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The hottest part of a candle flame is typically the blue inner cone or region near the base of the flame, just above the wick. This part of the flame is known as the "primary combustion zone" or "inner cone." The temperature in this zone can reach up to around 1400 degrees Celsius (2552 degrees Fahrenheit).

The outer, yellow portion of the flame is cooler than the inner blue cone. The outer region consists of unburned wax vapor and combustion byproducts, and its temperature is lower than that of the inner cone. The overall color and temperature variations in a candle flame are due to different combustion processes occurring in various regions of the flame.

Boiling water in a paper cup without causing it to catch fire is indeed an interesting phenomenon. This can be explained by understanding the specific conditions and processes involved in the experiment. Here's an explanation of the process:

1. Water Absorption:

   • The paper cup is made of cellulose fibers, which are not only good insulators but also have the ability to absorb water. When you pour water into the cup, the paper absorbs some of it. This absorbed water helps regulate the temperature of the paper during heating.

2. Low Thermal Conductivity:

   • Paper has a relatively low thermal conductivity, meaning it doesn't conduct heat very well. When you apply heat to the bottom of the paper cup, the absorbed water helps distribute the heat. The water in the cup absorbs a significant amount of heat before it starts to boil, preventing the paper from reaching its ignition temperature.

3. Boiling Point of Water:

   • The temperature needed to boil water is significantly lower than the temperature required to ignite paper. Water boils at 100 degrees Celsius (212 degrees Fahrenheit), while paper typically ignites at a much higher temperature.

4. Continuous Water Supply:

   • As long as there is water in the cup, the temperature of the paper remains below its ignition point. The process of boiling water consumes heat energy, which further helps keep the temperature of the paper in check.

5. Limited Exposure to Heat:

   • Boiling water is a relatively short-duration process. The time the paper cup is exposed to heat is brief, reducing the chances of the paper reaching its ignition temperature.

While it may seem counterintuitive to heat water in a paper cup, this experiment highlights the heat-absorbing and insulating properties of water and the limitations of thermal conductivity in materials like paper. However, it's important to note that attempting similar experiments with different materials or under different conditions could lead to fire hazards, so caution is always advised.