Heated sawdust catches fire when a drop of concentrated nitric acid is added to it. Chemically, which type of process is mainly responsible for this combustion?

Introduction / Context:
Sawdust is largely made of organic material such as cellulose, which contains carbon and hydrogen. Concentrated nitric acid is a strong oxidising agent. When the two come into contact, especially with some heating, a vigorous chemical reaction can occur that releases heat and may cause ignition. Understanding this reaction involves recognising the role of oxidation and the ability of nitric acid to act as an oxidiser in organic reactions.

Given Data / Assumptions:

• Sawdust is heated and a drop of concentrated nitric acid is added.
• The sawdust catches fire after the acid is applied.
• Concentrated nitric acid is known to be a strong oxidising agent.
• The question asks which type of process (dehydration, oxidation, reduction or dehydrogenation) is mainly responsible for ignition.

Concept / Approach:
Oxidation, in a broad sense, involves the gain of oxygen, loss of hydrogen or loss of electrons by a substance. Organic materials such as sawdust can be oxidised vigorously by strong oxidising agents, releasing heat and sometimes leading to combustion. Concentrated nitric acid can oxidise many organic compounds rapidly, sometimes explosively. While dehydration and dehydrogenation can occur in some reactions, the key process that leads to catching fire is rapid oxidation of the organic matter, which releases enough heat to ignite it.

Step-by-Step Solution:
Step 1: Recognise that sawdust consists mainly of cellulose, an organic compound rich in carbon and hydrogen.Step 2: Recall that concentrated nitric acid is a strong oxidising agent capable of oxidising organic materials.Step 3: When concentrated nitric acid comes into contact with heated sawdust, it promotes rapid oxidation of the cellulose.Step 4: This oxidation reaction is highly exothermic, meaning it releases a large amount of heat.Step 5: The heat generated can raise the temperature of the sawdust to its ignition point, causing it to catch fire.Step 6: Therefore, the dominant process responsible for the combustion is oxidation.

Verification / Alternative check:
Strong oxidising acids such as nitric acid and concentrated sulphuric acid are known hazards when mixed with organic materials like paper, sugar or sawdust. Safety manuals warn that such combinations can cause charring, ignition or even explosions due to rapid oxidation. If dehydration alone were the main effect, you might see drying or charring without open flames. The occurrence of actual fire indicates that substantial oxidation and combustion are taking place, not just loss of water or hydrogen in a controlled way.

Why Other Options Are Wrong:
Option a, dehydration, refers to the removal of water from a substance. Although concentrated acids can dehydrate some organics, dehydration by itself usually leads to charring rather than vigorous ignition in this context. Option c, reduction, would require the sawdust to gain electrons, which is the opposite of what happens when it is oxidised. Option d, dehydrogenation, is a specific type of oxidation involving loss of hydrogen, but the broader process here is overall oxidation and combustion, not just selective dehydrogenation. Hence these options do not capture the main cause of the fire.

Common Pitfalls:
Students sometimes focus on the strong acidic nature of nitric acid and conclude that dehydration must be the key process, confusing it with concentrated sulphuric acid, which is more famously a dehydrating agent. Another pitfall is to think of any reaction with acid as "acid base" rather than redox. To avoid these mistakes, remember that nitric acid is a powerful oxidising agent and that burning is a form of rapid oxidation. When organic matter catches fire after treatment with concentrated nitric acid, oxidation is the central process.

Final Answer:
Heated sawdust catches fire when concentrated nitric acid is added mainly due to rapid oxidation of the organic material by the strong oxidising agent.