CHM 151LL. States of Matter. Physical and Chemical Changes

Name: ________________________ Partner(s): _____________________ Section: _______________________ CHM 151LL: States of Matter: Physical and Chemical Changes In this lab you will investigate the three states of matter, explore the nature of physical versus chemical changes, and then use that knowledge to identify the type of change involved in several experiments. Note: Bring 1 clean, dry empty aluminum can per person for this lab. Electrolysis of Water: Demonstration Your instructor will demonstrate the electrolysis of water to produce hydrogen and oxygen gas. Record your observations of this demonstration in your notebook. Making/Recording Observations Everything of potential importance that you observe during the lab period should be recorded. Each component of each material should be described in terms of color, clarity and state of matter. Reactants should be described before they are mixed. During reactions, changes are often observable, like bubbles escaping the solution, color changes or loud noises. Make sure to use all of your senses except taste (never taste chemicals during an experiment) when making observations. At the end of a reaction, the product mixture should be described. It is often useful to create a table of observations for reactions that includes columns for “before”, “during” and “after” the reaction. Observations of electrolysis. Before During After Electrolysis of H2O Test of the gas(es) produced during electrolysis. Lit Splint Glowing Splint Electrolysis of H2O Left gas sample Electrolysis of water You should interpret your data and identify what the two gases are. Write a balanced equation for this reaction. page 1 of 10 States of Matter Substances can exist in three physical states: solid, liquid, and gas. Some of the differences between these states of matter are 1) the atoms’ or molecules’ freedom of movement and 2) the amount of space between the atoms or molecules. The physical state of a substance at a specific temperature depends on the attraction between atoms or molecules. Although more than one hundred elements exist; only two naturally occur in the liquid state. The two liquid elements are bromine and mercury. Several elements occur as gases at room temperature: hydrogen, nitrogen, oxygen, fluorine, chlorine, and all the noble gases (Group VIIIA elements). If an element is not a liquid or gas at room temperature, then it is a solid at room temperature. Most elements are solids at room temperature. Only the noble gases contain atoms that are not bonded to other atoms. Some pure elements are diatomic elements, in which atoms are bonded to each other in pairs as follows: H2(g), N2(g), O2(g), F2(g), Cl2(g), Br2(l), and I2(s). Note the subscript 2 represents how many atoms of that element are bonded together in one molecule. Chemists classify matter using the terms atom, molecule, and compound, pure substance and mixture. Being able to classify a substance using these terms will help you identify the properties of the material. It helps to form a mental picture of a materialso you can better understand how to describe it. For example, when someone says “atom”, you should visualize a single sphere. The word molecule means that two or more atoms are held together very tightly by bonds. Molecules have defined formulas and connections between atoms. When you hear “molecule”, you should visualize multiple atoms bonded together in a defined way. In a pure substance every particle in the sample would look the same. In a drawing of a pure substance, every particle in the material looks the same. In a drawing of a mixture, different components of the mixture will look different. A mixture contains different atoms, molecules, and/or compounds and each component will have its own chemical identity and chemical formula. A sample contains an element if there is only one type of atom in the sample. A compound occurs when there are two or more different atoms bonded together. We can represent elements and compounds by drawing pictures of them called molecular-level representations. This picture represents helium gas, He(g). The atoms are spread out like a gas and are not bonded together. The sample is pure since each atom is the same. This picture represents a diatomic element like nitrogen gas, N2(g). The molecules are spread out and contain two of the same type of atom. This sample is pure since all particles are identical. This picture represents a solid element like pure gold, Au(s), because the atoms are very ordered and are all the same type. This picture represents a gas, but it would be a compound not an element since the bonded atoms are different from each other. The sample is pure since each molecule is the same. It could be carbon monoxide gas, CO(g). Post lab questions will explore your understanding of the states of matter. page 2 of 10 Separation Methods A mixture is matter that contains two or more chemicals (atoms, molecules or compounds) that are combined, and that have not reacted with each other to form a pure substance. Mixtures can be separated into their individual compounds by physical methods, that is, by methods that don’t require the breaking or making of chemical bonds. Common separation methods include filtration, decanting, distillation, evaporation, sublimation, precipitation and chromatography. There are two kinds of mixtures, heterogeneous and homogeneous mixtures. A homogeneous mixture is uniform in composition throughout the mixture. When looking at a sample of a homogeneous mixture, there is no visible sign that it is a mixture. For example, salt water looks the same as a pure sample of water. A heterogeneous mixture often has visible indicators that it is a mixture. For instance, pizza, salad dressing and chicken soup are clearly mixtures. The observation that there are red solids imbedded in a yellow solid indicates that pizza is not homogeneous. In this section of the lab, you will separate a mixture of sand and salt. Obtain one vial of the sand/salt mixture per group and develop a procedure to separate the mixture. You do not have to obtain pure salt or sand, but the two components of the mixture must be separated from each other. You can write a rough draft of your procedure here for your instructor to discuss with you. Before you execute your procedure, obtain your instructor’s approval that your procedure is appropriate, safe, feasible, and contains sufficient detail. Do not perform the separation until your instructor gives you permission. In your notebook you should record 1) Detailed observations of the sand/salt mixture before you do anything to it. Observations are descriptions of what you see. For example, you can observe that a substance is a clear, colorless liquid. 