If we wish to write an equation for the chemical reaction between the elements phosphorus and oxygen, we begin with the formulae for the molecules of phosphorus and oxygen．

　　P4 (s) + O2 (g)--->　　　　[※s = solid , g = gas ．]

　　Again, we state the formulae of the molecules which react;by writing P4 and O2, we indicate or show that a molecule of phosphorus has four atoms, while a molecule of oxygen has two atoms ．

　　The product of this reaction is the compound phosphorus oxide:

　　P4 (s) + O2 (g)--->　　　　P4O10 (s)

　　Again wesee that a chemical unit of the new compound has two types of atoms． Each chemical unit contains or has four phosphorus atoms and ten oxygen atoms． This equation correctly gives the formulae of the reactants and the products, but, as it is written, mass is not conserved or kept． Only two atoms of oxygen enter the reaction, but ten atoms of oxygen appear in the product． This equation cannot be correct． Ten atoms of oxygen must have come from five molecules of oxygen． So we must make the following correction:

　　P4 (s) + 5O2 (g)---> P4O10 (s)

　　Now the mass of oxygen is conserved;all the oxygen atoms in the product are present in the reactants．

　　Now the equation is correct． Mass is conserved, and both the reactants and the products are shown by correct formulae． This process or step by which we explain all the atoms in a reaction, and so show that mass is conserved or kept throughout the reaction, is called balancing a chemical equation． A chemical equation is not correctly written until it shows that the reactants and products obey or keep to the Law of Conservation of Mass． This law of chemistry states or says that: In an ordinary chemical reaction, atoms are neither created (= newly made) nor destroyed．

　　In many cases or very often, both the reactants and the products of chemical reactions are compounds． But, as always, the reaction results in a rearrangement or replacing of the atoms in the reactants to form new substances or things． For example, copper (II) oxide will react with hydrochloric acid to produce a salt, copper(II) chloride, and water．

　　CuO(s) + HCl (2q)---Cucl2(aq) + H2O (1)

　　[※aq = aqua/‘æ kw¶ / =liquid solution溶液;l = liquid ．]

　　Since we know the reactants and the products, we begin our equation by writing correct formulae for these compounds． Then we check or examine to see whether the Law of Conservation of Mass has been observed． We can see immediately that it has not, for two atoms of chlorine appear in the products while only one atom of chlorine is shown in the reactants． So we balance the numbers of chlorine atoms and check again．

　　CuO(s) + 2HCl (2q)---CuCl2(aq) + H2O (l)

　　Now the equation is correct． It tells us that each compound has two different types of atoms． During the reaction, these atoms have “changed patterns”, so to speak, and new arrangements have been formed to produce different compounds．

　　We have seen that elements may react with one another to form compounds, and since compounds may also react with one another to form new substances, the number and variety of chemical reactions which may occur are almost limitless． Chemical equations are a type of chemical shorthand which we use to show these reactions, but they do not tell us what will happen when elements or compounds react． We can write a correct equation only if we know both the reactants and the products． Suppose you knew that the element zinc reacts with hydrochloric acid to produce a salt, zinc chloride, and the element hydrogen． You would begin the chemical equation by writing correct formulae to show the chemical units of the reactants and the products:

　　Zn(s) + HC1(aq) ® ZnCl2 (aq) +H2(g)

　　Correct formulae for the chemical units of both elements and compounds are necessary． (All the common gases have two atoms per molecule, and you are fairly safe in supposing that most other elements have only one atom per molecule． However, you have seen a few exceptions to this rule earlier in the chemistry book you learned at school．) If you are satisfied that your formulae are correct, balance the equation to show that the Law of Conservation of Mass is observed during the reaction． In our example you can see that the two atoms of hydrogen and the two atoms of chlorine in the chemical units of the products must have come from two chemical units of the reactant, hydrochloric acid．

　　We show this fact as follows:

　　Zn(s) + 2HC1(aq) ® ZnCl2 (aq) +H2(g)

1．Which of the following is a correct equation?

A．P4B． P4O10 C． P4 + O2 ---> P4O10　　D． P4 + 5O2 ---> P4O10

2．Which of the following is an element?

A．P B． P4O10　　C． H2O D． CuO + 2HCl ---> Cucl2 + H2O

3．P4 shows that it 　　　　 ．

A．has four atoms　　B． is a new compound

C．is the formula of an element D． contains or has ten oxygen atoms

4．Whichof the following is a reactant in a chemical reaction?

A．P4O10 in P4 + 5O2 ---> P4O10　　B． O2 in P4 + O2 --->

C．CuO in CuO + 2HCl---> CuCl2 + H2O

D．H2 in Zn + 2HC1 ---> ZnCl2 +H2

5．Which of the following is NOT correct?

A．ZnS + HCl ---> ZnCl2 +H2S　　B． P4 + 5O2 ---> P4O10

C．CuO + 2HCl---> CuCl2 + H2O　　D． Zn + 2HCl---> ZnCl2 +H2

6．Matter is made up of 　　　　　　　　 ．

A．substances　　B． reactants and products

C．elements and compounds　　D． equations

7．Which of the following sayings is true?

A．Most elements have only one atom each．

B．Most gases have only one atom each．

C．Almost all elements have two atoms each．

D．Almost all gases or gas elements have two atoms each．

8．What quantities are the same on both sides of a chemical equation?

A．Molecule quantities　　B． Atom quantities．

C．Reactant quantities and product quantities． D． Element quantities．

9．The difference between a compound and an element is that 　　 ．

A．they have different number of atoms each

B．they have different quality of atoms each

C．the element has the same kind of atoms while the compound has not

D．the element is made up of atoms while the compound is not

10．How many molecules of sodium would be required to produce six molecules of hydrogen according to the following equation?

　　2Na + 2H2O ---> 2NaOH + H2

A．6．　　B．8．　　C．10．　　 D．12．