1 00:00:00,000 --> 00:00:07,105 [MUSIC PLAYING] 2 00:00:07,105 --> 00:00:08,980 JESSICA HARROP: Hi, I'm Jessica and today I'm 3 00:00:08,980 --> 00:00:11,860 going to shed some light on a chemical demonstration I 4 00:00:11,860 --> 00:00:15,490 like to call anatomy of a glow stick. 5 00:00:15,490 --> 00:00:18,400 There are many different kinds of chemical reactions. 6 00:00:18,400 --> 00:00:21,190 Some absorb energy and some release energy. 7 00:00:21,190 --> 00:00:24,400 Some release energy as heat and others as light. 8 00:00:24,400 --> 00:00:26,110 And when a reaction produces light, 9 00:00:26,110 --> 00:00:28,570 the process is called chemiluminescence. 10 00:00:31,240 --> 00:00:34,480 Let's watch MIT's Dr. John Dolhun, his assistant, 11 00:00:34,480 --> 00:00:37,840 Shannon, and volunteers, Sarah, Catherine, and Trevor. 12 00:00:37,840 --> 00:00:40,030 They are going to show us some chemiluminescence 13 00:00:40,030 --> 00:00:42,066 at the Cambridge Science Festival. 14 00:00:42,066 --> 00:00:42,565 [WHOOSH] 15 00:00:42,565 --> 00:00:44,860 SHANNON MOREY: So not all reactions have this ability 16 00:00:44,860 --> 00:00:45,650 to produce light. 17 00:00:45,650 --> 00:00:48,880 And so we have to use some very special chemicals in order 18 00:00:48,880 --> 00:00:49,420 to do this. 19 00:00:49,420 --> 00:00:52,690 And we have some light sticks that I broke apart here, 20 00:00:52,690 --> 00:00:56,440 if you guys just want to take these solutions. 21 00:00:56,440 --> 00:00:57,620 Hold them carefully. 22 00:01:01,690 --> 00:01:04,510 And so in a light stick, usually there's 23 00:01:04,510 --> 00:01:05,920 actually hydrogen peroxide. 24 00:01:05,920 --> 00:01:08,499 And it oxidizes different chemicals-- oh, yeah, 25 00:01:08,499 --> 00:01:09,790 you can just face the audience. 26 00:01:09,790 --> 00:01:10,566 That's cool. 27 00:01:10,566 --> 00:01:12,190 And it will oxidize different chemicals 28 00:01:12,190 --> 00:01:15,310 depending on what color you want to give off light. 29 00:01:15,310 --> 00:01:15,810 All right. 30 00:01:15,810 --> 00:01:17,268 So just pour one solution together. 31 00:01:17,268 --> 00:01:19,989 It doesn't matter which way. 32 00:01:19,989 --> 00:01:20,530 AUDIENCE: Oh. 33 00:01:20,530 --> 00:01:23,550 SHANNON MOREY: And there we go, OK. 34 00:01:23,550 --> 00:01:25,090 Beautiful, beautiful. 35 00:01:25,090 --> 00:01:25,950 All right. 36 00:01:25,950 --> 00:01:28,110 So in each of these, we have a different chemical 37 00:01:28,110 --> 00:01:32,740 that gets oxidized to give us a different color. 38 00:01:32,740 --> 00:01:34,820 All right, thank you guys so much. 39 00:01:34,820 --> 00:01:37,568 [APPLAUSE] 40 00:01:37,568 --> 00:01:38,490 [WHOOSH] 41 00:01:38,490 --> 00:01:40,820 JESSICA: So how does a reaction produce light? 42 00:01:40,820 --> 00:01:42,540 Let's start with two molecules. 43 00:01:42,540 --> 00:01:45,230 Let's call them A and B. They react 44 00:01:45,230 --> 00:01:50,300 to produce C and D. Energy that's produced in the reaction 45 00:01:50,300 --> 00:01:53,270 causes the electrons in D to become excited. 46 00:01:53,270 --> 00:01:55,310 As the electrons relax back down, 47 00:01:55,310 --> 00:01:57,890 they give off energy as light. 48 00:01:57,890 --> 00:02:01,520 So basically, chemical energy is transformed into light energy. 49 00:02:01,520 --> 00:02:05,060 This all happens in one step as the reaction is progressing. 