![]() When a flammable substance comes in contact with a supporter of combustion, under certain conditions burning takes place. The necessary conditions are:-
This is a chemical reaction. as the bonds between the molecules breakdown resulting in new molecular bonds. Observing a flame from an oil lamp or a candle is a very good way to start the study of combustion. If you can light a candle you can compare what you see with the diagram given here. A candle is made from a wax which can easily melt and evaporate. Wax is a fairly simple compound consisting of the two elements Carbon and Hydrogen. To start the process wax molecules must be split up into its elements. This requires energy; which is called ‘activation energy’. The supporter of combustion, which is Oxygen, comes from the air.
These observations make it clear that the shape of the flame is determined by the wind. So the upward directed flame should also be due to an upward wind. In the absence of wind the flame will not waver and the rising air currents will cool the edge and form a wall to store the liquid wax. If the wind blows this wall will melt and the wax will drip.
Take a square shaped paper and cut along the diagonals leaving off 1 cm from the centre.These lines are shown as dotted lines. Paste the edges at the centre and attach a piece of thread. Fig.3 Folding the edges to make a fan. The thread is in red. Hang the fan just above the flame and observe what happens. Fig.4 The fan will rotate due to the rising current. Activity 3:- Tilt the candle and observe what happens
The liquid wax will spill yet the flame will remain vertical. This shows that the shape of the flame is determined by the air currents around the flame. The arrows in the diagram indicate the rising air due to heat. These are called convection currents. Activity 4:- An Experiment :- To obtain a burning gas from a flame. Find a glass or a metal tube with one narrow end. Insert the broad end into the flame and try to ignite any gas that may come out. You will be able to find out in which areas the unburnt gas is available. Fig. 6. A flame from a flame. Activity 5:- Touch the end of the glowing wick with an iron nail. Fig.7 Why does the end of the wick glow? A white smoke will be produced. What's this smoke? Fig.8 The flame gets sucked in to the extinguished candle. Chemistry of the candle flame. Fig.9. Hold a metal spoon or an egg shell inside the flame. Considering the change as a complete combustion, and taking the simplest hydrocarbon, we may write the equation as follows:-
FORUM Nelly - Burn a candle; matter changes to energy . Ali- No ! Never. Nelly- Why not ? The mass of the used candle is less than the original. Ali- True, but the loss is not due to a conversion of matter to energy. Nelly- Then How’d you get heat and light. Ali- This is what you call an exothermic (heat producing) chemical reaction. The energy that comes, is from a kind of potential (stored up) energy called chemical energy, Nelly- Then how does burning destroy matter. Ali- Up to the time of Antoine Lavoisier, 1743 -1794, (Fr) everybody thought like you. Lavoisier showed that burning does not destroy matter. What happens is changing wax, which is a solid to an invisible gases. Lavoisier burnt magnesium and phosphorus and showed that they actually gained weight. That is due to the addition of oxygen . By that time Joseph Prriestly had obtained oxygen and named it de-pholgisticated air. Nelly- Oh yes I remember. “Matter can neither be destroyed nor created” Ali- Exactly ! This law, “The law of conservation of mass” was established by Lavoisier who was guillotined by the French people. This law does not hold for nuclear reactions.
The Law of conservation of mass During a chemical reaction,the total mass of the products is equal to the Total Mass of the Reactants. |
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