The beautiful eye image on the front cover is a microscale experiment, which is a combined effort of Andres Tretiakov, Kensington Park School and Bob Worley at CLEAPSS1 in the United Kingdom. This “puddle chemistry”, an affectionate term for microscale reactions in drops, takes place on a liquid crystal temperature sensor. The sensor is a wonderful visual aid for microscale reactions to show whether they are endothermic or exothermic. The thermochromic liquid crystal changes colour to indicate different temperatures — you may remember mood rings based on the same chemistry. The thermochromic plastic appears red to yellow to green to blue as the temperature increases.
Photo 1.
Cover photo shows a puddle of water on a liquid crystal temperature sensor with KCl crystals being pushed in from the side.
The cover photo (Photo 1) shows the process of dissolving solid potassium chloride in a puddle of water — approximately 1 cm in diameter. The puddle is placed on a circular 2-cm diameter thermochromic plastic, which initially appears as a blue circle inside the grey square due to the warm room temperature — the activity was done in late spring. Bob pushed KCl crystals with a wooden splint from the side into the puddle. The temperature drops as KCl crystals start to dissolve, resulting in the thermochromic crystal underneath now appearing red. The dissolution of KCl is an endothermic process. The ions quickly diffused around the “shallowest” outside rim of the puddle resulting in the temperature sensor appearing as a circular ring of yellow and red. The deeper centre of the puddle is the warmest, which can be seen as the thermochromic plastic remains blue, because no crystals have reached this far into the puddle. The photo is difficult to describe as it captures a snapshot at one instant of time. The actual process is mesmerising to watch. Try it!
Bob Worley has also experimented with dissolving different salts: NaCl and LiCl. Photo 2 shows the addition of lithium chloride to the centre of a puddle of water on the thermochromic plastic (20 to 25 oC). This is an exothermic process. The experiment was done in the fall on a cooler day, so the surrounding room temperature gives thermochromic liquid crystals in the plastic a green appearance. At room temperature the water puddle appears reddish on the thermochromic plastic because energy is required for the evaporation of water. Solid LiCl is added to the centre of the reddish-looking water puddle using a wooden splint. The splint is cut to a point, inserted into water to make it damp, then into a small quantity of crystals in a weighing boat and finally into the centre of the puddle.
The centre appears blue (hottest) on the thermochromic plastic underneath. The radiating heat results in the water puddle appearing blue in the centre, moving outwards green and then to red.
Dissolving sodium chloride had little effect on the temperature (not shown) and therefore little change in colour. These three puddle experiments illustrate the relative effect of increasing cation radius on the energy of dissolution.
Photo 2.
A puddle of water on a thermochromic plastic with LiCl crystals added to the centre.
It is interesting to note the initial appearance of the liquid crystal temperature sensors in these experiments appear different. Photo 1 shows a “lab-made” liquid crystal temperature sensor, whereas Photo 2 is a purchased thermochromic plastic. The sheets have different transition temperature ranges. Also the room temperature was different. This is a great example to share with students on the importance of recording the initial conditions.
Andres Tretiakov has performed a simple acid/base reaction as seen in the accompanying photos. He places a drop of dilute sodium hydroxide and a drop of hydrochloric acid on the liquid crystal temperature sensor. When the reactants are mixed together with a wooden splint, the heat of neutralization can be observed by the colour change in the sensor.
Andres Tretiakov posted this video of thermochromic liquid crystal sensor showing heat of neutralization.
The photos below are taken from a video of this reaction, posted online.
Puddles of dilute NaOH and HCl on a liquid crystal.
The puddles are pushed together with a wooden splint.
Where the two puddles of reactants meet, the thermochromic plastic appears blue, indicating an increase in temperature.
The energy released in the neutralization reaction spreads; the colour changes can be seen in the underlying liquid crystals.
As the reaction comes to completion, there is a decrease in temperature — the colouring of thermochromic plastic appears yellow-red around the edge of the puddle.
Andres has taken this activity one step further. He makes his own liquid crystal temperature sensor — an engaging project for students. He used a plastic square plano-convex magnifying lens and laminated pouches with a black background, and different cholesteryl ester liquid crystal mixtures. By changing the ratio between these esters, the range of temperature changes — and therefore colour — can be fine-tuned. Sound difficult? University of Wisconsin-Madison has valuable online resources
Andres followed these step-by-step instructions and videos and has made these liquid crystal temperature sensors with students. Students thoroughly enjoy making their own temperature sensors, and it is a nice science club activity or investigation. The procedure is simple enough. The amounts needed are very small and are applied with a paintbrush. Otherwise, the mixtures can be pre-made, and students can concentrate on designing and making the most versatile sensor. Students can be encouraged to consider possible applications of these sensors, such as embedding them in roads to warn of icy conditions.
There is a drawback — the cost of the chemicals. Luckily, the amounts are so small and these chemicals last for a long time, so sensors can be prepared again and again.
The chemicals can be bought from Sigma-Aldrich:
• Cholesteryl oleyl carbonate, Aldrich 151157,
• Cholesteryl pelargonate (Cholesteryl nonanoate),
• Aldrich C78801 and
• Cholesteryl benzoate, Aldrich C75802.
If you do not want to make the liquid crystal sensors, Educational Innovations sells Liquid Crystal Sheets. These 4 by 4” sheets come in a package of three different temperature ranges: 20-25 oC, 25-30 oC and 30-35 oC. For these reactions, it is recommended to have 20-25 oC or even 25-30 oC for the heat of neutralization.
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The puzzle prize this month is a set of assorted temperature liquid crystal sensor sheets. Photo (left) shows an ice cube melting on a liquid crystal sheet.
Notes
- Consortium of Local Education Authorities for the Provision of Science Services (CLEAPSS) in a UK resource that provides teachers ideas for exciting and engaging practical activities that fire pupils' imaginations and then, unlike many other sources of ideas, goes on to show teachers and technicians in detail how to translate the ideas into safe and exciting experiments in the classroom. CLEAPSS advice and documentation is recognised by the UK Health and Safety Executive and the UK Department for Education. Much of the material is behind a subscription wall, but the Youtube channel is open source.
Bob has more posted puddle ideas and videos.
www.youtube.com/channel/UCPotDWzaKehdDRW5Tl71PPw/videos [JLH]