50 00:02:05,060 --> 00:02:06,723 Here's what's happening. 51 00:02:25,470 --> 00:02:29,270 This is the energy level of our reactants, A and B. When 52 00:02:29,270 --> 00:02:34,920 they react, they form C and D in its excited state. 53 00:02:34,920 --> 00:02:37,690 Now to relax back down to where it needs to be, 54 00:02:37,690 --> 00:02:40,204 D emits energy as light. 55 00:02:40,204 --> 00:02:41,870 Now there's a different amount of energy 56 00:02:41,870 --> 00:02:43,850 in our reactants than our products, 57 00:02:43,850 --> 00:02:48,050 so that's why these two energy levels are different. 58 00:02:48,050 --> 00:02:50,120 All right, so in Dr. Dolhun's experiment, 59 00:02:50,120 --> 00:02:53,510 each volunteer has a cup containing a different chemical 60 00:02:53,510 --> 00:02:56,720 and adds hydrogen peroxide to each cup. 61 00:02:56,720 --> 00:03:01,880 And each cup glows a different color, red, blue, or green. 62 00:03:01,880 --> 00:03:03,830 This happens because the hydrogen peroxide 63 00:03:03,830 --> 00:03:07,080 is oxidizing each of the different chemicals. 64 00:03:07,080 --> 00:03:09,980 So oxidation is the chemical reaction that will ultimately 65 00:03:09,980 --> 00:03:12,650 cause the chemiluminescence. 66 00:03:12,650 --> 00:03:14,330 So why did each of these chemicals 67 00:03:14,330 --> 00:03:15,830 glow a different color? 68 00:03:15,830 --> 00:03:17,450 Well it's because they each emitted 69 00:03:17,450 --> 00:03:19,100 a different amount of energy. 70 00:03:19,100 --> 00:03:21,224 And energy equals color. 71 00:03:21,224 --> 00:03:23,390 So let's think of it in terms of a rainbow spectrum. 72 00:03:33,870 --> 00:03:35,420 So the amount of energy increases 73 00:03:35,420 --> 00:03:36,990 as you move up the spectrum. 74 00:03:36,990 --> 00:03:39,890 So red is lower energy than blue. 75 00:03:39,890 --> 00:03:42,680 So if the difference between the ground state and the excited 76 00:03:42,680 --> 00:03:45,200 state is small, there is a smaller amount 77 00:03:45,200 --> 00:03:46,850 of energy released, and the light 78 00:03:46,850 --> 00:03:49,220 is on the redder side of the spectrum. 79 00:03:49,220 --> 00:03:51,200 Conversely, if that gap is larger, 80 00:03:51,200 --> 00:03:52,890 there's more energy released. 81 00:03:52,890 --> 00:03:55,940 So the light is on the bluer side of the spectrum. 82 00:03:55,940 --> 00:03:57,650 And this chemiluminescence chemistry 83 00:03:57,650 --> 00:04:00,110 has practical uses beyond glowsticks 84 00:04:00,110 --> 00:04:02,030 that light up a good party. 85 00:04:02,030 --> 00:04:04,160 We also use reactions like these to create 86 00:04:04,160 --> 00:04:08,120 flares that don't get too hot in use of emergency situations. 87 00:04:08,120 --> 00:04:10,160 And nature uses chemiluminescence too. 88 00:04:10,160 --> 00:04:12,440 [WHOOSH] If you've ever seen a firefly light up 89 00:04:12,440 --> 00:04:16,010 at night or an [WHOOSH] Angler fish hunt for prey, you've seen 90 00:04:16,010 --> 00:04:18,350 bioluminescence which is chemiluminescence 91 00:04:18,350 --> 00:04:20,329 in a biological system. 92 00:04:20,329 --> 00:04:23,030 It's the exact same principle as the chemical reactions 93 00:04:23,030 --> 00:04:24,759 that Dr. Dolhun showed us. 94 00:04:24,759 --> 00:04:26,550 [MUSIC PLAYING] Hope you enjoyed the video. 95 00:04:26,550 --> 00:04:28,510 I'll see you next time.