First Science Encyclopedia

During the day, plants The Earth’s cycles
take in carbon dioxide
and give out oxygen. From atmosphere
to the Earth
CARBON DIOXIDE
OXYGEN During an electrical storm,
some nitrogen is washed out
YGE N of the atmosphere and falls
to the ground. Plants can
then draw the nitrogen in
through their roots.

Oxygen cycle

Animals take in oxygen and use it to
release energy from their food. It is put
back into the air by green plants during
photosynthesis. Algae and plankton do

the same job in water.

Animals breathe in
oxygen and breathe
out carbon dioxide

all the time.

CARBON DIOXIDE OX

Our atmosphere contains 21 per cent oxygen and 78 per cent nitrogen. 49

50 In particular circumstances, carbon forms a hard crystal. What is it called? Carbon cycle It’s in the air Life science

Every living thing contains CARBON DIOXIDE RELEASEDPlants take in carbon Green plants take in carbon
carbon. Human beings CARBON DIOXIDEdioxide from thedioxide from the air and use
take in carbon through atmosphere it to make food, converting
carbohydrates, fats, and it into things such as
proteins in food, and release Animals eat plants and take in some carbohydrates. Animals take
it as carbon dioxide gas carbon. They breathe out carbon dioxide. in some of the carbon when
when breathing out. It is they eat plants.
also released from dead
matter, sometimes quite Animals
soon, sometimes millions
of years later in fuels Animals, such as these
such as oil and coal. sheep, contribute to the
carbon cycle by eating
An animal’s grass, breathing in air, and
droppings also dropping waste. They take
contribute to the in carbon from the plants
carbon cycle. they eat, and release it
when they breathe out.
Their bodies will release

more carbon when
they die.

CARBON

Waste matters DECOMPOSING Fossil fuels

Part of you might Sometimes
once have been the remains
part of a dinosaur. of organisms
Why? Because like are exposed to
all living things, extreme pressure
dinosaurs produced waste and heat. Over
and their waste became a millions of years,
part of the never-ending they turn into
carbon cycle. carbon-rich fuels,
A diamond. like coal and oil.

DECOMPOSINGWaste
disposal
Break it down DECOMPOSING Carbon cycle
When animals
51 Worms and bacteria die, their bodies
are an important part break down and
of the carbon cycle. decompose.
They help decompose
the soil, and break Plants and animals die
down organic matter and their bodies decay.
to its original elements,

such as carbon, and
other nutrients.

Materials science Four states
There are four main
What’s the states of matter.
matter?
Solids have a definite
Everything around you is made shape. Most of them
of matter, even the things you can’t are hard, such as rocks.
see. But everything looks and acts
differently – that’s because matter Liquids take on the
has different forms. shape of their container,
and have a fixed volume.
Solid, liquid, or gas
Gases have no fixed
The most common states of matter are solid, shape. They fill any
liquid, and gas. Each state behaves differently space they are in,
because the particles in their make-up move such as a balloon.
in different ways.
Plasma exists at very
high temperatures, like
inside the Sun.

Nearly everything on the Earth is solid, liquid, or

Planet Earth gas.

The Earth has a solid core, surrounded

by liquid rock, on which the solid

crust floats. Liquid water covers most

of the crust, and a layer of gas called

the atmosphere surrounds the planet.

Clouds are made of The blue areas are
liquid water droplets the oceans, which
and solid ice crystals – are liquid water.
they form from water

vapour, a gas.

The green areas are
land, which is made
of solid rocks.

52 Which form of matter are humans?

What’s the matter?

Picture detective
Look through the
Materials Science pages
and see if you can identify
the picture clues below.

Using gas

When air in a balloon
is heated, it becomes lighter
than the surrounding air and
quickly fills the balloon. The
lighter air rises, taking the
balloon and passengers with it.

No matter

A place with no matter,
not even air, is called
a vacuum. The closest
thing to a vacuum is
the space between stars.

Astronauts wear special Turn
suits in space because it and learn
is very cold and there is
no air to breathe. Amazing atoms:
pp. 58-59

The Universe:
pp. 94-95

We are three forms, our bones are solid, our blood is liquid, and we breathe in air. 53

Materials science

Properties of matter

Main properties Some materials are hard and
There are many different brittle, while others are flexible.
properties of matter. Some materials are colourful, while
others are transparent. These kinds
Boiling point is the of features are called “properties”.
hottest a liquid can get
before becoming a gas. A cork floats on oil.
Oil floats on water.
Freezing point is the
temperature at which Does it float?
a liquid becomes a solid.
It’s easy to learn
Plasticity is how well about some properties,
a solid can be reshaped. such as the ability
to float. The amount
Conductivity is how well of matter in a certain
a material lets electricity volume of an object
or heat travel through it. is called its density.
Objects and liquids
Malleability is how well float on liquids of a
a solid can be shaped higher density and
without breaking. sink through liquids
of a lower density.
Tensile strength is
how much a material can A plastic building brick sinks
stretch without breaking. through oil but floats on water.

Flammability is how An onion sinks through oil and
easily and quickly a water, but floats on syrup.
substance will catch fire. Syrup sinks below water.

Reflectivity is how well
a material reflects light.
Water reflects well.

Transparency is how A good insulator
well a material will let
light pass through it. Heat cannot easily pass
through some materials.
Flexibility is how easily
a material can be bent. These are known as
insulators. For example,
Solubility is how well aerogel can completely block
a substance will dissolve, the heat of a flame. But
such as salt in water.
don’t try this at home!

54

Is diamond harder than quartz?

Properties of matter

Compressibility

Gases can be

squashed, or

compressed, by

squeezing more

into the same

Broken space. This is what
glass happens when you
pump up a tyre.
Brittleness

Some materials, such as window Gas can be compressed Foot pump
glass, are particularly brittle. because its particles are far
They will break when pushed apart. A bicycle pump pushes
out of shape even a small amount. the particles closer together.

Hardness Gas particles Diamond is
the hardest
A scientist called Friedrich Mohs created a scale
mineral.
9
Corundum

using ten minerals to compare how hard they

are. Many materials are graded on this scale. 10
Diamond
5
Talc is the Apatite 6 7 8
softest mineral Feldspar Quartz Topaz
4
2 Fluorite
1 Gypsum
Talc 3
Calcite

hands on

Collect some
different pebbles and put
them in order of hardness. A
pebble is harder than another
if it scratches it. This is

how Mohs worked
out his scale.

A smooth flow

Some liquids flow more
easily than others. It depends
on their “stickiness”, or viscosity.
Hot lava from a volcano flows
slowly because it is sticky.

