WEBVTT 00:00:00.000 --> 00:00:06.825 align:middle line:90% [MUSIC PLAYING] 00:00:06.825 --> 00:00:08.700 align:middle line:84% JESSICA HARROP: Hi, I'm Jessica and today I'm 00:00:08.700 --> 00:00:11.340 align:middle line:84% going to talk to you about a chemical demonstration I like 00:00:11.340 --> 00:00:13.890 align:middle line:90% to call colorful indicators. 00:00:13.890 --> 00:00:16.650 align:middle line:90% 00:00:16.650 --> 00:00:18.540 align:middle line:84% Indicators show us what we normally 00:00:18.540 --> 00:00:21.840 align:middle line:84% wouldn't be able to see, from the canary in the coal mine 00:00:21.840 --> 00:00:24.420 align:middle line:84% to a paint that changes color with temperature 00:00:24.420 --> 00:00:27.510 align:middle line:84% to the acid base indicators we'll be seeing today. 00:00:27.510 --> 00:00:31.190 align:middle line:84% And chemist Dr. Bassam Shakhashiri here he is-- 00:00:31.190 --> 00:00:33.630 align:middle line:84% is going to be showing us the demo. 00:00:33.630 --> 00:00:36.090 align:middle line:84% He's the president of the American Chemical Society 00:00:36.090 --> 00:00:38.530 align:middle line:84% and performing this demo right here at MIT. 00:00:38.530 --> 00:00:39.030 align:middle line:90% [WHOOSH] 00:00:39.030 --> 00:00:40.863 align:middle line:84% BASSAM SHAKHASHIRI: Now what I'm going to do 00:00:40.863 --> 00:00:43.890 align:middle line:84% is an experiment using another form of carbon dioxide. 00:00:43.890 --> 00:00:45.480 align:middle line:90% It's called dry ice. 00:00:45.480 --> 00:00:48.390 align:middle line:90% Dry ice is solid carbon dioxide. 00:00:48.390 --> 00:00:50.950 align:middle line:84% And you'll notice I'm putting what on? 00:00:50.950 --> 00:00:52.225 align:middle line:90% AUDIENCE: Gloves. 00:00:52.225 --> 00:00:54.060 align:middle line:84% BASSAM SHAKHASHIRI: Putting gloves. 00:00:54.060 --> 00:00:56.280 align:middle line:84% And I'm going to open this bucket 00:00:56.280 --> 00:01:02.740 align:middle line:84% and pick up three chunks of carbon dioxide solid. 00:01:02.740 --> 00:01:05.650 align:middle line:90% This is solid carbon dioxide. 00:01:05.650 --> 00:01:10.280 align:middle line:84% Its temperature is minus 78 degrees Celsius. 00:01:10.280 --> 00:01:12.050 align:middle line:90% It's very cold. 00:01:12.050 --> 00:01:14.960 align:middle line:84% Dry ice changes from being a solid to a gas by a process we 00:01:14.960 --> 00:01:16.857 align:middle line:90% call sublimation. 00:01:16.857 --> 00:01:19.190 align:middle line:84% Sublimation is happening right now, but we can't see it. 00:01:19.190 --> 00:01:21.090 align:middle line:90% How come we can't see it? 00:01:21.090 --> 00:01:24.067 align:middle line:84% Because carbon dioxide as gas is what? 00:01:24.067 --> 00:01:24.900 align:middle line:90% AUDIENCE: Invisible. 00:01:24.900 --> 00:01:25.680 align:middle line:84% BASSAM SHAKHASHIRI: It's invisible. 00:01:25.680 --> 00:01:26.610 align:middle line:90% It has no color. 00:01:26.610 --> 00:01:29.040 align:middle line:84% So sublimation is happening right now. 00:01:29.040 --> 00:01:30.820 align:middle line:90% We can't see it. 00:01:30.820 --> 00:01:33.270 align:middle line:84% And I'm going to put those three back in here 00:01:33.270 --> 00:01:35.910 align:middle line:84% and ask you to focus your attention on what you 00:01:35.910 --> 00:01:37.519 align:middle line:90% see between my two hands here. 00:01:37.519 --> 00:01:39.060 align:middle line:84% What do you see between my two hands? 