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In other class materials you have learned about the types of substances and materials that there are. First, there are elements, which are pure substances made of only one kind of atom. Second, there are compounds, which are pure substances made of two or more kinds of atom chemically bound together in a specific ratio and with a specific structure. Third, there are mixtures. Mixtures are materials made of two or more substances. Mixtures are simply physical combinations of substances with different molecular structures. Salt dissolved in water is a mixture. Mud suspended in river water is also a mixture. Air is a mixture of oxygen, nitrogen, and argon. In all cases there are multiple pure substances present. Salt water is a mixture of two compounds. Mud suspended in river water is a mixture of the many compounds in soil with water, another compound. Air is a mixture mainly of three elements. Air and salt water are homogeneous mixtures, which have the same composition throughout. Mud suspended in water is a heterogeneous mixture because it has clumps of one material suspended in another. Anything with bubbles or clumps is a heterogeneous mixture.

There are two main kinds of changes that matter can undergo. Physical changes are the simplest kind. A physical change alters the appearance of a material without changing its molecular structure. For example, phase changes such as from a solid to a liquid to a gas are physical changes. Another type of physical change is sorting out the components of a mixture to separate them. Chemical changes are a bit more complex because they may also involve phase changes. What distinguishes a chemical change is that it involves a change to the molecular structure of the materials. It is literally a transformation of one or more materials into different substances. For example, if hydrogen gas reacts with oxygen gas there can be a fire or a big explosion. This reaction is in fact used in rockets when NASA launches astronauts into space. When the reaction is over no oxygen or hydrogen gases remain: instead, they have combined their atoms by forming new chemical bonds to form molecules of liquid water.

Chemical changes can be depicted using the chemical formulas of the materials involved in a chemical equation. The chemical equation for the formation of water from hydrogen and oxygen looks like this:

2H₂(g) + O₂(g) → 2H₂O(l)
The large numbers in front of the formulas tell you the number of molecules involved in the reaction. The number of each different type of atom is given by the small, subscripted numbers within each formula. The ‘2’ H₂O means that there are two H atoms in a molecule of water. The letters g and l stand for gas and liquid. Another way to use these equations is to depict physical changes, such as water freezing:
H₂O(l) → H₂O(s) Notice that when such an equation is used to show a physical change the beginning and end of the equation has the same chemical formula. Only the phase of matter symbol has changed so that at the end it is s, which means solid. One other phase of matter symbol is used for things dissolved in water, since that comes up so often in chemistry. The symbol aq stands for aqueous, which just means dissolved in water. This chemical equation uses this symbol: 2NaOH(aq) + MgCl₂(aq) → Mg(OH)₂(s) + 2NaCl(aq) Here, aqueous sodium hydroxide reacts with aqueous magnesium chloride to make solid magnesium hydroxide and aqueous sodium chloride.

Objectives

1. Learn about pure substances and mixtures
2. Learn about phases of matter
3. Learn about physical and chemical changes

4. Learn about chemical equations
5. Practice drawing molecular-level pictures

Pre-lab

Based on the following descriptions write equations using chemical symbols showing the correct formula and also the phase of matter using (s), (l), (g), or (aq).

1. Gaseous hydrogen (H₂) reacts with gaseous chlorine (Cl₂) to make gaseous hydrogen chloride (HCl).
2. Write an equation for the evaporation of water.
3. Liquid potassium chlorate (KClO₃) decomposes to form solid potassium chloride (KCl) and oxygen (O₂) gas.
4. Aqueous baking soda (sodium bicarbonate, NaHCO₃) reacts with aqueous hydrobromic acid (HBr) to form carbon dioxide gas, liquid water, and aqueous sodium bromide (NaBr).
5. Aqueous silver nitrate (AgNO₃) reacts with aqueous sodium chloride to form solid silver chloride (AgCl) and aqueous sodium nitrate (NaNO₃).
6. Draw one example of each of the molecules in the previous questions. Label your atoms clearly using colors you choose.