Yes, diomond is the hardest mineral of all. It can scratch quartz. 55

Materials science

Changing states

Liquid metal Many solids melt, to become liquids, when
they become hot enough. When liquids
Many substances melt get cold enough, they freeze and become
and boil at particular solid. This is called changing states and
temperatures (its melting it happens to all kinds of substances.
and boiling points). Most
metals are solid at everyday Changing states of water
temperatures because they
have a high melting point. Water exists as a solid, liquid, or gas.You can
But mercury has such a low find all three forms of water in your home.
melting point that it is liquid They are ice, water, and water vapour.
even at room temperature.

Condensation

As water vapour in the air is
cooled, it changes into liquid
water. This is called
condensation.
You can see it
on the outside
of a cold bottle.

When water Ice is solid water. It forms When ice is warmed, it As water is heated, bubbles of
vapour in the when liquid is cooled until melts and becomes liquid water vapour (gas) form. They
air touches a it freezes. Each piece of ice and takes on the shape of escape from the surface and
cold bottle, it has a definite shape. the container holding it. condense to form a mist of
condenses into liquid droplets called steam.

tiny drops Rivers of iron
of liquid.
Iron must be heated in
56 a furnace to make it melt.
Molten iron is so hot it
glows yellow. It is poured into
a mould and left to harden to
make solid iron objects.

Why does chocolate become soft and gooey in your mouth?

Changing states

Washing dries faster on a hot day, when
heat turns water into vapour very quickly.

Melting Evaporating
chocolate
In the open air, water slowly
turns into vapour – this is called
evaporation. Wet clothes dry on
a line because the water they
hold, evaporates.

mmm... frozen fruit in
M side

Freezing Melting Turn
and learn
Icicles are spikes of ice When you don’t eat your ice
that form when dripping cream quickly enough, it melts Actions and
water freezes.You often see reactions:
them on trees in winter. If and changes from a solid to a pp. 68-69
water keeps dripping down liquid! Chocolate melts too, and
and freezing, the icicle will The power of heat:
get longer and longer. makes your hands all sticky. pp. 86-87
Most solids will melt if the
57
temperature is high enough.

Because the warmth of your mouth makes it melt.

Materials science

Amazing atomsAtoms are tiny particles izz around the nucleus of the atom .

that make up everything Electrons wh Neutron

around us. Each atom of

a substance contains the

chemical properties

the substance is

made up of.

Inside an atom Electron Proton

Inside an atom are three tiny types
of particle: protons, neutrons, and
electrons. Protons and neutrons

make up the atom’s nucleus (core).
The electrons are outside this.

Oxygen atom

Hydrogen atom Hydrogen atom Au

A water molecule has GOLD Golden number
two hydrogen atoms 79
and one oxygen atom. An atomic number is the
number of protons
Molecules in an atom. The
atomic number
Substances are made from of gold is 79. This
little groups of atoms called means that each gold
molecules. The molecules of atom has 79 protons.
water have three atoms.

58

How many atoms are there in a drop of water?

Amazing atoms

Sunflower oil comes from the
seeds that grow in the middle

of a sunflower.

Oxygen atom

Big molecules

In natural substances like
vegetable oil, the atoms are
often joined in chains to make
very large molecules. The
molecules in sunflower oil
contain 50 atoms each.

Hydrogen atom Carbon atom

weird or what? The explosion
of a nuclear
An atom is mostly bomb can
empty space. If an atom create a
were the size of a sports spectacular
stadium, the nucleus would “mushroom
be the size of a marble cloud”.

in the middle.

The mighty atom

When the nucleus of an atom is split, it
releases a huge amount of energy. Nuclear

bombs use this “atomic energy” to create
huge explosions. Nuclear power stations use

the energy to produce electricity.

About 5 sextillion (5,000,000,000,000,000,000,000,000,000,000,000,000). 59

Materials science

Molecules
In most materials, atoms are joined in
tiny groups called molecules. The shapes Frozen solid
of molecules and the way they pack
together can help explain how different Cold molecules move slowly,
materials behave. allowing them to pack together
more easily. When water freezes,
the molecules line up in neat
rows, forming ice crystals.

Steaming ahead Snow may look
like white powder,
Molecules are always jiggling about.
When they get hot, they move further but if you look
and faster. When water heats up, the closely you can
molecules may start moving so fast that see thousands of
they escape into the air as water vapour. tiny crystals as

clear as glass.

Steam appears when
water vapour cools
down and becomes
liquid again. The steam
from this train is made
of millions of tiny
liquid droplets.

Melt: As a solid heats up, Liquid
its molecules move faster until
they break free from each other
and move separately, turning
the solid into a liquid.

Solid Solidify: As a liquid cools, its
molecules lose energy and move
more slowly. Eventually, they start

sticking together, turning
the liquid into a solid.

If a liquid is poured into
a jar or bottle, it takes

the shape of its container
and stays in place.

60 Are diamonds impossible to destroy?

Molecules

Diamond is made
into jewels that are
almost indestructible.

Diamond molecule

Diamond is the hardest
natural substance known.
Its hardness comes from the
way the carbon atoms in
diamond are arranged.
Each atom is joined by
strong bonds to four
neighbouring atoms.

Each group of five atoms in
diamond forms a pyramid
shape. This shape makes
diamond amazingly strong.

Turn Graphite molecule Each carbon atom
and learn in graphite is joined
Graphite, like diamond, to only three
Changing states: is also made of carbon neighbours. The
pp. 56-57 atoms, but the atoms are atoms form layers
Minerals: arranged in a different that slip over each
way, making graphite other, making
pp. 104-105 very soft.
graphite soft.

Graphite is used to make
the soft lead in pencils.

Evaporate: As a liquid Gas
heats up, its molecules speed
up until they move fast enough
to float away as gas.

Condense: When gas
molecules lose energy and slow

down, they stick together
and form liquid.

A gas can fill any container
it’s put in. If there’s no lid to
seal the container, the gas
will escape into the air.

