The correct option is (a).
Explanation
Combustion is a chemical process in which a substance reacts rapidly with oxygen to give off heat. Spontaneous combustion is a specific type of combustion where a material bursts into flames without the application of any apparent external cause, such as a spark or flame, typically due to internal heat accumulation raising the temperature to the substance's ignition point.
Statement 1 is Correct:
Spontaneous combustion is defined by the occurrence of fire without an external ignition source. It happens when a substance undergoes slow oxidation or other exothermic reactions that generate heat faster than it can be dissipated. Once the accumulated heat raises the substance's temperature to its ignition temperature, it bursts into flames.
Statement 2 is Incorrect:
Iron metal does not undergo spontaneous combustion at room temperature. While iron filings burn vigorously when sprinkled into a flame, solid iron requires significant heating to react with oxygen. Substances known for spontaneous combustion at room temperature include white phosphorus, which has a very low ignition temperature (approximately 35°C).
Statement 3 is Incorrect:
Sand (primarily composed of silicon dioxide, SiO2) is a non-combustible material. It does not burn or undergo spontaneous combustion. In fact, sand is commonly used as a fire-extinguishing agent because it cuts off the supply of oxygen to the fire.
Key Takeaway:
Spontaneous combustion occurs when a material heats up internally to its ignition temperature without external heating. Common examples include white phosphorus and coal dust in mines, whereas metals like iron and inert substances like sand do not exhibit this property at room temperature.