[O06-P01] Observation of phase transitions for ammonium nitrate using hot plate and polarizing sheets
Keywords:phase transition, In-situ observation, ammonium nitrate, polarizing microscope
Experimental observation of phase transitions of minerals will generally need expensive instruments, and also care is needed for safety. However, for educational and demonstration purposes, much simpler and cheaper approaches are needed. Ammonium nitrate (NH4NO3) has low melting point of 175 oC, and can be melted using a hot plate for kitchen purpose. It has four phases (cubic, tetragonal, orthorhombic I, and orthorhombic II) between the melting point and room temperature, and impressive phase transitions can be easily observed using polarizing sheets. I have demonstrated those phase transitions to students and general public, and here I will present how to conduct this experiment.
Ammonium nitrate, polarizing sheets (at least two sheets), slide glass, cover glass, an hot plate are necessary for the demonstration. First, place the slide glass on the heated hot plate. Put some powder on the slide glass, and then put the cover glass over the powder. When the powder melted, thin sample section is automatically produced. For non-in-situ study, just remove the slide glass from the hot plate, and let it cool. You can observe the orthorhombic I (if it is still a bit warm) or orthorhomibic II (if it is cooled to room temperature). Both are very colorful when the slide glass is inserted between two polarizing sheets with Cross Nicol orientation. Occasionally, you may see orthorhombic I to II transition (Tc: about 32 oC). This section is a good teaching aid to learn polarization. It can be also used as an alternative to real thin section of rocks. To observe the sample, you may hold it in your hand and observe it through sunlight or ceiling light. Alternatively, you can observe using a light box for tracing purpose which is now very cheap. Put the slide glass and two polarizing sheets on the light box, then more than one person can see same time, and it is much easy to explain. If a binocular microscope with transmission illumination is available, the sample can be observed more closely. Since the grain size and the texture will change depending on the cooling rate of the sample and the water content, the heating and cooling may be repeated to see difference.
If you observe the cooling process starting from the melted state, you can see phase transitions. In order to avoid direct contact between the heated slide glass and the polarizing sheet, thick paper with a central hole may be used to separate them. The paper may be fixed to the polarizing sheet by adhesive tape. The melted sample on the hot plate should be quickly moved on the thick paper with the polarizing sheet underneath. Then, quickly cover the slide glass with the polarizing sheet with the thick paper. To observe crystallization from melt, you better avoid Cross Nicol position as first crystallizing phase is cubic. After that, further transitions to tetragonal, orthorhombic I, and orthorhombic II can be observed as the temperature drops. By observing cooling of the melted sample, you may see four phase transitions (including melt/cubic phase). If available, use a binocular microscope for close observation. The transitions at higher temperature occur quickly, so it is necessary to reduce the cooling rate. Ask participants to come up with ideas for how to realize slower cooling rate. In the case of high school / college students, it is advisable to ask them questions; how many phase transitions can be observed, what is the crystal system of each phase based on observations with polarized light, and observe textural changes at the time of transition. A video of ammonium nitrate during cooling using a polarizing microscope is available from here (http://www.misasa.okayama-u.ac.jp/~masami/pukiwiki/index.php?%B6%B5%BA%E0).
It should be noted that ammonium nitrate absorbs moisture in the air, so that the current thin section state lasts only several hours to several days unless the sample is stored in a dry state, and the sample becomes an aqueous solution. Also, be careful not to get burned by a hot plate or heated slide glass. Use tweezers and instruct them not to directly touch the high-temperature objects.
Ammonium nitrate, polarizing sheets (at least two sheets), slide glass, cover glass, an hot plate are necessary for the demonstration. First, place the slide glass on the heated hot plate. Put some powder on the slide glass, and then put the cover glass over the powder. When the powder melted, thin sample section is automatically produced. For non-in-situ study, just remove the slide glass from the hot plate, and let it cool. You can observe the orthorhombic I (if it is still a bit warm) or orthorhomibic II (if it is cooled to room temperature). Both are very colorful when the slide glass is inserted between two polarizing sheets with Cross Nicol orientation. Occasionally, you may see orthorhombic I to II transition (Tc: about 32 oC). This section is a good teaching aid to learn polarization. It can be also used as an alternative to real thin section of rocks. To observe the sample, you may hold it in your hand and observe it through sunlight or ceiling light. Alternatively, you can observe using a light box for tracing purpose which is now very cheap. Put the slide glass and two polarizing sheets on the light box, then more than one person can see same time, and it is much easy to explain. If a binocular microscope with transmission illumination is available, the sample can be observed more closely. Since the grain size and the texture will change depending on the cooling rate of the sample and the water content, the heating and cooling may be repeated to see difference.
If you observe the cooling process starting from the melted state, you can see phase transitions. In order to avoid direct contact between the heated slide glass and the polarizing sheet, thick paper with a central hole may be used to separate them. The paper may be fixed to the polarizing sheet by adhesive tape. The melted sample on the hot plate should be quickly moved on the thick paper with the polarizing sheet underneath. Then, quickly cover the slide glass with the polarizing sheet with the thick paper. To observe crystallization from melt, you better avoid Cross Nicol position as first crystallizing phase is cubic. After that, further transitions to tetragonal, orthorhombic I, and orthorhombic II can be observed as the temperature drops. By observing cooling of the melted sample, you may see four phase transitions (including melt/cubic phase). If available, use a binocular microscope for close observation. The transitions at higher temperature occur quickly, so it is necessary to reduce the cooling rate. Ask participants to come up with ideas for how to realize slower cooling rate. In the case of high school / college students, it is advisable to ask them questions; how many phase transitions can be observed, what is the crystal system of each phase based on observations with polarized light, and observe textural changes at the time of transition. A video of ammonium nitrate during cooling using a polarizing microscope is available from here (http://www.misasa.okayama-u.ac.jp/~masami/pukiwiki/index.php?%B6%B5%BA%E0).
It should be noted that ammonium nitrate absorbs moisture in the air, so that the current thin section state lasts only several hours to several days unless the sample is stored in a dry state, and the sample becomes an aqueous solution. Also, be careful not to get burned by a hot plate or heated slide glass. Use tweezers and instruct them not to directly touch the high-temperature objects.