Grains and powders
Have you played in the sand on a beach? Perhaps you began by sitting in the sand at the top of the beach while you took off your shoes and socks. Then you felt the soft sand with your feet. If you buried your feet in the soft dry sand, you could let the sand grains pass between your toes. People often scoop up the sand in their hands and let it slip through their fingers. If the sand passes through just one gap, it forms a cone. This can be brushed away by the hand. As the hand sweeps through the sand the fingers make a pattern of wavy lines. All these properties of dry sand are due to the way grains flow over each other.

When people have finished playing in dry sand they usually run down the beach to paddle in the sea. As they run they can feel the soft, dry sand under their feet. When they get near the sea the sand feels different. It is hard and wet. This is due to the water in the sand holding the sand grains together.

The area of damp sand near the sea is the best place to build sand castles as the sand can be shaped into walls, turrets and towers. When a sand castle is complete, most people want to put a moat round it. As soon as the moat is finished they dig a trench from the moat to the sea. As they dig, the walls of the trench stand firm but once the sea water flows in they begin to collapse a little. This happens because too much water mixes with the sand in the walls and makes them weak. In a short time the trench has to be dredged with a spade to remove the sand that has fallen from the walls.

If the tide is going out, longer and longer trenches have to be built. After a while most people tire of digging trenches and leave their castles to play at other things. As the Sun shines on the castle, it dries the sand. Water no longer holds the grains together and the castle walls begin to fall. At first, battlements on the tops of the walls crumble, then smaller walls fall away. By the time the tide comes in again, the sand castle is reduced to a mound of sand sticking up on the beach. When the waves reach it, they mix with the sand again and swill it away up the beach. As the tide goes out no trace of the castle can be seen but soon people begin building new ones again.

What is sand made from?
It is made from fragments of a mineral called quartz. This mineral is found in many kinds of rock. A piece of rock is also made from many other minerals. The weather breaks them down and they dissolve in rainwater and are carried away into streams and rivers. Quartz cannot be broken down and dissolved in water. It is washed into streams and rivers and ground up by the water to make sand.

Why is sand yellow?
Pure quartz is white but it usually has other substances in it. They give the sand its colour. One substance contains iron. This makes the sand yellow.

Why is some sand black?
Black sands are found on islands which have been made by a volcano. The ash and lava from the volcano has been ground up by the seawater on the shore to make the sand.

Are there any other substances which make sand?
Yes. Gypsum is a mineral that was washed out of the San Andres Mountains in Mexico. It flowed away in streams and rivers to a lake. The lake dried up and the gypsum was left behind. The wind ground up the gypsum and it formed a desert of white sand. On some shores around the world there is white sand too. This is made from either ground up shells or coral.

Can you tell river sand from desert sand?
Yes, you can. If you look at them through a magnifying glass you can see that river sand has got jagged edges and desert sand is smoother and rounder. The desert sand has these features because it has been ground up by the wind.

How are sand dunes made?
They are made by the wind blowing the sand grains into huge mounds. One side of the dune has a gentle slope. This is facing the wind. The other side is much steeper but when it reaches an angle of 34° the sand collapses to make a gentler slope. As the sand dune collapses it may make a squeaking or roaring sound. These sounds are made by sands which have highly polished sand grains.

Can the wind move a sand dune?
Yes, it can. Some dunes can move a metre a day but most are much slower than this. When a sand dune moves, all the sand moves together so the sand dune keeps its shape.

Do sand grains ever stick together permanently?
Yes. If lightning strikes the sand, the heat in the flash sticks, or fuses, the sand grains together. They form a long tube which looks a little like a stalactite. This is called a fulgurite. A second, more common way of sticking sand grains together takes place over millions of years. In this time, deep layers of sand build up and become pressed together to make a rock called sandstone.

When did people start to use sandglasses?
The sandglass was invented about nine hundred years ago in the Mediterranean region. It was used by sailors to help them steer their ships safely across the sea. Since then, sand timers have been used to measure time in monasteries, schools and law courts. Sand timers have been made to measure different amounts of time. Some measure an hour and are called hourglasses. Other sand timers measure 45 minutes, 30 minutes or 15 minutes. Egg timers measure two minutes and some sand timers are adjustable and can measure between 30 seconds and three minutes. The speed at which the sand time measures time depends on the size of the grains and the size of the hole through which they pass.

How are powders made?
Putting small fragments in a drum called a mill makes many powders. The drum lies on its side and slowly turns. As the drum turns the fragments move up its walls then fall on each other. This grinds the fragments down. Heavy metal balls may be put in the mill to speed up its grinding action.

Is baking powder made from one substance?
No. It is a mixture of substances. One is called bicarbonate of soda. Another is made from a kind of rock which contains minerals called phosphates. These two substances take part in a change which helps cakes rise and become light in weight. There is also flour mixed with them. The flour helps to separate the two substances and to stop them making their change until they are added to the cake mix.

Can soil move slowly down a hill?
Yes. This is called creep. When the soil is wet or frozen it expands then when it dries it shrinks and falls a little further down the hill. This makes grass steps on the hillside and trees grow with bent trunks as they try to stay upright. The soil may move at less than 10 centimetres a year. A speed of 45 centimetres a year has been recorded.

Is soil creep the same as a landslide?
No. A landslide is caused by a mixture of water and soil which moves very quickly at speeds up to 44 metres per second.