Materials

The following list does not cover all possible hazards, just the ones that can be anticipated. Move slowly and carefully in the lab: haste and impatience have caused more than one accident.

• Wear chemical splash goggles, gloves, and a chemical-resistant apron.
• Keep work stations organized, neat, and uncluttered. These are shared spaces so be considerate. Clutter can lead to accidents.
• Sodium carbonate solution is corrosive and mildly toxic. Wear protective equipment when handling. If on hands, wash immediately with running water. If in eyes, rinse for up to 15 minutes in the eyewash.
• Hydrochloric acid solution is corrosive and toxic. Wear protective equipment when handling. If on hands, wash immediately with running water. If in eyes, rinse for up to 15 minutes in the eyewash.
• Copper(II) sulfate solution is mildly toxic. Wear protective equipment when handling. If on hands, wash immediately with running water. If in eyes, rinse for up to 15 minutes in the eyewash.
• Be careful when handling fire; keep flammable materials away from the work area. Report fires immediately.

Procedure

Lab Station I, Ice Melting

1. Observe the two black squares of material. In your lab notebook record your observations. Describe them with respect to appearance, weight, apparent temperature and make a guess about what they’re made of.
2. Use the infrared thermometer to measure the surface temperature of the two black squares. Record the temperatures in your lab notebook. Answer the following question in your notebook: Did the temperatures match your expectations? Why or why not?
3. If you place an ice cube on each of the black squares will there be a difference in how quickly the ice melts? Which one would you predict would make the ice cube melt faster? Why? Answer these questions using complete sentences before you try it.
4. From the styrofoam cooler retrieve two ice cubes. Place the rubber rings on each black square and set an ice cube down inside each ring. Observe what happens and record your observations in your lab notebook.
5. In your lab notebook explain what happened. Consider that metals are much more conductive of heat than non-metals. Better heat conductors feel cool to the touch because they are better at taking heat away from your skin.
6. In your lab notebook write an equation showing the change you witnessed. Be sure to use correct chemical formulas and to show the phase of matter.
7. Clean up the mess you made. Dry everything off and put the partially melted ice in the sink. Leave the station ready for the next lab group.

Lab Station II, Generating a Gas

1. Observe the chemicals you will be mixing. One of them is called calcium carbonate (CaCO₃) and you have it in the form of chips of marble. The other is hydrochloric acid (HCl) dissolved in water. You have it in a one molar concentration, which has 1 mol of HCl for every liter of solution. Describe these chemicals in your lab notebook.
2. Look up these two compounds on Wikipedia to find out about their molecular structure. Sketch that structure in your lab notebook. Note that calcium carbonate is an ionic compound which in solid form has a repeating pattern of calcium and carbonate ions. The hydrochloric acid molecules are dissolved in water so that the solution is actually mostly water. When HCl dissolves it separates into two ions: H⁺ and Cl^–.
3. Read all instructions before proceeding with the demonstration. First, place a few pieces of calcium carbonate into a test tube. Also, use a graduated cylinder to measure out 10 mL of hydrochloric acid.
4. Second, add the acid solution to the test tube and observe what happens.
5. Third, light a wood splint on fire with a match or butane lighter. Put the burning end of the piece of wood into the test tube being careful to keep it from touching anything but the glass.
6. Write down your observations of what happened when you added the acid solution to the calcium carbonate. Also, describe in your lab notebook what happened to the flame when you brought it into the test tube.
7. At this station you have observed a chemical change. The calcium carbonate has reacted with the hydrochloric acid to form water, calcium chloride, and carbon dioxide. Look up carbon dioxide on Wikipedia and sketch a sample of gaseous carbon dioxide in your lab notebook.
8. In your lab notebook answer this question: What was the purpose of the burning wood splint? What conclusions can you draw from its behavior when you put in into the test tube? Explain.
9. Before leaving the lab station please clean out and rinse thoroughly the test tube you used. Hang it up to dry.