2) Your interpretation of the observations. Interpretations are inferences or suppositions based on what you see. Is a material a pure substance or a mixture? If you see a clear, colorless liquid, you might interpret this data and conclude that water is present. Interpretations are often wrong, you be careful to support your interpretations with the reasoning used to make them. 3) A procedure for your separation. At each stage in the separation procedure, you should describe what you see in sufficient detail for someone who cannot see the experiment to visualize the experiment and reproduce your work. 4) Your instructor’s signature, confirming that you have successfully completed the separation. page 3 of 10 Physical Changes Matter can gain or lose energy. When heat is added to a substance, the kinetic energy (energy of motion) of atoms increases, and the atoms move faster. Increasing temperature causes the molecular motion of a substance to increase. Eventually, in the case of solids and liquids, the kinetic energy rises sufficiently to cause a change in physical state, also called a phase change. At the melting point a solid will melt into a liquid, and at the boiling point a liquid will boil and become a gas. If we decrease the temperature, which decreases the molecular motion, a gas will condense into a liquid at the boiling point, and a liquid will freeze to become a solid at the melting point (which can also be called the freezing point). A few substances, such as dry ice, go directly from the solid state to the gas state (sublimation) and vice versa (deposition). Melting, freezing, boiling, condensation, sublimation, and deposition are all phase changes. . Phase changes (or physical changes) do not change the chemical identity of a substance. They may alter its appearance, but not its chemical formula. To explore this, carry out the following two experiments. Aluminum Can Experiment: 1. Obtain 2 clean, dry aluminum cans. Get a blue tub and fill ¾ full with water. Take a dry can and hold it with crucible tongs upright over a lit Bunsen burner flame for one minute keeping the can in motion so it heats evenly. If the plastic coating on the inside of the can begins to burn, resulting in smoke, place the can farther from the flame. Do not allow the plastic coating to burn as it releases irritating and possibly toxic smoke. Immediately invert the can in a bucket of cool water making sure the mouth of the can is completely under water. Record your observations on your report sheet. 2. Add 10 mL of water into a different aluminum can. Repeat the procedure in step 2 above. Heat until steam is coming out of the top of the can. Again immediately invert the can in the bucket of cool water making sure the mouth of the can is completely under water. Record your observations in your notebook Observations During Al Can Experimnet During heating After submerged in water Al can with no water inside Al can with 10mL of water inside V. Chemical Changes Every pure substance can be described by its unique chemical and physical properties. Every sample of the same substance will behave in an identical fashion. Table salt, identified by the formula NaCl, will have the same physical properties and chemical behaviors whether it is in your kitchen or in a chemical laboratory in Russia. Chemists in industry and research are interested in the changes substances undergo in order to provide better products to the marketplace and to discover new potentially useful substances. Chemical changes do change the composition of the atoms or molecules in the substance. They result in new substances with a different arrangement of atoms. The products (substances formed by the chemical change or reaction) are different from the starting materials (called reactants), so they have different chemical and physical properties. page 4 of 10 One example of a chemical change is the slow decomposition of hydrogen peroxide H2O2 (H-O-O-H) into water H2O (H-O-H) and oxygen gas O2. Light speeds up this reaction, which is why hydrogen peroxide is sold in brown containers. Hydrogen peroxide is a potent bleach and antiseptic (it kills bacteria), but water has neither of these chemical properties. Chemists cannot actually see the individual atoms or molecules of a substance to determine if a substance changes, so they rely on observations to determine if two substances are the same or different. If two substances behave in the same way, they are the same. If they react differently, then they are different. In the following four experiments you will subject materials to extreme conditions (heat, exposure to corrosives, etc.) in order to try to change the materials. You can test the behavior of the materials before and after the change to determine if a chemical change has occurred. Record your observations carefully, before, during and after the experimental procedure. For each experiment the following general procedure will be followed: Obtain two identical samples of the substance to be tested. Subject one of the samples to the indicated conditions. The two samples (the unchanged, “before” sample and the possibly changed “after” sample) are tested to see how they react.  If the “before” sample and the “after” sample behave identically during the identity test, the change the sample underwent is most likely physical, if there was any change at all.  