**Why does plastic or paper burn off near a flame, but wood or glass do not?**

**Introduction:**
When exposed to a flame, different materials react differently. While plastic and paper burn off, wood and glass do not. This variation in behavior is due to the chemical composition and physical properties of these materials.

**1. Combustibility of Plastic and Paper:**
Plastic and paper are organic materials, primarily composed of carbon-based compounds. These compounds are known as hydrocarbons, which contain carbon and hydrogen atoms. When exposed to heat, the hydrocarbons in plastic and paper undergo combustion, a chemical reaction that produces heat, light, and various combustion byproducts such as smoke and ash. This combustion process is often referred to as burning.

**2. Ignition Temperature:**
Every material has an ignition temperature, which is the minimum temperature required to initiate combustion. Plastic and paper have relatively low ignition temperatures, making them susceptible to catching fire when exposed to a flame. When the heat from the flame raises the temperature of plastic or paper above their ignition temperature, the hydrocarbons start to break down, releasing volatile gases that can sustain the flame.

**3. Composition of Wood and Glass:**
Wood is primarily composed of cellulose, lignin, and other organic compounds. Cellulose, a polysaccharide, is the main structural component of wood. Lignin provides rigidity and additional strength to the wood. These organic compounds have a higher ignition temperature compared to plastics and papers.

On the other hand, glass is an inorganic material composed mainly of silica (silicon dioxide). Silica has a very high melting point, making it resistant to heat and flame. Glass does not undergo combustion because it lacks the organic compounds present in plastic, paper, or wood that can break down and release flammable gases.

**4. Heat Insulation and Conductivity:**
Wood is an excellent insulator and has low thermal conductivity. This means that it does not readily conduct heat and can act as a barrier, preventing the heat from reaching the ignition temperature. Additionally, the structure of wood contains moisture, which can absorb heat and hinder the combustion process.

Glass, being a poor conductor of heat, does not readily absorb the heat from the flame. Its high melting point further prevents the glass from reaching its ignition temperature and undergoing combustion.

**Conclusion:**
In conclusion, the varying behavior of different materials when exposed to a flame is a result of their chemical composition, ignition temperature, and physical properties. Plastic and paper, being organic compounds with low ignition temperatures, readily burn off when exposed to a flame. On the other hand, wood and glass, which have higher ignition temperatures and different chemical compositions, do not undergo combustion and remain unaffected by the flame.