My Science School

Almost three-quarters of the Earth’s surface is water. Water is found in rivers, lakes and oceans, and it can also be seen as tiny water droplets in clouds, or falling to Earth as rain or snow. Water is constantly moving from one place to another around the planet. This movement is called the water cycle.

Water on the move

The constant cycle of evaporation and condensation of water is almost entirely caused by heat from the Sun. the sun’s rays warm the land and the oceans. Heat from the Sun causes water in the ocean to evaporate, turning into invisible water vapour. Water also evaporates from rivers and lakes. As water vapour rises it cools, and condenses into tiny droplets, forming white clouds. Clouds are blown across the land by the wind. As clouds rise and cool, the tiny droplets come together and fall as rain or snow. Rain runs over the surface of the land and collects in streams and lakes. Snow from the mountains melts and forms streams. Streams join to form large rivers, which flow downhill. Rivers flow into the ocean. Groundwater eventually runs into the ocean. Water sinks into the ground, forming groundwater beneath the surface of the Earth.

Extreme conditions

Deserts are places where very little rain falls. The Atacama Desert in Chile is the driest place in the world, with only 15 mm (0.6 in) of rain every year. The wettest place on Earth is Mawsynram in India, which has 11,872 mm (467 in) of rain every year.

 

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Changes are taking place in the world around us all the time. Sometimes, after a change, things can be put back to the way they were before. Other types of change leave things altered forever.

Reversible change

Reversible changes are easy to reverse, or undo. For example, ice can melt to become water, and then freeze, turning back into ice again. A frozen lolly is solid. It is made of ice. If you take the ice lolly out of the freezer, it gets warm and the ice begins to melt. After a while, all of the ice has melted and has become a liquid. The liquid can be poured into a lolly mould and then put into a freezer. As the liquid gets cold it freezes and turns into ice.

Irreversible change

Irreversible changes cannot be undone. They are permanent. For example, once you have cooked an egg, it cannot be turned back into a raw one! The white of a raw egg is a clear, runny liquid. Heating the egg causes a change that cannot be undone. The egg white has now become a white solid. It cannot be turned back into a clear liquid.

Everyday changes

Here are some examples of common reversible and irreversible changes that you may see from time to time.

Steamed window

When invisible water vapour in the air hits a cold window, it condenses, turning into tiny water droplets. When the window gets warm, the droplets turn back into water vapour.

Autumn leaves

Most trees lose their leaves in autumn. Before the leaves fall off the tree, they charge from green to red to brown. This change cannot be undone, which means it is permanent.

Rusting

Rust forms slowly when iron comes into contact with air and water. Iron objects left in the rain become flaky and reddish-brown forever.

Rotting food

When food gets old, it can be attacked by tiny living things called mould and bacteria. As the food rots it turns brown, smells nasty, and shrivels up. It cannot be changed back into its fresh form again.

 

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Solids, liquids, and gas can sometimes be mixed together. For example, you can have a mixture of two or more solids, or you can mix a solid with a liquid. Sometimes, when a solid mixes with a liquid, the solid disappears. The solid is said to have dissolved, and this type of mixture is called a solution. It is fairly easy to separate any mixture back into the different things that formed it.

Separating mixtures

Mixtures can be separated back into their different parts. Choosing the correct method depends on what type of things the mixture is made up of.

Sieving

Sieving can be used to separate a mixture of solids, but the solids must be different sizes. Liquids containing large solids can also be separated by sieving. For example: a mixture of solids is passed through a sieve. The small bits of solid pass through the holes in the sieve. The sieve stops the big bits of solid from getting through.

Filtering

Mixtures of solids and liquids can be separated by a process called filtration. The mixture is passed through a fine mesh, such as filter paper, which holds back the solid bits, but allows the liquid through. For example: a mixture of a solid and a liquid is passed through filter paper. The liquid passes through the tiny holes in the filter paper. The filter paper stops the solid bits from getting through, even very small pieces of solid, such as grains of sand.

Evaporating

When a solid has dissolved in a liquid to form a solution, it cannot be separated by filtration. The solution must be heated until the liquid evaporates, leaving the solid behind. For example: a solution of sugary water is heated in a pan. The hot water evaporates. Eventually, all the water evaporates and solid sugar is left behind in the pan.

 

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Some materials are found naturally on Earth, others are made by humans. One material may be hard and brittle, while another may be soft and bendy. These types of features are called “properties”. We choose a material for a specific job based on its properties. For example, you wouldn’t build a castle out of chocolate!

Metal

Metals are usually strong, but not brittle, which means they do not break easily. Iron, for example, may be used to make strong chains. Metals are also good at allowing heat and electricity to pass through them.

