flammable substances
flammable
combustible
hot enough to burn it
Natural fibers such as cotton, linen, and silk burn more readily than other materials (silk being the most flammable). Synthetic materials like nylon, acrylic, and polyester do not ignite as readily as natural fibers but when they do ignite they melt causing severe burns. Natural and synthetic blends could be the most dangerous with the combination of high rate of burning and melting making it more hazardous than either fabric alone.
If there is enough oxygen, the hydrogen will ignite and burn rapidly, possible leading to an explosion.
Pure gold is a noble metal and does not combine readily with anything. It is inert in an oxygen atmosphere and will not burn.
flammable
Flammable substances are those gases, liquids and solids that will ignite and continue to burn in air if exposed to a source of ignition.
It depends on the substance. Yes, all flammable substances have a "flash-point". Every substance has a different flash point expressed in degrees Celsius or Fahrenheit. If the temperature is below the flash point you will not be able to ignite it. And the higher the temp above the flash-point the more readily it will burn.
The simplest, but not definitive, test is to burn a small amount. Wool will not ignite readily, but when it does burn smells like burning hair and what little ash it creates is "soft". Man-made fibres will generally ignite quickly, have an acrid smell and leave a hard or beaded ash.
If you're talking about burning, then no. Powdered aluminum has an extremely high surface area, and oxidizes very quickly. An aluminum can does not nearly the same amount of surface area, and oxidizes much more slowly, so it won't burn readily.
combustible
metal does not burn very well,
Acetone easily burn.
hot enough to burn it
There are many variables which affect the ease of combustion. The degree of mixing with the air is a very large factor. Powdered coal dust in air will ignite explosively, while lumps of coal are much harder to ignite. Different compounds have different activation energies for combustion and there require different temperatures to ignite. Sometimes combustible materials are mixed with non-combustible materials which also makes them harder to burn; for example, dry paper burns quite well, but wet paper is very difficult to burn.
Yes, wool is flammable, but is one of the most fire resistant textiles. It has a high ignition temperature, and once it finally reaches the temperature to burn, it burns slowly and is easily extinguishable.
Substances that do not burn on heating are classified into a group called non-combustible substance. Eg: Sand, Water, Iron nails, Glass, Stone, Asbestos, Cement.