No, you can burn them. 61

Materials science

Elements 1

An element is a H 2 The periodic table
substance made
up of just one type HYDROGEN In the periodic table, elements are arranged
of atom. Scientists 1 by the number of protons in their atoms,
have discovered starting with hydrogen. Elements with
117 different Li Be similar properties fall into groups,
elements. The chart which are shown in colour.
on this page, called LITHIUM BERYLLIUM
the periodic table, 3 4
shows most of them.
Na Mg

SODIUM MAGNESIUM 3456789
11 12

K Ca Sc Ti V Cr Mn Co

POTASSIUM CALCIUM SCANDIUM TITANIUM VANADIUM CHROMIUM MANGANESE IRON COBALT
19 20 21 22 23 24 25 26 27

Rb Sr Y Zr Nb Mo Tc Ru Rh

RUBIDIUM STRONTIUM YTTRIUM ZIRCONIUM NIOBIUM MOLYBDENUM TECHNETIUM RUTHENIUM RHODIUM
37 38 39 40 41 42 43 44 45

The elements in our Cs Ba LANTHANIDES Hf Ta W Re Os Ir
bforodmieswmhoasttlwyeceoamt.e or RARE-EARTH
CAESIUM BARIUM HAFNIUM TANTALUM TUNGSTEN RHENIUM OSMIUM IRIDIUM
55 56 METALS 72 73 74 75 76 77

57 – 71

Fr Ra ACTINIDES or Rf Db Sg Bh Hs Mt
RARE-EARTH
FRANCIUM RADIUM RADIOACTIVE RUTHERFORDIUM DUBNIUM SEABORGIUM BOHRIUM HASSIUM MEITNERIUM
87 88 104 105 106 107 108 109
METALS

89 – 103

Each vertical column La Ce Pr Nd Pm Sm
is called a GROUP,
or family, of elements. LANTHANUM CERIUM PRASEODYMIUM NEODYMIUM PROMETHIUM SAMARIUM
Some groups have 57 58 59 60 61 62
elements sharing
very similar properties. Ac Th Pa U Np Pu
Other groups have
elements with less ACTINIUM THORIUM PROTACTINIUM URANIUM NEPTUNIUM PLUTONIUM
in common. 89 90 91 92 93 94

Milk contains This bucket is made of Metal and non-metals
the element the element iron, coated
calcium, with the element zinc, Most elements are metals, and
which helps which stops iron rusting. the others are called non-metals.
form your Metals are normally solid, shiny,
teeth and and hard. They all conduct
bones. electricity and heat. Silver,
aluminium, and zinc are
metals. Carbon, oxygen,
and silicon are non-metals.

62

Which was the first element to be made artificially?

Every element has a name – Elements

a symbol made of usually Useful elements
We use elements to
one or two letters – and an make all sorts of useful
or decorative objects.
atomic number. The atomic Oxygen makes up 18
Gold is a precious
number is the number of about one-fifth of the air, and it’s He metal. It is used to
make jewellery.
protons in one atom so important that we have to take HELIUM
2 Copper is a metal that
of the element. it with us when we are underwater. conducts electricity well.
NEON It is used in electrical wires.
Kr Symbol 13 14 15 16 17 10
Silicon is a non-metal
KRYPTON Name B C N O F Ar used to make the chips
36 Atomic that power computers.
number BORON CARBON NITROGEN OXYGEN FLUORINE ARGON
5 6 7 8 9 18 Carbon fibres are strong
but light, so they are used
Al Si S Cl for tennis rackets.

10 11 12 ALUMINIUM SILICON PHOSPHORUS SULPHUR CHLORINE Iron is a strong, silvery
13 14 15 16 17 metal. It is magnetic
and has many uses.
Ni Cu Zn Ga Ge As Se Br Kr
Aluminium is a soft,
NICKEL COPPER ZINC GALLIUM GERMANIUM ARSENIC SELENIUM BROMINE KRYPTON shiny metal. It is used
28 29 30 31 32 33 34 35 36 to make drinks cans.

Pd Ag Cd In Sn Sb Te I Xe

PALLADIUM SILVER CADMIUM INDIUM TIN ANTIMONY TELLURIUM IODINE XENON
46 47 48 49 50 51 52 53 54

Pt Au Hg Tl Pb Bi Po At Rn

PLATINUM GOLD MERCURY THALLIUM LEAD BISMUTH POLONIUM ASTATINE RADON
78 79 80 81 82 83 84 85 86

Ds Rg

DARMSTADTIUM ROENTGENIUM
110 111

Eu Gd Tb Dy Ho Er Tm Yb Lu Sulphur is a yellow
non-metal used to harden
EUROPIUM GADOLINIUM TERBIUM DYSPROSIUM HOLMIUM ERBIUM THULIUM YTTERBIUM LUTETIUM rubber to make tyres.
63 64 65 66 67 68 69 70 71
Titanium is a very
Am Cm Bk Cf Es Fm Md No Lr strong, light metal used
in aeroplane bodies and
AMERICIUM CURIUM BERKELIUM CALIFORNIUM EINSTEINIUM FERMIUM MENDELEVIUM NOBELIUM LAWRENCIUM space rockets.
95 96 97 98 99 100 101 102 103
Helium is a gas used
KEY: Poor metals: These are Turn in balloons because it
Alkali metals: softer, weaker metals. and learn is less dense than air.
These silvery metals are
very reactive. Non-metals: Most Elements: Chlorine is a yellow-green
are gases at room pp. 64-65 gas, used in bleach and to
Alkaline-earth metals: temperature and Electricity: make some plastics.
These shiny, silvery white easily snap as solids. pp. 76-77
metals are reactive. Mercury is a liquid metal
Halogens: These used in dental fillings and
Transition metals: non-metals are is the gas inside fluorescent
Many are strong and highly reactive light bulbs.
have high boiling and harmful.
and melting points. 63
Noble gases:
Lanthanides: Many These non-metals
are soft, shiny, and are the least
silvery white metals. reactive of all
the elements.
Actinides: These are
radioactive heavy elements.

Technetium (Tc).

Materials science

Properties of elements

Alkali metals In the periodic table, elements with
similar properties are arranged in
These are soft, lightweight metals groups. Some groups are made up of
that react easily with other elements that react easily with other
chemicals, such as water. When chemicals to form new compounds.
put in water, they fizz and pop Other groups include elements that
violently. Sodium is an alkali barely react with anything at all.
metal. It reacts with the gas
chlorine to form common salt. What are transition metals?

Transition metals Forty elements make up a group
This group includes known as the transition metals.
well-known and These are typical metals, being
useful metals. solid, shiny, and mostly
hard. The precious
Silver is used in medals, metals gold, silver,
ornaments, jewellery, and and platinum are
cutlery (knives and forks). in this group.

Zinc protects things Pure gold is found as grains
from rusting. One of in rock or, more rarely, as
its many uses is in the whole rocks (nuggets) that
casing of batteries. are worth a small fortune.

Nickel is used in silver-
coloured coins because it
does not lose its shine.

Titanium is lightweight
yet incredibly strong.
It is used to repair
bones and joints.

Precious metals such
as gold are long-lasting
because they react
poorly with other
chemicals. Gold
is one of the least
reactive elements.

64

Which element is the most valuable precious metal?

