Candle-light dinner

D. Leela and class-mates

"Come in", shouted Leela, when she heard the knock at the door. Nivedita came in, looking scared. "Why, what happened, Nivu?" asked Leela.
"The power switched off just as I was leaving the lift!" she exclaimed. "If I had come even a second later, I would have been stuck in the lift. And I'm afraid of the dark," she confessed.
"I like it when there's a power-cut," said Maya. "I like to eat and play by candle-light." 
"Oh, no, it's so dark and gloomy," said Nivu. "I agree. I like to have all the lights on in the room," said Leela. "In fact, I like to have all the lights on in all the rooms of the house!" 
"Have you ever heard of global warming?" murmured Nivu. "It's not global warming I have to worry about; it's Mother!" shot back Leela. "She would never allow it!" 
By this time, Maya had found a candle and match-box. She lighted the candle and looked around for a surface to put it on. "Careful, Maya," said Nivu. "You'll hurt yourself." She picked up a plate and set it on the table. Maya carefully stuck the candle on the plate, with some melted wax. Then she sat on a chair and looked deep into the candle-flame. "I like candle-light" she repeated.
"Just imagine, years ago, when there was no electricity. People must have huddled around candles just as we are doing now," said Maya. Leela looked unhappy at the thought. "I am glad we have electricity and running water." 
"Ah, water!" said Maya. She got a glass of water and carefully poured it into the plate with the candle. "What are you doing, Maya?" asked Nivu. Maya looked at the candle in the water. "I am in a boat," she said, "and it's night and we have lighted the candle. There's water all around us..." 
"You funny thing!" exclaimed Nivedita, "Why, you seem lost in another world. Perhaps I can also close my eyes and believe that we are on a boat!" 
Leela said, "That sounds fun! Better than drawing in the dark, which we were trying to do." 
Maya said, "Mother showed me a nice trick with candle and water the other day. Do you want to see it?" "Sure," said Nivu. "Can I do it too?" 
"It's actually a puzzle," said Maya, "so you can certainly try it. Here's the question." Saying which she ran away and came back, with a coin clutched in her hand. "I went to ask Mother for a coin," she said. She put the coin into the water on the plate.
"For the puzzle to work you have to have just the right amount of water and I am not sure this is right. Let me ask the puzzle any way: how can you take the coin out of the water with your bare hands, and still not get your fingers wet?" 
Nivu thought for a moment. "Is it a trick?" "Of course it's a trick! All of them are," said Maya. "But it's not cheating, just a nice trick." Nivu looked at Leela. "Don't ask me" said Leela. Mother showed me this two years ago. I didn't know she had shown it to Maya too." 
Maya said, "It was when I was at home with cold and fever. I was bored so she showed me a whole lot of things. She showed me another trick with a candle flame, but I promised not to do it without an adult." 
"I know that one!" shouted Leela. "Hey, hold it!" said Nivedita. "I'm still on the first puzzle and I can't think of any idea. Don't go to the next one yet!" 
"Actually," said Maya, "it may not be easy to figure out. I think I'll show you the trick, and then Leela can do the other one." 
"That's a great idea," said Nivu. Maya went and got a glass tumbler. She held it upside-down over the candle flame. Nivu shouted out in alarm, "Stop! You'll put out the candle and we'll be in the dark!" 
Maya looked sheepish. "Sorry, I forgot you don't like the dark. Mother showed it to me in broad daylight, so it didn't occur to me that I need to put out the candle!" 
She brought another candle and lit it, setting it on the table. She again picked up the glass and this time, she inverted it over the flame and gently set it on the water-filled plate, covering the candle completely (See the figure).
As the children watched, the candle flame guttered and slowly went out. As it went out, the water around the glass started entering the glass rapidly. "Look, the water is bubbling into the glass!" shouted Nivu.
"Isn't it nice?" asked Maya, proudly, as if she had arranged for the phenomenon as a personal gift to Nivu. There was about 3-4 cm of water in the glass. The plate was nearly dry. Maya reached out and picked up the coin. "See," she said, "the coin was practically out of water because so much water has been sucked into the glass. If you arrange it carefully, starting out with the right amount of water, all the water in the plate will enter the glass. The coin will be dry and you can easily pick it up without getting your fingers wet. That's the trick!" 
"Do you know what makes the water rise?" asked Leela. "Well, I can guess," said Nivu. "When you cover the candle, there is no more air, so the candle goes out." 
"Fine so far," said Leela. "But what makes the water rise? Hm, hm ..., let me think," said Nivu. "Water must be going in because something is pulling it in. But what can that be?" 
Maya burst out, "Let me tell you. The air around a candle flame is hot, right? So when the candle goes out, the air starts to cool." 
"Yes, I see," said Nivu. "When you put the glass, there was hot air in the glass. When the candle goes out, the air starts to cool. And I know that hot air expands, which means that as the air cools, it contracts. Now what?" asked Nivu.
Leela continued, "I learned this part in school. When air is enclosed inside something and it cools, there is a partial vacuum inside. That's because the hot air filled the space, and the contracted air doesn't quite fill it." 
"Yes, you mean the pressure inside drops," said Nivu. "Now I've got it. The pressure drops and the outside air pressure is more than the inside pressure, so air tries to come in to equalise the pressure. But the glass is sitting in water, not in air, so it's the water that comes in instead. Very nice indeed." 
"Before you try it again, do you want to see another experiment with a candle flame?" asked Leela. "Sure," said Nivu. "But we must get Mother. I can't do this without an adult." 
