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Practical Homeschooling® :

Water from Gases

By Janice VanCleave
Printed in Practical Homeschooling #63, 2005.

How can we get liquid by combining two gases?
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Janice VanCleave

Fun Time

Purpose. To make models of atoms and molecules.

Materials. Two lemon-size pieces of modeling clay of different colors - one red, one yellow; ruler; pen; one sheet of copy paper; two crayons - one red, one yellow; toothpicks.


  1. Divide the red clay into four equal parts. Shape each clay piece into a ball. Each part represents an atom of hydrogen.

  2. Divide the yellow clay into three equal parts. Shape each clay piece into a ball. Each part represents an atom of oxygen.

  3. Use the ruler, pen and copy paper to prepare a matter data table, such as the one shown in Fig. 1.

  4. In the data table, make a colored drawing of each of the two kinds of atoms: hydrogen (red clay ball) and oxygen (yellow clay ball).

  5. Form molecules using the clay balls for atoms and toothpicks for bonds. The structures listed here indicate the number and location of the bonds between the atoms. For example, there are double bonds between the oxygen atoms in an oxygen molecule.

  6. In the "Model" column of the table, make a colored drawing of each kind of molecule.


You have made models of atoms and molecules and described the models on a data sheet.


The individual clay balls represent atoms and each kind of clay ball "atom" differs from other types of clay ball "atoms" by its size and/or color. Clay ball "molecules" are made up of clay ball "atoms" linked by toothpicks that represent bonds. Some of the molecules are made of two atoms of the same kind and represent diatomic molecules. The water molecule is made of two different kinds of atoms and represents a molecular compound.

This experiment was taken from Janice VanCleave's Super Science Models and used by permission of the publisher, John Wiley & Sons, Inc. For more information about chemistry, check out Janice VanCleave's Chemistry for Every Kid (ages 8-12) and Janice VanCleave's A+ Projects in Chemistry (ages 13 and up).

Q: How can water be a liquid when it is made of two gases, oxygen and hydrogen?

A: Water is a compound and hydrogen and oxygen are elements. The physical properties of the elements making up a compound are generally not the same as those of the compound. Physical properties describe the features of a substance.

For example, the elements oxygen and hydrogen are both odorless, colorless gases. Water's physical properties include being a colorless liquid.

Matter is anything that occupies space and has mass (an amount of matter making up a material). Matter is the stuff that makes up the universe. The term substance is used by scientists to mean the basic parts of matter. A substance is made of one kind of matter, an element or a compound.

An element is a substance that cannot be broken down into simpler substances by ordinary means. Elements are made of only one kind of atom, which is the smallest building block of matter. Atoms are so small that about 150 billion atoms could fit on the period at the end of this sentence. There are more than 100 different elements that have been identified, and hydrogen and oxygen are two of them.

Atoms combine with each other in two general ways. One is by sharing electrons. When atoms share electrons they form molecules. Hydrogen and oxygen atoms form diatomic molecules, meaning a hydrogen molecule is made of two atoms of hydrogen and an oxygen molecule is made of two atoms of oxygen.

Water is an example of a molecule made up of different kinds of atoms. This type of substance is called a molecular compound. All compounds are made of two or more kinds of atoms combined in a fixed proportion.

The physical properties of a compound and the elements it is composed of are generally different because of the differences in the atoms bonded (linked) together. Hydrogen molecules have only hydrogen atoms bonded together, and oxygen molecules have only oxygen atoms bonded together. For these two gases to combine and form water, a chemical change must occur. A chemical change involves the rearrangement of atoms as a result of the breaking and reforming of bonds between the atoms. In the formation of water from the gases, hydrogen and oxygen, first the bonds between the atoms of each gas must be broken. Then, new bonds form between the oxygen and hydrogen atoms. The ratio of combination of hydrogen and oxygen is 2 to 1, thus the formula for water is H2O.

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