Calcium is found Properties of elements
in seashells in
the compound Alkaline-earth metals

calcium carbonate. Five elements, including magnesium and
calcium, are called alkaline-earth metals.
The dazzling light Like alkali metals, they are soft and light.
of fireworks comes They don’t react as strongly with water,
but they join with other chemicals to make
from burning many compounds important in nature.
magnesium.
Noble gases
Poor metals ing your eyes.
The six noble gases
The elements in this get their name because
group are soft and weak. they hardly react with
They are called poor
metals but are very other chemicals, as
useful. Tin, lead, and though staying aloof.
aluminium are examples They include neon and
of poor metals. argon, which are used

Tin cans are actually to make lasers and
made of steel with coloured lights.
a thin coating of tin.
Chlorine sme lls strongly and can st

Halogens

Five elements make up a group
called the halogens. These are
all highly reactive chemicals.
The gas chlorine is one of
the best-known halogens.
It is added to the water
in swimming pools
because it kills germs.

Rhodium. It is about ten times more expensive than gold. 65

Materials science

Mixtures

A mixture is Suspension Colorado River, Arizona, USA
created when two
Mixture of or more things are A muddy river is a type of mixture called a

milk and suspension. Small particles of soil are “suspended”

cereal combined together, in the water, making it brown and cloudy.
without bonding. A
mixture can usually be Alloy
easily separated back
into its original bits. Different types of metal can be
When atoms of different melted and mixed together to make
elements join, or bond, a kind of mixture called an alloy.
a compound is formed.
The alloy has different properties
from the original metals. This tankard Pewter
tankard
is made of pewter, which is an alloy
of tin and lead. Pewter is much
harder than tin or lead.

Water Solute molecule Solution

Salt forming Solution
on the shore of
the Dead Sea If you stir sugar into water, the sugar
molecules spread out and fit between the
in Jordan. water molecules, making the sugar seem to
disappear. We say the sugar (a solute) has
dissolved in the water (a solvent). This kind
of mixture is called a solution. Seawater
is a solution of water and salt. If you let
seawater dry out, the salt reappears.

66

What is an 18-carat gold ring made of?

Mixtures

Separating compounds

It takes a great deal of effort to
separate a compound into pure
elements. To make pure iron, you
have to separate the compound iron
oxide, into iron and oxygen. This is
done in a very hot blast furnace.

Creabmy and cheese marielkm. a
separating

de

Iron ore Pure iron

The mineral iron ore is rich in iron oxide.
It is mined to extract iron, which is mostly
used to make steel.

Separating mixtures Milk Separating milk
A mixture can be
easily separated in Strawberries Whole milk can be separated
several ways. and cream into cream and skimmed milk
by spinning it in a bowl. The
Evaporation removes heavier skimmed milk spins
water from a mixture away from the lighter cream,
by turning it into a gas which stays in the centre of
(water vapour). the spinning bowl.

Filtration separates large 67
particles, such as coffee
grinds, from a suspension.

Spinning at high speed
separates blood cells from
blood, in a device called
a centrifuge.

Distillation separates
mixtures of liquids by
making them evaporate
and condense.

It is an alloy containing 75 per cent gold and 25 per cent other metals, such as silver.

Materials science

Reactions and changes

Materials change as a Melting is not a
result of physical processes chemical reaction.
or chemical reactions. In a
chemical reaction, atoms
join with or break away from
other atoms, forming different
compounds. Chemical reactions
often lead to a dramatic change.

Chemical change Physical change

Burning is a chemical reaction Not all dramatic changes are caused by chemical
involving oxygen (O). Wood is made reactions. When ice lollies melt, the atoms in the
of compounds containing carbon (C) water molecules do not get rearranged into new
and hydrogen (H). When it burns, molecules – they remain water molecules. Melting
the carbon and hydrogen react with is simply a physical change.
oxygen to produce carbon dioxide
(CO2) and water (H2O). Burning is a
chemical reaction.

Escaping energy

Chemical reactions can
release energy as heat
and light. A sparkler

contains chemicals that
release a lot of energy as
light to create a dazzling

shower of sparks.

68

What chemical reaction makes silver objects slowly turn grey and dull?

Reactions and changes

Speeding up reactions

Cooking makes carrots softer
because the heat causes a
chemical reaction. Chopping
carrots into small bits speeds
up the reaction because it
increases the area of contact
between the carrots and
the hot water.

Sliced carrots
cook faster than
whole carrots.

Glow in the dark

Light sticks glow in
the dark thanks to a
chemical reaction that
releases energy as light.
You can slow down this
reaction by putting a
light stick in a fridge,
which makes it
last longer.

hands on Soda volcano

Ask an adult If you drop mints into a
to boil some red cabbage bottle of fizzy drink, the drink
and save the coloured water.
Let the water cool. Then add turns to foam and explodes
out in an instant. This is a
acid (vinegar) or alkali
(bicarbonate) and watch for physical change rather than
a chemical reaction. The
a spectacular change
of colour! rough surface of the mints
helps gas, dissolved in the
drink, to turn into bubbles
much more quickly than it

normally would.

Tarnishing. It happens when silver atoms react with the oxygen atoms in air. 69

Materials science

Irreversible changes

Nylon Physical changes are reversible – for
jacket
example, you can freeze water, and heat
Manmade
materials can turn the ice into liquid water again.

Chemical reactions can However, many chemical reactions are
be used to create new
materials that don’t irreversible because they involve atoms
exist in nature. Nylon,
for example, is a fabric joining together in new ways.
made using chemicals
from oil. Many types Cooking
of clothes, from socks to
coats, are made of nylon. When food is cooked,
heat triggers chemical
reactions that change
it permanently. When
a freshly baked cake
cools down, it doesn’t
turn back into gooey
cake mixture.

Baking powder

Baking powder makes cakes light
and fluffy. It contains chemicals
that react when they’re wet to
produce bubbles of gas.

A fresh pepper looks plump An old pepper darkens Rotting
and brightly coloured. and shrivels up as it rots.
Rotting food is full of tiny organisms
such as a bacteria and fungi. These
organisms trigger chemical reactions
that break down food molecules,
changing them permanently.

70

Why are some parts of cars covered with a layer of shiny chrome (chromium)?

Irreversible changes

Ready to fall Maple leaves
turn orange
Maple trees shed their as they die.
leaves in autumn. Before the leaves
die, they change from green to golden, Turn Rust
orange, or red. The colour changes and learn
because a chemical reaction in Iron reacts chemically with
the leaves breaks down a green Plants: oxygen in the air to form
compound called chlorophyll pp. 20-21
inside leaf cells. Ecosystems: rust – a flaky, reddish
pp. 44-45 brown compound. Rust
Solid as a rock can ruin cars, so the
Severe rust metal is painted to
Concrete is made by mixing gravel, protect it.
sand, cement powder, and water.
A chemical reaction between
the water and cement
makes the mixture
harden permanently
to become as solid
as rock – ideal for
building dams
and houses.