"Mother!" called Maya, "can you please come at once?" "What is it," asked Mother. "We want to show Nivedita the candle-flame experiment," said Maya. "May we?" 
"Leela can do it," said Mother. Maya made a face but sat back, ready to watch a treat. "Now," said Mother, "it's dark and so you must be very careful indeed." Just then the lights came back on and every one laughed.
"Oh, no!" said Maya. "What a pity." Leela said, "Don't blow out that candle. I'll use it for my experiment." She took a piece of paper that they had been drawing on when the lights went off. She tore a small square of it. She carefully held it flat in front of the flame, about one hand's length away. She checked that the paper was being held at a height about half-way through the candle-flame.
Then, with a swift movement, she slid the paper forward so that it was held in the central part of the flame (see figure). She and Maya chanted together, "One, two, three!" On the count of three, Leela slid the paper out of the flame, and flapped it once or twice. "Look," she told Nivu, with pardonable pride at her immediate success, "see how the flame has burnt the paper." 
Nivu took the paper from her and looked at it. "Why, it's burnt in a circle!" "Yes, indeed, although I usually take three or four tries before I get it right," answered Leela, honestly. "If you hold the paper in the flame for just the right amount of time (3-4 seconds), and the flame is nice and steady, the paper burns out in a ring." 
"I thought the whole paper would catch fire," confessed Nivu. "It can," said Leela, "that's why you must always have a bowl of water nearby so you can dip the paper into it if it catches fire." She pointed to the water in the plate that had been used for the experiment. "And you must have an adult with you," piped up Maya and Mother smiled.
"But do you know why it burns like that?" asked Leela. "Sure," said Nivedita, "No, don't look so sad, it's just that I learned it in school, the parts of a candle-flame, I mean. So I know that the centre part of the flame is the coldest." 
Maya said, "That's because it doesn't get enough oxy.. oxy...," "Oxygen," said Mother. "Yes," said Maya, "it doesn't get enough oxygen to burn. Because there's so much flame and so there's very little air. And the oxygen in the air is needed to burn. You know," she meditated thoughtfully, "this air is such a funny thing. It's all around us, but if any thing else (like flame or water) comes along, it promptly gives it place!" 
Every one laughed at her, but kindly. "Well," said Nivu. "We learned the parts of the candle-flame in class. But I didn't know you could do such a simple experiment to see it so clearly. I shall go and show all my friends." "Certainly," said Mother. "But be careful, and whatever you do, don't hold the paper above the flame. It will burn instantly, because that's the hottest part of the flame. In fact, if you look carefully, you will see that different parts (the bottom, the part near the wick, the outer edges and the top) of the flame have different colours, so they are at different temperatures." 
"Yes, the outside edge is hot," said Leela. "In the beginning I used to to hesitate and moved the paper slowly into the flame. But then the edge of the paper would catch fire! It took me a while to figure out that you have to do it quickly." 
Nivu tried it a few times and after a few tries when she either burnt the paper or didn't hold it long enough, she got a nice burnt ring. "Yeah!" said Nivu. "Now let me try the first experiment with water." She lit the candle that was sitting in the plate of water and overturned the glass over it. They all watched as water bubbled into the glass. "This is an easy experiment," said Nivu.
Just then Kavita came in. "Look," said Nivu. And she showed the experiment to Kavita. "I know that," said Kavita. "It's in my text-book. The water rises because there's oxygen in the air. In fact, there's 20% oxygen in air which is burnt up by the candle. After that the candle goes out, and so the water that comes in fills about 20% of the glass." 
"But it's not 20%," said Maya. Every one looked. "Yes, it looks a lot less than 20%," said Kavita. "Actually," said Leela, "I've noticed that more water rises with a candle that has been burning for some time than if you have just lit it." 
"That's a nice obervation," said Mother. "Actually, several text books talk about the 20%, but I don't think that explanation is correct. If the air is hotter, it has expanded more and so when it cools, it leaves more of a vacuum so more water enters." 
"But, Aunty, oxygen is definitely needed to burn," said Nivu. "But wait, the water doesn't begin to rise until the candle actually goes out, or, maybe just before the candle goes out. If it had been because of the oxygen, as the oxygen gets used up, the water should have risen continuously." 
"No, it doesn't do that," said Leela. "It sort of rushes in as the candle goes out." "Full marks, both of you," said Mother.
"Let me see," said Kavita. They did the experiment again as she watched. "You are right," said Kavita, "so it definitely can't be the oxygen." Mother said, "Yes, Kavita, oxygen burns into carbon-dioxide, so actually the oxygen in the air is simply replaced by carbon-dioxide as the candle burns. Since carbon-dioxide doesn't burn, the candle goes out, but the amount of air is the same." 
"I know carbon-dioxide," said Maya, "it's the fizz in coke and soda." "Hey!" said Kavita, "the carbon-dioxide has dissolved in the water, that's why the pressure drops inside the glass." Maya sniffed the water to see if it smelt like soda. "Actually," said Mother, "Carbon-dioxide doesn't actually like to dissolve in water. It does so under pressure, which is why coke and soda go "flat" if you leave them open for a while." 
Maya looked disappointed. "So it's just the hot air cooling that causes this effect?" asked Kavita. "I certainly think so," said Mother. "There's been a lot of debate about it. Perhaps you children can come up with a nice way to see how much water enters by changing the amount of heat? Think about it." 
Please think about it too. And if you've made your own experiments and have more results, please write to Jantar Mantar at the address provided and tell us about it.