Types of metal

Different metals have different properties and so are used to make different things.

• Gold is a shiny metal from which crowns and jewellery are made.


• Iron is heavy. Heated flatirons were once used to press clothes.


• Aluminium is a light metal. It is used to make cans and aircraft.


• Copper is easy to bend. It is often used for water pipes or wires.

Brick

Bricks are rectangular blocks of clay mixed with sand. They are heated to a high temperature to make them very hard. Because we can make as many bricks as we want that are exactly the same size, they are an ideal building material.

Glass

Glass is made by melting sand. It is transparent, which means we can see through it. So it is often used to make windows, it can also be made in different colours. Glass is hard, but brittle and so can easily be broken.

Wood

Wood comes from the trunks of trees. It is strong and light. Wood is easy to cut into different shapes, so it is sued to make bridges, buildings, and furniture. Wood is also easy to set on fire. Burning wood is useful because it produces heat.

Cloth

Cloth can be made by knitting or weaving cotton, wool, or silk. It is a soft, light material, and people have made clothes from it for thousands of years. It can also be used to make coloured flags.

Rock

Rock is a natural material. For example, mountains are made of rock. Some rocks are hard, but others are soft and crumbly such as chalk. Humans have often used hard rocks to build tall, strong walls.

 

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Matter is the “stuff” that all things are made of. Everything we see, touch, and breathe is made of matter. Matter has different forms called states. The three most common states on Earth are solids, liquids, and gases. Matter behaves differently in each of these states.

Solid

A solid is something that holds its shape, and stops you from moving through it. For example: a rock, a spoon, or a book.

• Solids can be cut or shaped.


• Most solids are hard, like metal.

Liquid

Liquids do not hold their shape on their own. They will take on the shape of their container, like water in a glass.

• Liquids flow, like river, and they can be poured, like milk and juice.


• Liquids fall down, as they are pulled towards the Earth by gravity.

Gas

Gases have no fixed shape at all. They fill any space that they are in, for example, air in a room.

• Gases are always moving.


• Gases are lighter than solids and liquids.

Switching states

Most things change from one state to another when their temperature changes. When a solid gets hot, it melts and changes into a liquid. When this liquid is heated, it evaporates, changing into a gas or vapour. For example: Ice is a solid. It is very cold. When ice is heated above 0ºC  (32ºF) , it melts and becomes a liquid called water. When water is heated to 100ºC  (212ºF), it boils and changes into a vapour called steam.

 

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Science isn’t just a lot of facts in a book. Science is a way of thinking. Science is about asking questions and finding out the answers. Scientists come up with new ideas, and invent new ways to make life easier for everyone. Here are some examples of things that can be explained by science.

• Where does electricity come from?


• What materials conduct electricity?


• Where does lightning come from?


• Why do plants need sunshine?


• Why do humans have bones?


• How do you know if something is alive?


• How are shadows made?


• Where does light come from?


• Why do only some things stick to magnets?


• What is a magnet?


• What is the hardest material on Earth?


• What is gas?

Bright sparks

Electricity is one of several different types of energy that have been studies by scientists. Electrical energy flows through wires, and is used to power things like lights and computers.

Life on Earth

Cats aren’t the only curious animals that live on Earth! Scientists have already discovered animals, plants, and other living things, and they think that there may be many more.

Material matters

Everything around you is made of matter, and that includes body! Scientists have explained how matter can take different forms, and why it behaves differently when it is a solid, liquid or gas.

Forces of attraction

Magnets produce an invisible pulling and pushing force called magnetism. The Ancient Greeks knew this force existed, but it took until 1279 for a scientist to explain how it works!

Casting shadows

Have you ever wondered why you only sometimes have a shadow, and why its length changes? Well, science has the answer.

 

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Glaucoma is a group of eye conditions that damage the optic nerve, the health of which is vital for good vision. This damage is often caused by an abnormally high pressure in your eye. Many forms of glaucoma have no warning signs. The effect is so gradual that you may not notice a change in vision until the condition is at an advanced stage.

Glaucoma is the result of damage to the optic nerve. As this nerve gradually deteriorates, blind spots develop in your visual field. For reasons that doctors don't fully understand, this nerve damage is usually related to increased pressure in the eye.

Glaucoma tends to run in families. In some people, scientists have identified genes related to high eye pressure and optic nerve damage. Promptly go to an emergency room or an eye doctor's (ophthalmologist's) office if you experience some of the symptoms of acute angle-closure glaucoma, such as severe headache, eye pain and blurred vision.

 

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Lakes in cold region freeze in winter. What happens to the fish and other animals living in the lake?

