Law of Definite Composition

Background information Many compounds are made of exactly the same elements but have very different physical and chemical properties. For example, carbon dioxide and carbon monoxide are both gases consisting of carbon and oxygen, but carbon monoxide is a deadly gas. The reason for the difference between these two compounds is explained by the Law of Definite Composition. This law states that the masses of the elements in a compound always occur in the same proportion. In other words, a carbon monoxide molecule consists of only one carbon and one oxygen atom while carbon dioxide always consists of one carbon and two oxygen atoms. Even though water and hydrogen peroxide are made of the same type of atoms, the ratio of atoms is different. It is this ratio that gives these molecules different chemical properties.

KNOW: What do you know about the Law of Definite Composition?

Objectives -Compare and contrast two chemicals with similar physical properties. -Better understand the Law of Definite Composition. Materials: graduated cylinder, hydrogen peroxide, manganese dioxide, water, safety goggles, two test tubes, matches, two wooden splints chemical scoop

Procedure: 1. Measure 5 mL of hydrogen peroxide and water and place in two separate test tubes. 2. Light a wooden splint, and blow it out so it is still glowing. 3. Place one small scoop of the manganese dioxide in one of your test tubes.

Test one molecule at a time, and DO NOT use too much manganese dioxide. A small scoop is all that is needed for this chemical reaction.

4. Place the glowing stick into the test tube as close to the liquid as possible without getting it wet. 5. Record your observations. 6. Repeat the process with the second test tube and record your observations.

Data Compound properties reaction with Manganese dioxide

WHAT: What did you learn about the Law of Definite Composition?

Answer four of the analysis questions with complete sentences. 1. How do the formulas of the two compounds differ?

2. How do the physical properties of the two compounds differ?

3. How do the chemical properties of the two compounds differ?

4. Why do water and hydrogen peroxide have different properties? 5. What caused your observations in the test tube containing hydrogen peroxide?

LEARN: What do you still want to learn about the Law of definite Composition?