The chrome protects the iron underneath from rusting. 71

Physical science Sources of energy
Energy comes from lots
What is energy? of different sources.

Energy is what makes everything Wind drives wind
happen.Your body needs energy so turbines, which convert
that you can move, grow, and keep movement energy
warm. We also need energy to power into electricity.
our cars, light our homes, and do
thousands of other jobs. Geothermal energy
is heat from deep
Sunshine underground.

We get nearly all our energy from the Sun. Plants Dried plants can be
absorb the energy in sunlight and store it as chemical burnt to provide energy
energy. The stored energy enters our body through food for cooking, heating,
and is released inside our body’s cells. Sunlight absorbed and lighting.
through our skin is also necessary to produce certain
vitamins and minerals in our body. The Sun is the Waves can be used to
ultimate source of energy for all plants and animals. generate large amounts
of electricity.
Only a tiny fraction
of the Sun’s energy Dams harness
reaches the Earth. the energy in rivers
flowing downhill to
A bow stores energy make electricity.
by bending. When
you let go, the bow The Sun’s energy can be
springs back into captured by solar panels
shape and releases to make electricity.
the stored energy.
Fossil fuels, such as oil,
are used to power cars
and to make electricity.

Stored energy

An object can store energy
and release it later. When
you wind up a clockwork

toy, energy is stored in a
spring. A bow and arrow
uses stored energy to shoot
the arrow. Stored energy is
also called potential energy
because it has the potential
to make things happen.

72

Is energy destroyed when we use it?

Movement energy What is energy?

Rollercoasters start from Picture detective
the top of a hill, where
their height gives them Look through the Physical
a lot of potential Science pages to identify each
energy. As they of the picture clues below.
move downhill,
the potential
energy turns into
movement energy
(kinetic energy),
making them go
faster and faster.

Nuclear energy

Matter is made up of tiny
particles called atoms.
The centre of an atom,
called a nucleus, stores

huge amounts of energy.
This nuclear energy is
used in power stations
to make electricity.

Electrical energy

Lightning is caused by

electrical energy in a

storm cloud. The

electrical energy Turn
turns into the and learn
heat and light
energy of Light:
lightning and pp. 82-83
the sound energy
of thunder. Heat:
pp. 86-87

Energy cannot be destroyed. It turns into another form of energy when it’s used. 73

Physical science Energy changes

Energy chain All around you, energy is being
converted from one form to another.
Changing energy from You can see these changes happen –
one type to another is called switching on a light turns electrical
“energy conversion”. The energy into light energy.
steps can be linked to make
an energy chain. Driving force

Coal contains Car fuel is full of
chemical energy. chemical energy. When

Burning the engine starts, the
coal chemical energy is
produces
heat energy, changed to heat energy.
which is used This is the first in a
to boil water.
Boiling water series of energy changes
creates steam. that make cars run.

Moving
steam is
a form of
kinetic (motion)
energy, which
operates
turbines.

The kinetic
energy
produced by
the moving
turbines
creates
electricity.

Electrical energy used by television sets Heat to sound
changes into light, sound, and heat energy.
Some heat energy
becomes sound energy.
The roar of a racing car

engine can be deafening!

74

What are energy sources like coal, oil, and gas called?

Energy currents Energy changes

Energy is transported Energy savings
through wires as currents Energy is precious, so
of electricity. The electrical people are finding extra
energy in this circuit comes ways to limit energy use.
from chemical energy in
the battery. Roof insulation
stops heat energy from
80 100 140 Moving on escaping and helps keep
60 houses warm.
Some heat energy Energy-saving light
180 is changed to kinetic bulbs last longer and
40 energy as the pistons use less energy than
move. The movement standard ones.
20 220 of the car is also
kinetic energy. Washing clothes at
0 low temperatures saves
the energy needed to
heat water.
Boiling only as much
water as you need in
the kettle saves time
and energy.

Wheels of fire

Some of the
kinetic energy
in the wheels
becomes heat
energy. The
hottest parts
are shown
white and yellow.

Fossil fuels. Turn
and learn

Types of energy:
pp. 72-73
Resources:

pp. 110-111

75

Physical science

Electricity

Have you ever thought Power supply
about what powers your
television, your computer, Electricity travels to your home along wires
above and sometimes below the ground. The
or the lights in your
bedroom? A flow of wires above the ground hang on metal
electricity makes all towers called pylons.
these things work.

Making electricity

Electricity is a form of energy. It can be
made using any source of energy, such as
coal, gas, oil, wind, or sunlight. On a wind
farm, wind turbines use the energy of
moving air to create electricity.

Everyday electricity
We use electricity in
all sorts of ways in
our everyday lives.

Electricity is used to heat
up household appliances
such as irons and cookers.

Electricity is used to light
up our homes, schools,
offices, and streets.

Electricity helps in
communication by
powering telephones
and computers.

Electricity helps
in transportation
by powering certain
vehicles, such as trains.

76

What’s the name of a small object that can store electricity?

Circuits of power Electricity

An electric circuit is a loop that electricity Electrical cables
can travel around. An electric current
moves through the wires in this circuit, Electrical cables are made of
and lights up the bulb. metal and plastic. Electricity flows
through the metal (which is called
Circuits usually include an a conductor). The plastic (which
energy source (battery) is called an insulator) stops
and load (lamp). electricity escaping.

hands on Lightning
strikes
Rub a party
balloon up and down on Electric charge
your clothes. The balloon will building up in one
now stick to the wall. This place is called “static
electricity”. Lightning
is because rubbing it is an electric current
gives the balloon an
caused by static
electric charge. electricity building up

High voltage in thunderclouds.

Electricity can be very Food battery
dangerous. This triangle
is an international Food that contains water and weak
warning symbol. acid will conduct electricity. In a food
It means “Caution: battery, a chemical reaction between
risk of electric shock”. the metal and the acid in the food
creates an electric current.

A conductor
attached to
food (containing
acid) forms an
electric circuit.

A battery. 77

Physical science This magnet has attracted
a clump of steel paperclips
Magnetism because steel has an iron content.

Magnets exert a force called The Northern Lights are
magnetism, which can attract partly due to magnetic
certain objects – especially forces in our atmosphere.
those containing iron.

Attract or repel?

Magnets attract materials
containing iron, and they
can also attract other magnets.
Two magnets can also push
apart, or “repel”.

Opposite poles of a magnet
attract each other.

SN SN

Magnet rules

The ends of a magnet are
called the north and south poles.
Opposite poles attract each other.
Similar poles repel each other.