00:01:39.060 --> 00:01:41.580 align:middle line:84% In order to sharpen your powers of observation 00:01:41.580 --> 00:01:44.910 align:middle line:84% and develop the skills of reporting these observations, 00:01:44.910 --> 00:01:49.420 align:middle line:84% I ask you to pretend to be the play by play radio announcer, 00:01:49.420 --> 00:01:52.260 align:middle line:84% describing to someone who was not with us what's going on. 00:01:52.260 --> 00:01:54.212 align:middle line:84% Not the TV announcer, that person 00:01:54.212 --> 00:01:55.920 align:middle line:84% has got it made because the picture tells 00:01:55.920 --> 00:01:57.070 align:middle line:90% almost everything. 00:01:57.070 --> 00:01:59.242 align:middle line:84% So, there are how many cylinders? 00:01:59.242 --> 00:02:00.159 align:middle line:90% AUDIENCE: Six. 00:02:00.159 --> 00:02:02.700 align:middle line:84% BASSAM SHAKHASHIRI: And what do you see inside the cylinders? 00:02:02.700 --> 00:02:03.870 align:middle line:90% AUDIENCE: Colored liquid. 00:02:03.870 --> 00:02:05.369 align:middle line:84% BASSAM SHAKHASHIRI: Colored liquids. 00:02:05.369 --> 00:02:07.060 align:middle line:84% OK, I'm listening to you on the radio. 00:02:07.060 --> 00:02:10.259 align:middle line:84% And what I hear you say is that there are six cylinders. 00:02:10.259 --> 00:02:12.150 align:middle line:84% And they have in them colored liquids. 00:02:12.150 --> 00:02:12.990 align:middle line:90% Come on. 00:02:12.990 --> 00:02:15.190 align:middle line:84% Your brain learned a lot more information 00:02:15.190 --> 00:02:17.240 align:middle line:84% than those two statements, right? 00:02:17.240 --> 00:02:17.850 align:middle line:90% Right? 00:02:17.850 --> 00:02:19.520 align:middle line:84% So they're about this big, you said. 00:02:19.520 --> 00:02:21.810 align:middle line:84% I can see you on the radio saying it's about this big, 00:02:21.810 --> 00:02:22.410 align:middle line:90% right? 00:02:22.410 --> 00:02:24.120 align:middle line:84% You've got to do better than this, OK? 00:02:24.120 --> 00:02:27.030 align:middle line:84% Are they 100 milliliters in size? 00:02:27.030 --> 00:02:29.340 align:middle line:90% Are they 10 milliliters in size? 00:02:29.340 --> 00:02:30.940 align:middle line:90% Are they somewhere in between? 00:02:30.940 --> 00:02:31.440 align:middle line:90% Yes. 00:02:31.440 --> 00:02:33.690 align:middle line:84% We put a bracket on it when we estimate in science. 00:02:33.690 --> 00:02:34.650 align:middle line:90% We put a bracket on it. 00:02:34.650 --> 00:02:35.490 align:middle line:90% All right. 00:02:35.490 --> 00:02:37.830 align:middle line:84% And they have, yes, colored liquids. 00:02:37.830 --> 00:02:40.900 align:middle line:90% How do you know they're liquids? 00:02:40.900 --> 00:02:41.770 align:middle line:90% They could be gels. 00:02:41.770 --> 00:02:42.610 align:middle line:90% How do we find out? 00:02:42.610 --> 00:02:46.770 align:middle line:90% 00:02:46.770 --> 00:02:48.020 align:middle line:90% We shake them up a little bit. 00:02:48.020 --> 00:02:50.750 align:middle line:84% Because we know from experience, it's a keyword, 00:02:50.750 --> 00:02:54.560 align:middle line:84% we learn things-- a brain learns things, so we use them, right? 00:02:54.560 --> 00:02:56.600 align:middle line:90% So they're liquids. 00:02:56.600 --> 00:02:59.240 align:middle line:84% So I'm going to take chunks of dry ice 00:02:59.240 --> 00:03:02.630 align:middle line:84% and put them in the cylinders in a very special way. 00:03:02.630 --> 00:03:08.916 align:middle line:84% And when I get done, you tell me what the special way is. 00:03:08.916 --> 00:03:11.840 align:middle line:90% AUDIENCE: It's blowing out. 00:03:11.840 --> 00:03:15.170 align:middle line:84% BASSAM SHAKHASHIRI: What's blowing out? 00:03:15.170 --> 00:03:16.982 align:middle line:90% You see any bubbles? 00:03:16.982 --> 00:03:20.571 align:middle line:90% What kind of bubbles are those? 00:03:20.571 --> 00:03:22.455 align:middle line:90% AUDIENCE: Blue. 00:03:22.455 --> 00:03:23.868 align:middle line:90% Green. 00:03:23.868 --> 00:03:25.889 align:middle line:90% Green. 