Lab Station III, Generating a Different Gas

1. Half-fill a clean test tube with 3% hydrogen peroxide solution. (This is 3% H₂O₂ and 97% H₂O). Draw a picture of what this solution looks like at the molecular level in your lab notebook. If necessary, look up their molecular structures on the Internet. The solution will have mostly water molecules with only a few hydrogen peroxide molecules.
2. Read all instructions before proceeding with the demonstration.
3. Use a dropper to add a little more than one milliliter of the orange iron(III) chloride solution. Add it by squirting it strongly to get it to mix well with the hydrogen peroxide solution.
4. Write down your observations of what is happening inside the test tube in your lab notebook. You should see the solution begin to bubble slowly but eventually effervesce like a freshly poured glass of seltzer water.
5. Use a lighter or match to light a wood splint on fire. Let it burn for a few seconds—just long enough to get a glowing red coal at the top. Blow out the flame, leaving the glowing coal.
6. Test for the nature of the gas being produced by the process in the test tube by inserting the glowing coal into the test tube. Write down your observations of what you see in your lab notebook.
7. At this station you have observed a chemical change. The hydrogen peroxide has decomposed to form oxygen gas (O₂) and water. Sketch a sample of gaseous oxygen in your lab notebook.
8. In your lab notebook explain what happened to the wood splint when you put the glowing red coal into the oxygen. Why did it happen?
9. Before leaving the lab station please clean out and rinse thoroughly the test tube you used. Hang it up to dry.

Lab Station IV, Precipitation

1. At this lab station you will find a bottle filled with a solution of dissolved sodium carbonate (Na₂CO₃). You will also find a bottle that contains a solution of copper(II) sulfate (CuSO₄). Describe these chemicals’ appearance in your lab notebook.
2. Look up these two compounds on Wikipedia to find out about their molecular structure. Sketch that structure in your lab notebook. Note that sodium carbonate is an ionic compound which in solid form has a repeating pattern of sodium and carbonate ions. Dissolved in water these ions separate to form Na⁺ ions and carbonate ions (CO₃^2–). The copper(II) sulfate ions are dissolved in water so that the solution is actually mostly water. When CuSO₄ dissolves it separates into two ions: a Cu²⁺ ion and one SO₄^2– ion.
   Draw a picture of each of these solutions in your lab notebook.
3. Read all instructions before proceeding with the demonstration. Measure out then add 5 mL of blue copper(II) sulfate solution to the test tube.
4. Measure then add 25 mL of tap water.
5. Use a plastic pipet to add draw up about two mL of the clear sodium carbonate solution. An approximate amount is fine.
6. Use the pipet to slowly drop the clear sodium carbonate solution into the blue copper(II) sulfate solution in the test tube. In order to see what happens bring your eyes to within a few inches of the test tube before dropping the sodium carbonate solution into the test tube. Write down what you observe in your lab notebook.
7. At this lab station you have witnessed a chemical change. The copper ions from the blue solution have combined with the carbonate ions in the clear solution to form a solid material called copper(II) carbonate (CuCO₃). The sodium ions and sulfate ions remain unchanged in the solution, leaving sodium sulfate (Na₂SO₄) dissolved in the water. Look up copper(II) carbonate on the Internet to learn about its chemical structure. Draw this structure in your lab notebook.
8. In your lab notebook, construct a visual chemical equation to show what happened in this reaction. Draw molecules of sodium carbonate and copper(II) sulfate next to one another with a plus-sign in between them. Draw an arrow toward molecular drawings of copper(II) carbonate and sodium sulfate (Na₂SO₄). When you are done drawing, write in the chemical formula for each of these compounds underneath its picture.
9. This type of reaction is called a precipitation reaction. Look up the definition of this type of reaction on the Internet and describe it in your own words in your lab notebook. Discuss it with your lab partner in order to help you express the idea in your own words.
10. Please clean up at this lab station according to your teacher’s instructions. The copper ions are toxic and should not go down the drain or into the trash can.

Post-lab