In old books, browning and yellowing of paper pages are mainly caused by which chemical process acting on the paper over time?

Introduction / Context:
Old books often show brown, brittle pages, especially at the edges. This familiar observation has a chemical explanation related to the composition of paper and its reaction with air. This question asks you to identify the main chemical process responsible for browning of paper in old books.

Given Data / Assumptions:
- Paper is primarily made of cellulose, a natural polymer from plant fibres.
- Old paper may also contain lignin and other impurities from the original pulp.
- Over time, environmental factors such as oxygen, light, and pollutants can affect the paper.

Concept / Approach:
Cellulose and lignin in paper are susceptible to slow oxidation when exposed to oxygen and light. This oxidation breaks chemical bonds and forms coloured compounds, such as quinones and other oxidised products, that give paper a yellow or brown hue. The process is similar to the browning of some foods or wood exposed to air. While dust accumulation, handling, and ventilation may influence appearance and preservation, the core reason for colour change is chemical oxidation of cellulose and related components.

Step-by-Step Solution:
Step 1: Recall that paper is mainly composed of cellulose fibres produced from wood or other plant sources. Step 2: Understand that cellulose is an organic compound that can react slowly with oxygen and environmental oxidants. Step 3: Recognise that oxidation reactions can generate coloured products that absorb light differently from the original material. Step 4: Consider external factors like dust or handling and note that while they may dirty or damage paper, they do not primarily cause the chemical browning. Step 5: Conclude that oxidation of cellulose and related compounds is the major cause of browning and yellowing of old paper.

Verification / Alternative check:
Conservation science studies show that papers containing high lignin content brown more quickly, especially when exposed to light and air. Archival quality papers are manufactured with lower lignin and better control of chemical impurities to reduce oxidation and ageing. Experiments that store paper in inert atmospheres or in low oxygen environments show slower browning, supporting the conclusion that oxidation is the key driver of colour change.

Why Other Options Are Wrong:
- Collection of dust particles on the surface: Dust can make pages dirty or greyish, but it does not account for the characteristic brown colour throughout the paper.
- Frequent use and handling by readers: Handling causes mechanical wear and tear, but the colour change happens even in rarely opened books kept in air.
- Lack of aeration in closed rooms: Reduced air circulation might slightly change ageing conditions, but browning is still driven by chemical reactions with oxygen present in the air.
- Exposure only to moisture without air: Moisture can contribute to deterioration and mould growth, but without oxygen, oxidation would be limited; browning is closely linked to oxidative processes.

Common Pitfalls:
Learners may attribute all changes in old materials to dirt or handling because these are easily observed, underestimating slow chemical reactions like oxidation. Another mistake is to forget that paper is an organic material subject to similar ageing processes as wood. Recognising oxidation as a universal process affecting many organic materials helps in understanding why books, wooden furniture, and many other objects change colour over time.

Final Answer:
Browning of paper in old books is mainly caused by Oxidation of cellulose in the paper over time.