The first time I do the demonstration, I prefer not to add phenolphthalein, so that there will be more scope for inquiry. 4 The collection of gas from a small piece of sodium reacts with water. Michel Bultingaire (previously of the Lycée Saint Exupéry, Fameck, France), also does it in much the same way.įig. I devised this demonstration some years ago but recently saw that a French chemistry teacher, M. If potassium is used, the teacher has to be that much more skillful! On igniting the hydrogen, one gets a light purple flame. If less sodium is used, then on lifting the cylinder out of the water, air goes in, and on lighting the gas one gets a very satisfying "bark" (not an explosion), along with a characteristically orange flame. If the cylinder is almost full of hydrogen it burns quietly. It is best to hold the cylinder nearly horizontal, to enable the air to mix with the hydrogen. The sodium forms a ball, a sign that it has melted (exothermic reaction).Īfter all the sodium has reacted the gas can be tested by igniting it, using a long match or splint. The sodium reacts and floats on the surface gas is released and forces the water level down. Teachers must practice this before demonstrating it! It is not a disaster if the sodium escapes and reacts on the surface of the water another piece can be inserted, but first make sure that the tweezers are quite dry! Using tweezers or forceps that have an angled tip might help facilitate this maneuver. Using tweezers, the sodium has to be rapidly placed underneath the mouth of the cylinder as shown in Fig. ![]() This is important, as the oil slows down the initial reaction with the water. A small piece of sodium - a cube of about 4x4x4 mm - is cut on a Petri dish, and then re-immersed in paraffin oil. The cylinder should be held by a clamp so that its mouth is just below the surface - for simplicity I have not drawn the clamp.Īll safety precautions must be in place before starting. The polythene is removed, and the cylinder remains filled with water. However, there is another way with which to demonstrate the reaction.Ī large, preferably glass, bowl or evaporating dish is three-quarters filled with water and a 100 mL glass measuring cylinder is filled with water- the mouth covered by a piece of a polythene bag* so that it can be inverted and placed in the bowl (Fig. 3 A small piece of sodium is added to the water under the graduated cylinder. 2 A graduated cylinder inverted in water.įig. 1 A layer of paraffin oil above water in a test tube is shown with a small piece of sodium.įig. The procedure cannot be carried out with lithium (why not?).įig. Students themselves can carry out the reaction, provided that they are warned neither to shake the test tube nor stopper it tightly. ![]() Excess metal remaining at the end of the reaction can be destroyed by adding ethanol or propanol. The test tube can be loosely closed with a stopper, and the gas ignited after a couple of minutes. Once they have been released, the metal sinks and the process is repeated. 1 – 1) it releases bubbles which themselves carry the metal up to the surface (Fig. ![]() It sinks down, and on reacting with the water (Fig. This can be accomplished by placing a layer of paraffin oil or turpentine above a layer of water in a test tube and then dropping a small piece of sodium or potassium into the upper layer. There is a simple and safe way to demonstrate the reaction. As the piece of metal skitters across the surface of the water in a beaker and - particularly in the case of potassium - it appears to catch fire, it is not obvious that the explanation for both phenomena lies in the production of hydrogen gas.
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