Lesson 2.1 - Elements and Compounds

You communicate with others using the English language. Have you ever thought how such complex communication is accomplished using only 26 simple letters? The 26 letters can be arranged in various ways to produce millions of words, each different from the others in shape, sound, and meaning. Similarly, your world is made up of only 92 elements that are combined in various ways to make up the millions of substances that make up you and the universe in which you live.

Remember that an element is defined as a substance that contains only one kind of atom and that cannot be broken down into other substances. This is similar to letters in the alphabet that cannot be broken down into other letters. Read Section 2.2 Elements and Compounds on pages 30 and 31 up to Testing for Compounds on page 31.

Question 1. What is the definition of a compound?

Question 2. Just by looking at the name of a substance, how can you tell easily if it is an element or a compound?

Question 3. According to the flowchart in Figure 2.9, can a mixture be a compound?

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Because compounds are composed of two or more elements, it should follow that you can separate a compound into the elements from which it is made. Scientists call this process a decomposition reaction. It is a chemical reaction that usually requires energy. Read the topic titled Testing for Compounds on page 31.

Question 4. What is a decomposition reaction as defined in the textbook?

Question 5. Figure 2.10 shows a person dismantling an alarm clock. Do you think it would be possible to break down a chemical compound using the tools that this person is using?

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In question 5, you learned that you cannot break down a chemical compound using ordinary tools. How are compounds broken down?

One way is to use a chemical reaction. An example of this method is the production of iron. Compounds of iron and oxygen (iron oxides) are common and are called iron ore. The iron ore is mined; then, it is reacted with carbon in a blast furnace. The carbon reacts with the oxygen in the iron oxide leaving iron. This is a type of decomposition chemical reaction.

Another way to force a decomposition reaction is to use electricity in a process called electrolysis. Water can be decomposed into its elements, hydrogen and oxygen, using a decomposition reaction powered by electricity. Read the investigation Decomposition Reaction on page 32 up to Procedure.

Question 6. Sir Humphry Davy forced electricity through molten compounds in early electrolysis experiments. In this experiment, what is electricity being forced through?

Question 7. What is the source of electricity in this experiment?

Question 8. What compound is being decomposed into its elements in this reaction? What elements do you predict will be produced?

Question 9. What do you think is the purpose of dissolving sodium sulphate in the water?

Question 10. The setup in the diagram shows two inverted test tubes submerged in the water. Why?

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Read the Procedure on pages 32 and 33 in your textbook. Then, return here to continue.

The test tubes collected the gases hydrogen and oxygen. Because one electrode is positive and one is negative, the hydrogen is collected in one test tube while the oxygen is collected in the other test tube. There are two lab tests for these gases. In step 11 of the procedure, a burning splint is held underneath the test tube that has been removed carefully from the water. The result is a "pop" sound as the hydrogen makes a mini-explosion. This is a common test for hydrogen. In step 12 of the procedure, a glowing splint is held under the test tube that has been removed carefully from the water. The result is that the glowing splint bursts into flame. This is a common test for oxygen gas.

Question 11. One of the test tubes contains twice the volume of gas as the other test tube. Given that the chemical formula for water is H2O (two hydrogen and one oxygen), what gas would be in the test tube that had twice as much gas as the other?

Question 12. What is the test for hydrogen gas?

Question 13. What is the test for oxygen gas?

Question 14. What do you predict would happen if you performed the experiment again but switched the wires on the battery?

Question 15. How would you use a decomposition reaction to help you tell if a pure substance was an element or a compound?

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Answers to Questions:

Question 1. What is the definition of a compound?

A compound is defined as a substance made up of two or more elements that are chemically combined.

Question 2. Just by looking at the name of a substance, how can you tell easily if it is an element or a compound?

If the name of the substance is on the periodic table, it is an element; if the name does not appear on the periodic table, it is a compound.

Question 3. According to the flowchart in Figure 2.9, can a mixture be a compound?

No, a compound is a pure substance, and a mixture is made up of more than one type of particle. You could have a mixture of elements, a mixture of elements and compounds, or a mixture of compounds, but you cannot have a compound that is a mixture.


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Answers to Questions:

Question 4.What is a decomposition reaction as defined in the textbook?

A decomposition reaction is a chemical reaction that breaks down compounds into simpler substances. Note that it does not say that it is a reaction that breaks a compounds into elements. The reason is that some compounds can be broken down into smaller compounds. Therefore, they use the term simpler substances.

Question 5. Figure 2.10 shows a person dismantling an alarm clock. Do you think it would be possible to break down a chemical compound using the tools that this person is using?

It would not be possible to break down a chemical compound using pliers or any tool that the person is using. Compounds are bigger than elements but still far too small to use any tool to break them into simpler substances.


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Answers to Questions:

Question 6. Sir Humphry Davy forced electricity through molten compounds in early electrolysis experiments. In this experiment, what is electricity being forced through?

In this experiment, electricity is being forced through a water solution containing dissolved sodium sulphate.

Question 7. What is the source of electricity in this experiment?

The source of electricity in this experiment is a 9V battery that might be used in a smoke detector or battery-powered toy.

Question 8. What compound is being decomposed into its elements in this reaction? What elements do you predict will be produced?

Water is being decomposed in this reaction. Because water is a compound composed of hydrogen and oxygen, you should predict that the elements hydrogen and oxygen will be produced.

Question 9. What do you think is the purpose of dissolving sodium sulphate in the water?

Pure water is a non-conductor of electricity. The sodium sulphate is dissolved in the water so that electricity will conduct through the water.

Question 10. The setup in the diagram shows two inverted test tubes submerged in the water. Why?

The test tubes are submerged in the water to collect the gases, hydrogen and oxygen, from the decomposition reaction. Notice that they are filled with water so that only the pure gases will be collected and not mixed with ordinary air. The gases produced at the electrodes rise up in the water solution and collect in the test tubes. The water in the test tubes is displaced by the rising gases.


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Answers to Questions:

Question 11.One of the test tubes contains twice the volume of gas as the other test tube. Given that the chemical formula for water is H2O (two hydrogen and one oxygen), what gas would be in the test tube that had twice as much gas as the other?

The test tube that had twice as much gas in it as the other would contain hydrogen gas because there are two hydrogen atoms in the chemical formula.

Question 12. What is the test for hydrogen gas?

A burning splint is held underneath the test tube of gas. If there is a "pop" sound, the gas is hydrogen.

Question 13. What is the test for oxygen gas?

A glowing splint is held underneath the test tube of gas. If the splint bursts into flame, the gas is oxygen.

Question 14. What do you predict would happen if you performed the experiment again but switched the wires on the battery?

You would predict that the gases collected in the test tubes would be the opposite of the first experiment. That is, oxygen would be collected in the test tube that hydrogen has been collected in and hydrogen would be in the test tube that oxygen had been in.

Question 15. How would you use a decomposition reaction to help you tell if a pure substance was an element or a compound?

If the decomposition reaction produced two or more substances, it must have been a compound. If you could not decompose a substance, it is likely an element.



Go to the next page to learn about chemical names and formulas.