SN N S Lights in the sky The Earth has a
magnetic field.
Amazing lights are caused

Similar poles of a magnet when particles in the solar wind
repel each other.
(streaming from the Sun) travel

into the atmosphere along force

lines in the Earth’s magnetic field.

SN The Earth as a magnet

Iron filings show the magnetic field between The Earth behaves as if there
the two magnets. is a giant invisible magnet

78 between the North and South
poles. That’s why we can use

a compass to find our way.

What is a magnetometer?

Magnetism

Electromagnets

When an electric current flows through a wire
coil, the coil becomes magnetic. This creates an
electromagnet. Automatic doors, loudspeakers,

and electric motors all use electromagnets.

Lifting with magnets

Some cranes use magnetic force,
in the form of giant electromagnets,
instead of hooks. The electromagnet
can be switched on or off.

Electromagnets are
used in speakers.

When switched on, the crane’s electromagnet attracts hands on
huge pieces of iron and steel.
Use a
Magnetic rails magnet to find out which
things in your home are made
Maglev trains are held above from magnetic materials.
a track by a magnetic force.
Maglev is short for “magnetic Your magnet will be
levitation”. The trains attracted to objects
literally travel on air.
containing iron.
There are maglev
trains in Japan, South 79
Korea, and China –
and others are being
developed elsewhere.

A device that measures the strength of a magnetic field.

Physical science

Energy waves

A form of energy called “electromagnetic radiation” travels
in waves, like waves on the surface of a pond. Just as waves in
a pond can be close together or far apart, different types of
electromagnetic radiation have different wavelengths.

The spectrum Radio waves

Visible light is a type of energy Radio waves have the
wave that we can see. There are longest wavelengths and are
other waves that are not visible to us, good at travelling far. Radio
such as radio waves. The spectrum is
made up of different types of waves, and TV programmes are
with varying wavelengths. broadcast as radio waves.

The distance between
the peaks of two waves is
called the “wavelength”.

Low energy

Radio waves can be
hundreds of miles long.

Radio waves Microwaves Infrared waves

Microwaves

Microwaves are used to heat up
food in microwave ovens. They
are also used by mobile phones
and by satellites in space.

Infrared waves

Hot objects give off invisible
rays of heat called infrared
waves. An infrared camera
can detect these waves
to create images.

80

What is invisible to human eyes but visible to the eyes of a bee?

Energy waves

Visible light X-rays

Light waves bounce off every object X-rays are invisible
around us, allowing us to see things. waves that pass
Visible light includes all the colours
of the rainbow, each of which has a through soft parts
particular wavelength. of the body but

not bone. This is
why doctors can
use X-rays to take

images of bones.

High energy

Visible UV rays X-rays Gamma rays

Ultraviolet (UV) light Gamma rays

As well as producing visible light, The wavelengths of gamma
the Sun produces invisible rays of rays can be as small as the
ultraviolet light. UV light makes nucleus of an atom. Gamma
you tan but too much of it can rays are packed with energy,
cause skin cancer and eye damage. which makes them
powerful. They are
used in hospitals to
kill cancer cells.

This man is being
treated with gamma

rays to kill cancer
cells inside his body.

Ultraviolet light. 81

Physical science Light

82 Light is a form of energy that
our eyes can detect. It comes
in all the colours of the rainbow,
but when the colours are mixed
together, light is white.

Where does light Fireflies
come from?
Some animals create their
Light is produced by own light. Fireflies flash a
electrically charged particles yellowish-green colour from
in atoms – especially their abdomen at night to
negatively-charged electrons.
attract mates.
Candlelight is produced by hot
atoms in tiny particles of soot Using light
inside the flame. We can use light for
many different things.
Casting shadows
CDs and DVDs store
Light can only travel in digital information that
straight lines. If something can be read by laser beam.
blocks its path, it casts a
Cameras capture light
shadow – a dark area that in a split second to
the light cannot reach. create photographs.

Telescopes collect
the light from stars
and planets, and produce
magnified images of them.

Mirrors reflect light
so we can see images
of ourselves.

Periscopes bend the
path of light so we can
see around corners.

Torches shine a beam
of light to help us see
in the dark.

What’s the fastest thing in the Universe?

Light

Bright Dark

Light enters your eyes through your pupils (the black circles in the
middle). Pupils can change size. When it’s dark they get bigger to let
more light in, and when it’s bright they shrink so you don’t get dazzled.

How your eye works

The human eye works like a camera. The front
parts of the eye focus light rays just as a camera
lens does. The focused rays form an upside-down
image in the back of your eyeball.

1. Light rays 3. An image forms Light beams
from the tree on the back of the
enter your eye. eye. Light-sensing Unless it enters
cells send the your eyes, light
image to the brain. is invisible. The
beam of light from
Cornea a lighthouse can
Lens only be seen from
the side if it catches
Tree 4. The brain mist or dust in the
2. The cornea turns the image air, causing some
(front of eye) the right of the light rays to
and lens focus way up. bounce off towards
the rays. you. Lighthouse
Convex mirrors bulge beams sweep round
Reflecting light outwards. They in circles and can
make things look be seen from far
When light hits a smaller but let you out at sea.
mirror, it bounces see a wider area.
straight back off.
Concave mirrors
If you look into bulge inwards. They
a mirror, you see make things look
this bounced light bigger but show
a smaller area.
as a reflection.

Light. It travels at a billion kph (620,000,000 mph). 83

Physical science Measuring sound
Loudness is measured
Sound in decibels.

Every sound starts with a vibration, Leaves rustling nearby
like the quivering of a guitar string. make a sound of only
The vibration squeezes and stretches 10 decibels.
the air, sending its energy out in waves
in all directions. This is a sound wave. Somebody whispering
close by measures about
Sound notes 20 decibels.

When you blow across City traffic reaches
a bottle, the air inside approximately
vibrates. Small air spaces 85 decibels.
vibrate more quickly than
large spaces, making higher Drums being played
notes. So partly empty nearby makes a sound
bottles produce lower of around 105 decibels.
notes than fuller ones.
Road-drills measure
Silent space about 110 decibels
from a close distance.
Sound can travel through
solids, liquids, and gases, but A lion’s roar would
measure 114 decibels if
it can’t travel where there is you were close enough.
no matter. There is no sound
Fireworks can measure
in space because there is 120 decibels or more.
no air.
The sound of jet
engines sometimes hit
140 decibels if heard
from nearby.

Sound waves travel through air
like a wave along a coiled spring.

How hearing works

When a sound reaches your ears,
it makes your eardrums vibrate.
The vibrations are passed to
your inner ear through tiny bones.
From here, nerves send messages
to your brain that allow you to
recognize the sound.

84

Do all animals hear sounds in the same way?

