Challenge Question
About 500 billion billion* gallons of surface water is held in Earth's oceans, lakes, rivers, icecaps, groundwater and atmosphere. The salt water of the oceans (covering three quarters of Earth's surface) accounts for most of this total supply of surface water. About two percent is frozen in the ice caps at the North and South Poles.
To the nearest whole number, approximately what percentage of Earth's total supply of surface water is usable, fresh water?
[* This number which is 500 x 1024 is expressed in the UK as 500 billion billion or 500 quadrillion]


Atoms, the building blocks of all matter on Earth, are very tiny. One million atoms placed end to end would form a line no longer than a period (full stop) on this page. Even though they are so small, each atom is made up of even smaller particles such as protons, neutrons and electrons.
Atoms in nature generally bond together in certain predictable ways (determined by the electrons of each atom) to form molecules. Molecules can be composed of all one kind of atom (elements have this sort of molecule), or of several different kinds of atoms.
Carbon dioxide (normally a gas) is very simple type of chemical compound. Each molecule of carbon dioxide has three atoms, one carbon atom and two oxygen atoms.
Sugar is a more complex chemical compound. A simple sugar molecule has 24 (twenty four) atoms in each molecule. What three different kinds of atoms are there in such a molecule of sugar?

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