because its internal
the crust is the thin outer later of the earth where we live. well, it looks thin on the picture and it is thin relative to the other layers, but don't worry, we're not going to fall through by accident anytime soon. the crust varies from around 5km thick (in the ocean floor) to around 70km thick (on land where we live called the continental crust). the continental crust is made up of rocks that consist primarily of silica and alumina called the "sial".
the next layer of the earth is called the mantle. the mantle is much thicker than the crust at almost 3000km deep. it's made up of slightly different silicate rocks with more magnesium and iron.
the earth's outer core is made up of iron and nickel and is very hot (4400 to 5000+ degrees c). this is so hot that the iron and nickel metals are liquid! the outer core is very important to earth as it creates something called a magnetic field. the magnetic field the outer core creates goes way out in to space and makes a protective barrier around the earth that shields us from the sun's damaging solar wind.
the earth's inner core is made up of iron and nickel, just like the outer core, however, the inner core is different. the inner core is so deep within the earth that it's under immense pressure. so much pressure that, even though it is so hot, it is solid. the inner core is the hottest part of the earth, and, at over 5000 degrees c, is about as hot as the surface of the sun.
to search for tiny particles of gold in rivers, a mixture of sand, mud, and gravel is scooped up in a pan and swirled around. gold particles are heavier than the other particles, so they settle to the bottom of the pan. the lighter particles stay suspended in the water, and are decanted (poured off). this technique of panning for gold is called decanting. cream is also separated from milk by decanting – the cream is less dense than the milk.
in distillation, a mixture of liquids is heated in a flask. the liquid with the lower boiling point evaporates (changes to a vapour) first, and is condensed (changes back to a liquid) and collected. the liquid with the higher boiling point and any solid particles are left behind in the flask. fractional distillation separates liquids one by one as they boil. the oil industry separates crude oil using this technique.
when the substances in a mixture have different particle sizes, they are separated by filtration. the mixture is poured through a sieve or filter. the smaller particles slip through the holes, but the larger particles do not. filtration is the first stage in water recycling. chemists use filters called zeolites, which have holes so tiny that they can remove microscopic particles from water.
filtering dirty water
you can turn dirty water into clear water using a filter. place a container with a hole in the bottom inside another container and line it with filter paper. fill the container with layers of charcoal, sand, and gravel. pour dirty water into the container. the layers will filter out smaller and smaller particles of dirt. the result is clearer (but not necessarily drinkable) water.
centrifuging a mixture
a centrifuge is like an extra-fast spin dryer. it spins a liquid so quickly that the particles separate out. the heavier particles sink to the bottom and the lighter particles collect at the top. doctors separate blood samples for analysis (study) using a centrifuge.
scientists separate many liquid mixtures using chromatography. the mixture is dissolved in a liquid or a gas to make a solution. the solution is put on a solid material and the substances that dissolved most easily travel farthest up the solid material. the separated substances form bands of colour called chromatograms. food scientists study chromatograms to discover which colourings a food contains.
food scientists separate food colouring for analysis using paper chromatography. a drop of colouring is put onto filter paper. the edge of the filter paper is dipped in water. as the water flows up through the paper, it carries the colours with it. some colours travel faster than others, so the substances split into different coloured bands.
thin layer chromatography
genetic scientists use thin layer chromatography (tlc) to study the substances that make up our genes. in tlc, the solid material is a plate of glass or plastic coated with a chemical, usually aluminium oxide or silicon oxide. when the liquid mixture travels up the plate, some of the substances move farther up the plate than others. the substances appear as spots on the plate. scientists study genes to learn about inherited characteristics.