Speeding sound Sound

Sounds travel through air at 85
about 1,200 kph (750 mph).
It travels faster through
solids and liquids than
through gases. Supersonic
jets fly faster than the
speed of sound, so they
can pass over you before
you hear their sound.

When a supersonic jet breaks
the speed of sound, it catches

up with the sound waves in
front of it and squashes

them. As the air is squashed,
it produces a sound called a

“sonic boom”.

The echo effect

Some animals use sound to communicate
or to hunt. Dolphins “talk” by making
clicks, barks, and other sounds that
other dolphins recognize. They
also use clicks to find food –
the sound bounces back
off objects as an echo,
so the dolphin can
establish their shape
and position. This is
called echolocation.

When sounds
bounce back, the
dolphin can tell if
the object is a
yummy fish or
another dolphin!

No – dogs and bats can hear higher notes than people.

Physical science Sources of heat
Heat can be produced in
Heat several different ways.

Atoms and molecules are always Friction (rubbing) makes
jiggling about. The faster they move, heat. If you pull on a rope,
the more energy an object has. We feel your hands will feel warm.
this energy as heat. When something
is hot, its atoms are moving quickly. Combustion means
When something is cold, its atoms burning. When something
are moving slower. burns, it produces heat.

Feel the heat Electricity is used to
create heat in electric
Heat always tries ovens and heaters.
to spread from hot
things to cooler things.
When you touch a hot
object, heat energy flows
into your skin, triggering
sense cells that make your skin
feel hot. When you touch a cold
object, heat flows out of your skin,
triggering a different feeling.

Temperature Warm glow

The temperature of an object tells Heat escapes from warm objects as invisible
you how hot it is on a numbered rays that travel like light. We call this infrared
scale. A device called a thermometer
is used to measure temperature. radiation. Special cameras use infrared rays
rather than light to take photos. Hot areas
appear white or red and cold areas, such
as this ice lolly, appear black.

Keep your cool

Heat travels from the Sun as infrared rays. Just like light,
infrared rays are reflected away by white objects but
absorbed by black objects. In hot countries, people paint
houses white to reflect the heat and keep the indoors cool.

86

Can snow keep you warm?

Free ride Heat

When land gets hot, it warms Conduction
the air above it. The warm air
Heat spreads through solids
rises. Birds use these areas by a process called conduction.
of rising air (thermals) Hot atoms, which jiggle about
to lift them high in a lot, knock into cooler atoms
the sky. and make them jiggle faster,
passing on the heat energy.

Eagles can fly
without flapping
when they catch

a thermal.

Convection weirSdnakeosrcalwledhat? Heat is spreading
along this metal
When air or water warms pit vipers have heat bar. Metal is good
up, it rises, and cool air sensors on their heads. The at conducting
or water sinks to take its heat sensors work like eyes, heat quickly.
place. This process is called allowing the snakes to “see”
convection. Convection
helps keep the ocean the warmth of mice
currents moving, spreading when hunting in
heat around the world. the dark.

This satellite image shows the temperature of the world’s oceans.

Keeping warm

Emperor penguins live in the icy
Antarctic. Their feathers trap air,
which stops too much heat escaping
from their body by conduction. This
trapping layer is called insulation.

Yes, if you use it to build an igloo. Snow is a good insulator. 87

Physical science

Forces

Gravity A force is simply a push or a pull. When
you push or pull something to make it
The force that makes things move, you are using forces. Some forces
fall to the ground is gravity. work only when objects are touching, but
Gravity keeps the Earth in others, such as gravity and magnetism,
orbit around the Sun and work at a distance.
keeps the Moon in orbit
around the Earth. It is one of This NASA space shuttle,
which was in operation
the most important forces
in the Universe. until 2011, needed three
rockets to help it escape

from the Earth’s
gravitational pull.

Lift-off

A huge force is needed to

make a spacecraft take off

and escape the Earth’s gravity.

In a spin A force called thrust is provided
by rockets. The rockets make

On a merry-go-round, the riders feel hot gases, which expand and

they’re being pushed outwards. This stream out at the bottom

pushing, called centrifugal force, isn’t to push the spacecraft up

a real force. It’s caused by the riders’ into the air at great speed.

bodies trying to move in a straight line

while the chains are holding them back.

88

What force makes compass needles point towards the North Pole?

hands on Forces

Rub your Friction
hands together as hard
and fast as you can for When objects rub or slide
10 seconds and see how hot against each other, they
they get. The heat is caused
by the force of friction create a force called friction.
Friction slows down moving
acting on your skin. objects and wastes their
energy, turning the
energy into heat.

To reduce friction,
the bottom surface
of these skis is very
smooth and coated
with slippery wax.

Friction slows down a skier.

Electric forces

When objects become charged with electricity,
they pull on each other with an invisible force
that is a bit like magnetism. If you rub a
balloon on your hair, the balloon becomes
charged and will stick to your shirt.

Buoyancy

What makes objects float? The answer
is a force called buoyancy. If an object is
lighter than water, the force of buoyancy

outweighs gravity and the object floats.

Gravity pulls
the duck down.

Upthrust from the water
keeps the duck afloat.

Magnetism. 89

Physical science

Forces and motion

It can be difficult to make an object move, The football
but once it is moving, it will go on moving would stay still
until something stops it. Force is needed if the footballer
to start something moving, make it move
faster, and make it stop. didn’t kick it.

Newton’s laws of motion

In 1687, Isaac Newton presented three
important rules that explain how forces make
things move. They have become the foundation
of physics and work for just about everything,
from footballs to frogs.

Newton’s first law

An object stays still, or keeps moving in

a straight line at a constant speed, if it

isn’t being pushed or pulled by a force.

Newton’s second law Forces make things
accelerate. In this case,
The bigger the force and the lighter the force is created by
the object, the greater the acceleration. the cyclist’s powerful legs.
A professional cyclist with a lightweight
bike will accelerate faster than a normal Newton’s third law
person cycling to work.
Every action has an equal and opposite
reaction. The leaf moves away as the
frog leaps in the opposite direction.

90

How fast can a skydiver fall?

Forces and motion

Speed and velocity

Speed is different from velocity. Speed is how fast you are going
and is easy to work out – divide how far you travel by the time it
takes.Your velocity is how fast you travel in a particular direction.
Changing direction without slowing reduces your velocity, but
your speed stays the same.

If you drive 80 km (50 miles) Accelerating is fun, but defining it
in two hours, your speed is in scientific terms can be confusing.
This is because acceleration doesn’t just
40 kph (25 mph). mean speeding up. It is any change in
velocity. So, it is also used to describe
slowing down and changing direction.

