You must have observed how a candle burns when lights go off. We know that to produce fire, we need a fuel and oxygen to support combustion. Here the fuel is the paraffin (candle wax) which gives out light and heat when candle is lit. There is also a wick which is made of some kind of absorbent material. The heat generated when the wick is lit turns the solid wax into liquid wax. As the wick is made of absorbent substance, it absorbs the liquid wax causing the liquid wax to be drawn up the wick. Due to heat, the liquid wax now turns into vapor. This is how the candle burns. Also as wax burns completely there is less candle wax after burning than before.
The flame color varies because of the temperature difference. The hottest part of the flame is blue in color and cooler parts are in orange or yellow color. The blue zone is rich in oxygen compared to cooler zones. Paraffin wax is composed mainly of carbon atoms and hydrogen atoms. The carbon and hydrogen atoms break free in the blue zone. Hydrogen atoms react with oxygen to form water vapor and some of the carbon atoms form carbon dioxide. Carbon atoms in the orange or yellow zone with less oxygen forms soot.
When the candle burns, the vapor reacts with oxygen to give out carbon dioxide, water vapor along with heat and light. The flame of the candle heats up the air around the flame. Heated air being light rises up. To take its place, cold air rushes in. This cold air in turn gets heated up and rises. The incoming air comes from all the directions – from the sides as well the burning material. This activity of less dense air (hot air) rising up and more dense air (cold air) taking its place, happens in a continuous cycle causing the elongated shape of flame.
The flame points upwards on earth due to gravity. Gravity causes the warm air rise upwards to a lower pressure. The same candle flame in the space where there is no gravity (micro gravity region), doesn’t point up but is in the form of a sphere.