A lake may freeze when it is very cold but fortunately for the fish and other animals living in it, it freezes only at the surface. The water below the surface remains unaffected. This happens because of the odd behaviour of water. Like any substance water also contracts and becomes denser as it cools; but once it cools to 4  its behaviour changes. Below 4  water actually starts becoming lighter. At 0 , water freezes to ice. Ice is lighter than water and it floats on water. The ice layer insulates the water below from the cool air above. This keeps the water below the surface, warm and in liquid state.

 

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My friend had a small magnet and he warned me not to use it to pull things towards it too often as that would exhaust its magnetic powers.

Can a magnet become demagnetized by overuse?

In materials like iron, the atoms themselves are little magnets. In the natural or neutral state, these atoms all point in different directions. In a magnet, the atoms all point in the same direction.

To de-magnetise a magnet, the alignment of the atoms needs to be disturbed in such a way that they are pointing randomly in different direction, as in the neutral metal. This can be done in either of three ways:

(a) by heating the magnet over a gas flame.

(b) by hammering it for a few minutes, or

(c) by dropping the magnet from a height of over 100 ft.

The magnet cannot lose its magnetic properties due to over use.

 

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The electric train gets electricity from an overhead wire. But how does a train running underground obtain electricity? The electric train has a device called a pantograph through which it draws electricity from a live wire above the tracks. In a suburban train 3 or 4 pantographs are used to run electric motors located in various compartments.

But in many underground train systems, the current is not obtained from an overhead wire. Instead, a third rail that runs parallel to the existing two, is laid and charged with electricity. The train picks up electricity from this track. This is done to reduce the height of the tunnel. In India, the Calcutta metro railway is an example of such an arrangement. Abroad, it is used in the London Underground.

 

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My friend says his uncle has invented a machine that once started can run forever without any assistance or additional energy. Are there such machines?

Many scientists have tried to build perpetual motion machines but without success. The force of friction ultimately brings every moving thing to a stop. So far the only machine that comes close to being a perpetual motion machine, is an artificial satellite circling the earth. It circles the planet for years but ultimately friction with dust particles slows it down and it falls into the upper atmosphere where t burns up.

 

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Glass is a bad conductor of heat. If a glass vessel is heated directly over a flame, the heat is not readily conducted to other parts of the vessel and even at the point of contact with the flame, the inside surface does not get as hot as the outside surface.

Consequently the outside surface expands more than the inside surface and the unequal expansions cause the glass to crack.

To avoid this the glass vessel is usually heated by keeping it on a wire gauze made of a good conductor of heat such as iron or copper. This ensures that heat is transferred indirectly and slowly to the glass vessel and the vessel does not crack.

 

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The ink is stored in the barrel of the pen. When we start writing, the ink flows down and collects at the hole near the middle of the nib. When we press the nib against the paper while writing, the slit in the nib opens slightly and behaves as a very narrow tube or capillary. The ink is pulled through the tube by capillary force and flows onto the paper through the tip of the nib.

Why isn’t it possible to write on an extremely smooth surface with a ballpoint pen?

The tip of the ballpoint pen consists of a small ball which projects from a metallic socket. Fast-drying, oil-based ink is supplied to it through a narrow tube. When we write, the ball rotates and transfers the ink onto the paper. If the surface on which we want to write is very smooth, there will not roll efficiently. Consequently the ink will not get transferred to the paper. Sometimes, the ball may be rotating but the ink may not be able to adhere to the surface.

 

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The speed of sound in air varies. At sea level it is about 1220 kmph but at higher altitudes, where air is cooler, it slows down becoming about 1,070 kmph at a height of 11,000 metres. Because the speed of sound changes with height, planes that fly faster than the speed of sound use a Mach number to measure their speed. The Mach number is the aeroplane’s speed divided by the speed of sound at the height the aeroplane is flying.

Mach 2 indicates a speed of twice the speed of sound. At Mach 1 the plane is flying at the speed of sound. Mach 0.5 is half the speed of sound, or subsonic.

Many fighter aircraft are capable of flying at high Mach numbers.

The unit is named after Ernst Mach, an Austrian physicist.

 

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The freezing of water starts at the walls of the tray and at the open surface as these are the areas in contact with a cooler material. As water is a bad conductor, the cooling process of freezing moves from outside to the inside. Water has some air dissolved in it. These air molecules occupy the spaces between the water molecules. When the water starts freezing, i.e. when liquid water changes to solid state, the air molecules are forced out from the inter-molecular spaces. They can now move only to the interior where some liquid still exists. As finally, the whole volume of water freezes, the air molecules get trapped in the middle as tiny bubbles. It is these air bubbles that make the centre of the ice cube opaque. The rest of the cube, being pure ice, is transparent.

 

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