Inertia The golf ball will carry
on rolling until friction,
When things are standing still or moving, gravity, and air resistance
they continue to remain in the state they are in slow it down.
(unless force is applied to them to change it).
This tendency to be as they are called inertia. Rescue helicopters
balance forces so
LIFT they can hover
above the waves.

DRAG/
FRICTION

Turn Balanced forces THRUST
and learn GRAVITY
Forces act on objects
Magnetism: all the time. Opposing 91
pp. 78-79 forces can be balanced out.
Gravity: When this happens, the object
pp. 88-89 won’t be pushed in any direction.

The maximum velocity of falling through air is 200 kph (124 mph).

Physical science

Load Machines

Effort required Machines make tasks easier. They
to push down reduce the effort you need to move
is needed here. something, or the time it takes. They
work either by spreading the load, or
Fulcrum by concentrating your efforts. All the
machines you see here are called
Levers simple machines.
A lever is a bar that can
turn around a fixed point Axle
(fulcrum). If you apply a
force (effort) to one part Wheel and axle
of a lever, another part
exerts a force (load). An axle goes through the centre of a wheel.
Together they work as a simple rotating
One type of lever works machine that makes it easier to move
like a seesaw with the something from one place to another.
fulcrum between the
load and the effort.

Another type places
the load between the
fulcrum and the effort
(as on a wheelbarrow).

A third type of lever,
shown by tongs, places
the effort between the
fulcrum and the load.

hands on

Try walking

Gears straight up a

hill and then zig-zag your

Gears are wheels with teeth that way up. The winding path

interlock so that one wheel turns works like a simple machine.

another. They increase speed or force. It increases the distance you

Gears on a bicycle affect how much you walk, but decreases the

must turn the pedal to spin the wheel. The pedal turns a wheel, effort you use.
which turns a smaller

wheel at a greater speed.

92
What is another name for gear?

Machines

It takes just one man to
pull a stone up the slope,
but four men are needed
to lift a stone straight up.

Wedge Inclined plane

An axe blade is an It is easier to push or pull something up a slope than
efficient but simple lift it straight up. A slope, or inclined plane, therefore
increases force. In ancient Egypt, stones were
machine that dragged up slopes to build the pyramids.
increases force.
When it hits the
wood, the wedge
forces the wood to
split apart between

its fibres.

The crane lifts The screw turns around a
up heavy loads greater distance than it moves
with a system into the cork. So it moves into
of pulleys. the cork with more force than

is used to turn it. 93

Pulley

A pulley makes it easier to lift
something straight up. It consists
of a piece of rope wound around
a wheel. One end of the rope is
attached to the load and force is
applied to the other end to pull up
the load. When a pulley has more
than one wheel, the pulling force
is increased.

Screw

A screw is a machine. It is really
an inclined plane, or slope, going

round and up. A corkscrew uses
a screw. It is easier to twist the

point of a screw into a cork than
to push a spike straight in.

Cogwheel.

Earth and space science 9 billion years fosrOymsutsermsolarBang

The Universe billioUennxsiihvstearipdsnGeeifstafhenleraoexwniets after the Big

The Universe is everything of
that exists. It includes the Earth,
the Sun, and the other stars in 1
our galaxy. Beyond our galaxy
are countless other galaxies.
The Universe was created
in the “Big Bang”, around
13.7 billion years ago.

gatolafxoTirehmsebfeirgsitn years

mill after

500 the

Big

200 million yea tsotafroTsrhmbeegfiirnst Bang
ion years

after

rs after the Big Bang the

tBeinxigpyTlBohpdeaoeinUnsgtnfriinovemrse Big

a Bang
the

94

How many stars are there in the Milky Way?

Galaxies The Universe

Galaxies are groups Picture detective
of stars held together
by gravity. There are Look through the Earth
more than 100 billion and Space Science pages.
stars in a typical Can you identify the
galaxy. Galaxies picture clues below?
are different shapes.
Some are spirals Near neighbour
and some are oval.
The nearest galaxy
to our own is the
spiral-shaped
Andromeda galaxy.
It would take around
2.2 million years
to get there – if you
were travelling at
the speed of light!

The Milky Way Turn
and learn
Our solar system is part of a galaxy called
How stars form:
the Milky Way. From the inside (where we are), pp. 96-97

it looks like a haze of light in the sky. The solar system:
pp. 98–99

Between 200 billion and 400 billion. 95

Earth and space science

Starry skies

There are many more stars in the
Universe than there are grains
of sand on all the beaches
on the Earth. Many are
far brighter than our Sun.

The lives of stars

The lives of stars begin
inside thick clouds
of gas in space,
called nebulae.

Supernovae
The most massive
stars end their lives
in huge supernovae
explosions.

Red giants White dwarfs
Stars are fuelled by the gas The outer layers of the star are
hydrogen. They burn until eventually thrown off into space.
the hydrogen starts to run The cooling core is left behind.
out. Then they expand, This is called a white dwarf.
forming a red giant star. White dwarfs are no bigger
than the Earth.

Nebulae
Gravity pulls together little
knots of dust and gas inside
the nebulae. Each one could
become a star, as gravity
squeezes it tighter and it
becomes hotter.

Leave a camera shutter Stars in motion
open for a few hours on a
The position of the stars
clear night, and you can seems to change throughout
see the stars leave trails the night. The stars are not
really moving, though. It is
as the Earth rotates. the Earth that is turning
beneath them.

96

How many stars can you see on a clear, dark night?

Remnants The Sun is Starry skies
The fragments of the
star can remain glowing made mostly of hydrogen.
in space for hundreds
of years. Core
Nuclear reactions

Outer layer

Starshine

Our Sun is a star that is
halfway through its life.
In the life cycle, it sits
between being formed
within a nebula and
becoming a red giant.

Black holes Shapes in the sky
When the biggest stars explode, most
material is blown outwards. But the Hundreds of years ago, people grouped stars that
core is crushed and collapses to form appear close together in the sky into shapes called
a black hole. constellations. They all have names – often related
to their shapes. This is the Plough, in Ursa Major.

97
About 2,000.

Earth and space science

Our solar system

The solar system is our own Family of worlds
small part of space. It is made
up of the Sun, eight planets, The planets all move
and countless smaller objects,
such as comets. around the Sun in paths

called orbits. They are held

in these orbits by gravity. Neptune is the furthest
planet from the Sun in
the solar system.

Uranus has
13 rings and
27 moons.

Mars is red because
the soil on its surface
is full of iron oxide.

Jupiter is the largest planet
in the solar system. It has
more than 60 moons.

The Sun

The Sun is our closest star. All the
heat and light we need to survive on
the Earth come from it. Although the
Sun is about halfway through its life,
it will continue to burn for another
five billion years.

98

How old is the solar system?