.

Burning, rusting, and cooking are permanent changes. They can’t

be undone by reversing the conditions that brought them about.

Changing Materials

4 LAVA FLOW

Liquid lava pouring from a

volcano is solid rock melted by

heat from the Earth’s core at

about 700°C (1,292°F). The lava’s

surface cools first, setting into a

thin skin that wrinkles as the lava

moves. When it is completely

cool, it sets back into solid rock.

PERMANENT CHANGES

REVERSIBLE CHANGES

CONCRETE 3

An electron microscope picture

of setting concrete shows the

changes that take place as the

concrete hardens. As cement

reacts with water, crystals form,

bonding sand and gravel

particles together.

RUSTING IRON 1

Any iron object left outside in

the wet becomes covered with

an orange-brown substance

called rust. Rusting is a

chemical reaction between

iron, oxygen, and water.

ROT AND DECAY 3

The complex molecules in living

things are decomposed by

micro-organisms after death.

Water escapes

into the

atmosphere and

the nectarine

shrivels and dies

Fungus spores

multiply on the

nectarine skin,

breaking down

its structure as

they feed

FRESH NECTARINE ROTTEN NECTARINE

changing

materials