00:03:25.889 --> 00:03:26.930 align:middle line:90% BASSAM SHAKHASHIRI: What? 00:03:26.930 --> 00:03:28.370 align:middle line:84% You want to know what's in there? 00:03:28.370 --> 00:03:29.744 align:middle line:84% You already know what's in there. 00:03:29.744 --> 00:03:31.910 align:middle line:84% Dry ice, I put the dry ice in there. 00:03:31.910 --> 00:03:33.780 align:middle line:90% What did I put the dry ice into? 00:03:33.780 --> 00:03:36.500 align:middle line:84% Into two cylinders that have colored liquids in them, right? 00:03:36.500 --> 00:03:38.510 align:middle line:84% And I put the dry ice in every other cylinder. 00:03:38.510 --> 00:03:40.460 align:middle line:84% I didn't put it in every cylinder, right? 00:03:40.460 --> 00:03:43.600 align:middle line:84% Every other cylinder, leaving one for comparison purposes. 00:03:43.600 --> 00:03:47.540 align:middle line:84% So these are dyes that change color when the pH of the liquid 00:03:47.540 --> 00:03:48.050 align:middle line:90% changes. 00:03:48.050 --> 00:03:48.750 align:middle line:90% [WHOOSH] 00:03:48.750 --> 00:03:49.010 align:middle line:90% JESSICA HARROP: All right. 00:03:49.010 --> 00:03:50.070 align:middle line:90% So let's break down. 00:03:50.070 --> 00:03:52.560 align:middle line:84% What's happening in these cylinders. 00:03:52.560 --> 00:03:53.500 align:middle line:90% There's six of them. 00:03:53.500 --> 00:03:57.720 align:middle line:90% 00:03:57.720 --> 00:03:59.490 align:middle line:84% And each set is a different color 00:03:59.490 --> 00:04:03.610 align:middle line:84% because they're different acid base indicators inside them. 00:04:03.610 --> 00:04:06.510 align:middle line:90% So this set starts out blue. 00:04:06.510 --> 00:04:09.360 align:middle line:90% This one starts out pink. 00:04:09.360 --> 00:04:13.530 align:middle line:90% And the final one is purple. 00:04:13.530 --> 00:04:17.589 align:middle line:84% And every cylinder contains a few drops of sodium hydroxide, 00:04:17.589 --> 00:04:19.529 align:middle line:90% which is a base. 00:04:19.529 --> 00:04:26.140 align:middle line:84% NaOH, sodium hydroxide, and it's a base. 00:04:26.140 --> 00:04:27.630 align:middle line:90% So let's start with the basics. 00:04:27.630 --> 00:04:29.790 align:middle line:90% What is an acid and a base? 00:04:29.790 --> 00:04:32.160 align:middle line:84% One definition is that acids are compounds 00:04:32.160 --> 00:04:39.260 align:middle line:84% that donate a hydrogen ion or H plus to another compound. 00:04:39.260 --> 00:04:43.260 align:middle line:84% And the compound that accepts that hydrogen ion is a base. 00:04:43.260 --> 00:04:45.780 align:middle line:84% So acids are hydrogen ion donors. 00:04:45.780 --> 00:04:48.840 align:middle line:84% And bases are hydrogen ion acceptors. 00:04:48.840 --> 00:04:50.160 align:middle line:90% Here's some examples. 00:04:50.160 --> 00:04:52.080 align:middle line:90% Sodium hydroxide is a base. 00:04:52.080 --> 00:04:53.910 align:middle line:90% It's used to make soap. 00:04:53.910 --> 00:05:00.020 align:middle line:84% And acetic acid, which looks like this, 00:05:00.020 --> 00:05:02.370 align:middle line:90% it makes vinegar sour. 00:05:02.370 --> 00:05:04.970 align:middle line:84% All right, so let's look back at our cylinders here. 00:05:04.970 --> 00:05:07.830 align:middle line:84% This first set of cylinders contains the indicator 00:05:07.830 --> 00:05:09.560 align:middle line:90% bromothymol blue. 00:05:09.560 --> 00:05:12.180 align:middle line:84% Let's take a look at the spectrum. 00:05:12.180 --> 00:05:14.960 align:middle line:84% All right, so let's take a look at bromothymol blue, 00:05:14.960 --> 00:05:17.870 align:middle line:84% the indicator in the first set of cylinders. 00:05:17.870 --> 00:05:19.880 align:middle line:84% As you can see here, there's a scale of numbers. 