Properties of Matter: Investigation 3 –
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Remind students that by observing a sample of matter and measuring its chemical and physical properties, we gradually acquire enough information to characterize it and distinguish it from other kinds of matter. This is the first step in the development of chemical science, in which interest is focused on specific types of matter.
Explain to students that soil scientists might study the composition of soils on Mars to determine what elements they contain or chemically evaluate soil nutrient levels in a farming community to determine what crops would grow best there.r.
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- Tell students that during the course of this presentation we will:
- review the relationship between atoms, molecules and compounds,
- develop the concept of a chemical formula, and
- introduce two more important physical properties of matter, solubility and miscibility.
- Tell students that atoms can combine to form very different compounds.
- Explain to students that while there are a limited number of different elements, they bond with each other in varying combinations to form an almost endless variety of molecules and compounds.
- Discuss how this slide shows three elements, oxygen, hydrogen and sulfur combining to form different molecules and compounds.
- Oxygen and hydrogen each combine with themselves to form O2 and H2, respectively.
- Both O2 and H2 are gases. However, while breath O2 gas to stay alive, H2 gas is extremely flammable. It is used a fuel for spacecraft and was responsible for the famous explosion of the Hindenburg airship in 1937.
- Oxygen and hydrogen atoms combine with each other to form water.
- Oxygen, hydrogen and sulfur atoms combine to form the sulfuric acid molecule, a very caustic liquid.
- Explain to students that a chemical formula tells how many atoms of each element are in a compound molecule. Click the right arrow once.
- The example used in this slide is the common table sugar, sucrose:
- Tell students that the large molecular model of sucrose on this slide also shows that the individual atoms of C, H, and O are bond to each other in a very specific arrangement. The “sticks” joining the atomic “balls” together represent the chemical bonds that hold the molecule together.
- Explain to students that this chemical formula
- tells us which atoms are present in the sucrose molecule (carbon, hydrogen, and oxygen), and (Click right arrow once.)
- it also tells us how many atoms of each element are present (12 carbon, 22 hydrogen, and 11 oxygen).
- Tell students that the exact complement of atoms in any molecule dictates its physical and chemical properties.
Note: Students will learn much more about chemical bonds in high school, but it may be worth mentioning that physical changes that occur in matter do not involve breaking or rearranging chemical bonds, whereas chemical changes do. Thus, a chemical change results in one or more entirely different molecules than one(s) that existed prior to the chemical reaction.
- Tell students that the table in this slide lists all of the compounds that they will work within Investigation 3.
- The first column gives the common name of the compounds.
- The second column gives the chemical formula. Click the right arrow once.
- The third column shows the elements present. Click the right arrow once.
- The fourth column depicts the number of atoms of each element present.
Note: Atoms of the same three elements, carbon, hydrogen, and oxygen are found in different combinations in a number of the compounds.
- Work through the rest of the table as a class to help the students develop a familiarity with chemical formulas. Click on the right arrow (multiple times) to expose the remaining information in the third and fourth columns.
Note: Vegetable Oil is shown with a chemical formula of (CHO)x, which means that it contains a combination of different types of molecules that contain C, H, and O atoms. Vegetable oil is therefore not a pure oil as plants contain many different types of oil molecules.
- Review the signs of a chemical reaction listed on this slide.
Note: This slide was previously seen by students in the presentation for Investigation 2.
- Review the physical properties of matter.
Note: This slide was adapted from a slide previously seen by students in Investigation 2.
- Tell students that solubility is the ability of a solute to dissolve in a solvent.
Note: This is an important concept for students to grasp. The Lab for Investigation 3 will give them repeated practice mixing various solutes with a solvent (water). During Lab, it is worthwhile asking students which compound is a solute and which is a solvent at various points.
- Tell students that, in general, solutes are solids and solvents are liquids.
- Explain to students that in this Investigation, we will use the solvent water when studying solubility and then the solvents vinegar, rubbing alcohol and vegetable oil when studying miscibility.
Note: Later in sixth-grade students will learn much more about solubility in the CELL Solutes and Solubility. For now, we wish to establish solubility as an important physical property of a compound.
- Tell students that miscibility is the ability of two liquids to mix with each other.
- Explain to students that we can mix oil paints together to create new colors because oil paints are miscible with each other.
- We would get a very different result if we tried mixing oil paint with water-based paints like tempera or watercolors.
- In the latter case, the paints would not mix well together and the colors would likely remain separated from each other in globs.
- Tell students that salad oil and water or vinegar is also a common example of immiscible liquids.
- Explain that the degree to which various liquid compounds are miscible with each other is a physical property of the compounds and can tell chemists much about their molecular characteristics.
- Ask students, “Which of the two liquids do you think is more dense?”
- Explain to students that when two liquids that are not miscible with each other are mixed they often form two layers with the denser of the two liquids on the bottom (yellow) and the less dense liquid “floating” on top (blue).