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THINK ABOUT IT

Table salt, or sodium chloride, NaCl, can enhance the flavor of soup. Even though the salt dissolves in the soup, you can still taste it. This change is described by a chemical equation:

NaCl(s) → NaCl(aq)

It is also possible to change the salt in far more dramatic ways. For example, when an electric current is passed through a sodium chloride solution, a toxic, green gas bubbles out of the solution. The change is described by this chemical equation:

2NaCl(aq) + 2H₂O(l) → 2NaOH(aq) + Cl₂(g) + H₂(g)

How can you predict what you will observe based on a chemical equation?

To answer this question, you will explore

EXPLORING THE TOPIC

Predicting Change from Chemical Equations

While some chemical changes are difficult to see, many changes in matter are accompanied by observable evidence. For example, when a change takes place, you may hear fizzing, a pop, or an explosion. You might see changes in color or physical form. You may smell gases that escape, or you may feel heat. Perhaps there is even a fire.

But what if you don’t have a laboratory and a lot of chemicals around? How can you predict what you might observe if you are provided only with a chemical equation?

SUGAR DISSOLVING

Consider these two chemical equations. What type of change does each describe?

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The only difference between these two equations is the position of the (aq) and (s) symbols. Notice that the chemical formula does not change from one side of the equation to the other. The sugar molecule changes form but not identity. The first equation describes dissolving, in which a solid is mixed with water. The second equation describes removing water from an aqueous substance, leaving a solid.

Although the sugar changes appearance when it dissolves, the sugar itself has not changed into a different compound. The chemical equation indicates how the molecules have, or have not, changed.

SUGAR MELTING

Consider these two equations that involve changes to sugar.

Again the formulas are identical on both sides of the arrow, except for the symbols in parentheses, (l) and (s). These equations describe a phase change from a solid to a liquid and back to a solid. By heating sugar carefully, you can melt it to a clear liquid. When liquid sugar is cooled, it becomes a solid again.

The same compound shows up on both sides of the equation. Even though the tiny grains of sugar have been transformed into a single, large, chunk of solid sugar, the atoms within the sugar molecules have not rearranged into new compounds. Sugar is still sugar.

SUGAR DECOMPOSING

Consider another transformation of sugar.

C₁₂H₂₂O₁₁(s) → 12C(s) + 11H₂O(g)

Notice that the two sides of the equation look very different. There is no sugar on the right side of the arrow. The sugar has been decomposed. It has been converted into two products: solid carbon and water vapor.

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If you heat sugar sufficiently (beyond melting), it will turn black and it will release smoke. It will not taste sweet anymore, and there is no way to turn it back into sugar. These property changes, and the release of smoke, are signs that a more dramatic rearrangement of atoms has occurred.

This particular rearrangement of atoms is not easy to reverse. In other words, you cannot combine carbon with water and expect to form sugar.

SUGAR REACTING

Consider one more transformation of sugar, as described by this equation:

C₁₂H₂₂O₁₁(s) + 8KClO₃(s) → 12CO₂(g) + 11H₂O(g) + 8KCl(s)

Interpreting the chemical formulas tells you that sugar and potassium chlorate are added together. The result is the formation of two gases, carbon dioxide and water vapor, as well as the formation of some solid potassium chloride.

The sugar and potassium chlorate are converted to carbon dioxide gas, water vapor, and solid potassium chloride. This change happens very quickly and releases energy in the form of light and heat.

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Information in Chemical Equations

Just as a chemical equation can help you predict what you might observe, it can also provide more information than you can get by observation alone. Consider the reaction described at the beginning of this lesson. The change that happens upon passing an electric current through aqueous sodium chloride is described by this equation:

2NaCl(aq) + 2H₂O(l) → 2NaOH(aq) + Cl₂(g) + H₂(g)

If you were to observe this reaction without knowing the equation, you would not be able to identify the compounds before and after the reaction. Many solutions are clear and colorless, and you might not realize that two gases are produced. A chemical equation identifies the reactants and products in a reaction. This chemical equation indicates that aqueous sodium chloride combines with water to form aqueous sodium hydroxide, chlorine gas, and hydrogen gas. An equation also identifies the form or phase a substance is in. This equation indicates that a liquid of some sort will be observed before and after the reaction, and gas bubbles will be seen as the reaction takes place.

Some information is not contained in a chemical equation. The equation above, for example, does not indicate that chlorine gas is green and toxic. The reaction of sugar with potassium chlorate, mentioned previously, is quite spectacular. The equation does not indicate that the reaction is explosive or that a purple flame is produced. An equation does not tell you whether a procedure will result in hot or cold sensations. It won’t tell you how fast a change will occur or if a color change will be observed. It does not tell you if there will be an explosion, a loud noise, or an overflowing beaker. Only direct observation can provide you with all of the information about what you will see, hear, and feel.

LESSON SUMMARY

How can you predict what you will observe based on a chemical equation?

Chemical equations contain valuable information about what is happening during a change in matter. They track the identities of the substances involved in the change, using chemical formulas. They also identify the phase of each substance. By carefully examining a chemical equation, you can predict what you might observe if the procedure were completed. A chemical equation cannot tell you everything that you will observe. It does not inform you about the speed of a reaction, if a color change will occur, or if energy is transferred.

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