00:05:19.880 --> 00:05:21.620 align:middle line:90% This is a pH scale. 00:05:21.620 --> 00:05:23.840 align:middle line:84% The higher the number, the more basic the solution. 00:05:23.840 --> 00:05:25.910 align:middle line:84% The lower the number, the more acidic. 00:05:25.910 --> 00:05:27.920 align:middle line:84% So our solution started off basic. 00:05:27.920 --> 00:05:28.550 align:middle line:90% It was blue. 00:05:28.550 --> 00:05:31.250 align:middle line:90% It had sodium hydroxide in it. 00:05:31.250 --> 00:05:34.100 align:middle line:84% And this is what the chemical structure of the molecule 00:05:34.100 --> 00:05:36.740 align:middle line:90% looks like when it's blue. 00:05:36.740 --> 00:05:38.270 align:middle line:84% The chemical bonds in the molecule 00:05:38.270 --> 00:05:40.580 align:middle line:84% are absorbing colors of the visible spectrum that 00:05:40.580 --> 00:05:41.940 align:middle line:90% are not blue. 00:05:41.940 --> 00:05:46.270 align:middle line:84% So blue is reflected back to our eyes. 00:05:46.270 --> 00:05:49.630 align:middle line:84% Now, if our compound picks up a hydrogen ion which 00:05:49.630 --> 00:05:54.130 align:middle line:84% it would if you added an acid to it, the molecule changes color. 00:05:54.130 --> 00:05:57.460 align:middle line:84% Now it absorbs blue and reflects yellow. 00:05:57.460 --> 00:06:00.070 align:middle line:84% Since the colors depend on the structure of the molecule, 00:06:00.070 --> 00:06:02.110 align:middle line:84% a chemist could reprogram the molecule 00:06:02.110 --> 00:06:04.420 align:middle line:84% to switch its color at other pHs, 00:06:04.420 --> 00:06:07.000 align:middle line:90% other numbers on this scale. 00:06:07.000 --> 00:06:09.190 align:middle line:84% That's useful depending on the kind of reactions 00:06:09.190 --> 00:06:10.890 align:middle line:90% she may want to study. 00:06:10.890 --> 00:06:13.570 align:middle line:84% Now, nature experiments with color all the time. 00:06:13.570 --> 00:06:16.300 align:middle line:84% Animals use color to avoid predators, attract mates, 00:06:16.300 --> 00:06:17.890 align:middle line:90% scare off enemies, etc. 00:06:17.890 --> 00:06:18.502 align:middle line:90% [WHOOSH] 00:06:18.502 --> 00:06:19.960 align:middle line:84% All right, so now let's take a look 00:06:19.960 --> 00:06:22.270 align:middle line:84% at what's happening in the second set of cylinders 00:06:22.270 --> 00:06:23.680 align:middle line:90% that started off pink. 00:06:23.680 --> 00:06:24.420 align:middle line:90% [WHOOSH] 00:06:24.420 --> 00:06:27.490 align:middle line:84% The indicator in there is phenolphthalein. 00:06:27.490 --> 00:06:29.830 align:middle line:84% The same thing is happening with our bromothymol blue, 00:06:29.830 --> 00:06:33.700 align:middle line:84% but this time when the solution is basic, it's pink. 00:06:33.700 --> 00:06:36.235 align:middle line:84% And when it's acidic, it's colorless. 00:06:36.235 --> 00:06:37.450 align:middle line:90% [WHOOSH] 00:06:37.450 --> 00:06:40.030 align:middle line:84% But why does adding dry ice change the solution 00:06:40.030 --> 00:06:42.100 align:middle line:90% from basic to acidic? 00:06:42.100 --> 00:06:44.830 align:middle line:84% Well, dry ice is solid carbon dioxide. 00:06:44.830 --> 00:06:46.570 align:middle line:84% When you add it to water, it bubbles. 00:06:46.570 --> 00:06:49.540 align:middle line:84% And part of every bubble of carbon dioxide dissolves. 00:06:49.540 --> 00:06:51.854 align:middle line:84% When it does, it forms carbonic acid. 00:06:51.854 --> 00:06:53.020 align:middle line:90% Let's look at that equation. 00:06:53.020 --> 00:06:57.620 align:middle line:90% 00:06:57.620 --> 00:07:00.480 align:middle line:84% So there's more and more carbonic acid in the water. 