If the “before” sample and the “after” sample behave differently during the identity test, the change the sample underwent is most likely chemical. Waste disposal: Dispose of all waste in the waste container in the hood. Wash and rinse all of your test tubes, shake out any excess water from the inside of each, and dry the outside of each. Make sure the gas in your area is turned off. Use paper towels to wash then wipe up your entire lab bench before leaving lab. A. Question: Does silver nitrate react with copper metal? CAUTION: Silver nitrate stains skin and clothing. Rinse spills immediately. Note: You may use a wet test tube for this test as long as it is clean. 1. Clean a small piece of copper wire with sandpaper, coil it, and drop it into a small test tube. Add enough 0.1M AgNO3 to completely cover it. Also add an approximately equal volume of the 0.1M AgNO3 to a second test tube. Wait a few minutes and observe the contents of the first test tube carefully. 2. Pour the AgNO3 solution off the piece of copper into a third test tube. The identity test is: Add 3-4 drops of 3M NH4OH to the AgNO3 solution that was in contact with the copper metal, and also to the that was not in contact with the copper metal. Record your observations in your notebook. Observations of Cu and AgNO3 solution Before Cu is exposed to AgNO3 solution After Cu is exposed to AgNO3 solution Appearance of copper metal Appearance of AgNO3 solution page 5 of 10 Test of AgNO3 solutions with and without exposure to copper AgNO3 sample AgNO3 after exposure to Cu AgNO3 solution + 3M NH4OH In your notebook indicate whether you think copper chemically reacts with silver nitrate or not. Explain your reasoning. B. Magnesium Ribbon and Heat Note: You may use a wet test tube for this test as long as it is clean. 1. Obtain two short strips of Mg ribbon. Record the appearance of the metal. 2. Holding a test tube with a clamp, place one strip into the test tube. Carefully add 10 drops of 2M HCl into the test tube. Record your observations. 3. Grasp one end of the other strip with your crucible tongs, and hold the strip in the flame of your burner until the magnesium ignites. Do NOT look directly at the burning Mg. 4. Collect any combustion product (ignoring unburned metal), and record its appearance. 5. Put the product (minus any unburned metal) in a test tube and treat it with HCl as above. Record your observations in your notebook. Mg sample Mg sample after ignition Appearance sample + 2M HCl (aq) Identify whether you think that heating magnesium resulted in a physical or chemical change. C. Ammonium Chloride and Heat Note: You may use a wet test tube for this test as long as it is clean. 1. Cover the bottom of your evaporating dish with a thin layer of NH4Cl (ammonium chloride). Support this on a ring stand and wire gauze. Cover the evaporating dish with a watch glass. 2. Heat the bottom of the evaporating dish with your bunsen burner until you notice white fumes escaping from the spout of the evaporating dish or until a white coating forms on the watch glass. Turn off your bunsen burner, and let the equipment cool. You should notice a white deposit on the underside of the watch glass. 3. Wait at least 5 minutes for the watch glass to cool, and then carefully remove it with your hand. Holding the edges of the watch glass securely over a test tube, scrape a pea-sized amount of the white deposit into the test tube. Dissolve this in a minimum amount of deionized water. 4. In a second test tube, place the same exact pea-sized amount of unheated NH4Cl, and also dissolve this in a minimum amount of deionized water. 5. Add two drops of AgNO3 solution to each test tube and compare the results. Record your observations in your notebook. Indicate whether you think heating ammonium chloride resulted in a physical or chemical change. page 6 of 10 Unheated NH4Cl Sample (before) Heated NH4Cl Sample (after) Appearance sample + AgNO3 Conclusions: Your conclusion section for this lab might be a little longer than usual, because this week involved several different experiments. For each, you should identify whether a physical or chemical change was observed, since the goal of the lab is to gain experience observing and identifying physical and chemical changes. page 7 of 10 Post-lab questions: Name:________________________________ 1. What is the only naturally occurring liquid element that is not diatomic? ____ ________ 2. What element occurs naturally as a diatomic solid? _____________ 3. In the boxes below, draw A) molecules, B) a diatomic element, C) a pure compound, and D) a mixture of a compound and an atom. A 4. C D Match the following figures with their most likely sample by putting the letter for each figure in front of each description below the images. A 5. B B C D _______ Solid zinc metal _______ Pure H2O in the gas state _______ Pure liquid mercury _______ Mixture of oxygen and fluorine gases A student claims that all molecules are compounds. Write the definition of the word “compound” and give one example of a molecule that is not a compound. page 8 of 10 6. Animations In the box below draw a picture of ten atoms in the solid state. Describe the position and motion of atoms in the solid state. Do solids have constant shape and/or constant volume? Position: Motion: Constant shape? In the box below draw a picture of ten atoms in the liquid state. Constant volume? Describe the position and motion of atoms in the liquid state. Do liquids have constant shape and/or constant motion? Position: Motion: Constant shape? In the box below draw a picture of ten atoms in the gaseous state. Constant volume? Describe the position and motion of atoms in the gaseous state. Do gases have constant shape and/or constant volume? Position: Motion: Constant shape? Constant volume? page 9 of 10 7. Explain the differences in behavior of each aluminum can (one without water inside, one with water inside) after each can was heated and then cooled in the tub full of water. How are phase changes involved in what you observed for the can with water? Al can without water inside: Al can with water inside page 10 of 10