00:07:00.480 --> 00:07:03.230 align:middle line:84% And this acid reacts with the sodium hydroxide 00:07:03.230 --> 00:07:04.550 align:middle line:90% until there's none left. 00:07:04.550 --> 00:07:06.050 align:middle line:90% So the solution becomes acidic. 00:07:06.050 --> 00:07:13.010 align:middle line:90% 00:07:13.010 --> 00:07:16.820 align:middle line:84% So we saw our bromothymol blue turning from blue to yellow 00:07:16.820 --> 00:07:19.960 align:middle line:84% as the solution became more acidic. 00:07:19.960 --> 00:07:23.840 align:middle line:84% And we saw our phenolphthalein turn from pink to colorless 00:07:23.840 --> 00:07:25.352 align:middle line:90% as it became more acidic. 00:07:25.352 --> 00:07:26.810 align:middle line:84% Now, let's watch again what happens 00:07:26.810 --> 00:07:28.730 align:middle line:90% in the final set of cylinders. 00:07:28.730 --> 00:07:29.230 align:middle line:90% [WHOOSH] 00:07:29.230 --> 00:07:30.480 align:middle line:90% BASSAM SHAKHASHIRI: All right. 00:07:30.480 --> 00:07:35.000 align:middle line:84% So now I ask you to focus your attention on this cylinder. 00:07:35.000 --> 00:07:37.510 align:middle line:84% Actually, you can focus your attention 00:07:37.510 --> 00:07:39.660 align:middle line:90% on anything you want to, OK? 00:07:39.660 --> 00:07:41.810 align:middle line:84% You can even not pay attention if you want to. 00:07:41.810 --> 00:07:44.040 align:middle line:90% We live in a free country. 00:07:44.040 --> 00:07:46.560 align:middle line:84% But if you want to follow the experiment with me, 00:07:46.560 --> 00:07:48.630 align:middle line:84% I want you to focus your attention on this one 00:07:48.630 --> 00:07:50.910 align:middle line:84% and tell me, count them out, how many different color 00:07:50.910 --> 00:07:54.516 align:middle line:84% changes you see as I drop the dry ice in there. 00:07:54.516 --> 00:08:01.810 align:middle line:90% 00:08:01.810 --> 00:08:04.160 align:middle line:84% I'm listening to you on the radio. 00:08:04.160 --> 00:08:05.290 align:middle line:90% One. 00:08:05.290 --> 00:08:06.032 align:middle line:90% Wow, I heard wow. 00:08:06.032 --> 00:08:07.240 align:middle line:90% What kind of a count is that? 00:08:07.240 --> 00:08:10.610 align:middle line:90% 00:08:10.610 --> 00:08:12.260 align:middle line:90% Three so far. 00:08:12.260 --> 00:08:13.940 align:middle line:90% Three different color changes. 00:08:13.940 --> 00:08:17.340 align:middle line:84% But you know, I'm listening to you on the radio. 00:08:17.340 --> 00:08:19.340 align:middle line:84% You want me to appreciate what you're seeing, 00:08:19.340 --> 00:08:21.870 align:middle line:84% so what were the color changes that you saw? 00:08:21.870 --> 00:08:23.570 align:middle line:84% Why couldn't you say those, right? 00:08:23.570 --> 00:08:26.690 align:middle line:84% You see how we have to help our brain make 00:08:26.690 --> 00:08:29.600 align:middle line:84% the right observations and make the right reporting. 00:08:29.600 --> 00:08:30.362 align:middle line:90% [WHOOSH] 00:08:30.362 --> 00:08:32.570 align:middle line:84% JESSICA HARROP: OK, so in the final set of cylinders, 00:08:32.570 --> 00:08:35.990 align:middle line:84% Dr. Shakhashiri has a mixture of indicators. 00:08:35.990 --> 00:08:37.520 align:middle line:84% This created the color changes that 00:08:37.520 --> 00:08:41.059 align:middle line:84% occurred as the solution became more and more acidic. 00:08:41.059 --> 00:08:45.190 align:middle line:84% There were three changes that we saw. 00:08:45.190 --> 00:08:52.165 align:middle line:84% It went from purple to blue to green to yellow. 00:08:52.165 --> 00:08:52.790 align:middle line:90% [MUSIC PLAYING] 00:08:52.790 --> 00:08:54.206 align:middle line:84% All right, that's it for me today. 00:08:54.206 --> 00:08:56.200 align:middle line:90% I'll see you next time. 00:08:56.200 --> 00:09:26.101 align:middle line:90%