Why Rust Forms on Cars:

Rust is a type of corrosion that occurs when metal comes into contact with water and oxygen. In the case of a car, rust can form on the paint due to a combination of factors, such as:

1. Exposure to the elements: Cars are exposed to the elements all the time, and the paint can be damaged by exposure to rain, snow, salt, and other contaminants. If these substances are not removed in a timely manner, they can cause the paint to corrode and rust to form.

2. Road salt: In locations with cold winter climates, road crews often use salt to melt snow and ice on the roads. This salt can be kicked up onto cars while driving, and can cause rust to form if not washed off in a timely manner.

3. Damage to the paint: Cars can experience small chips, scratches, or other types of damage to the paint. If these areas are not properly repaired, they can become entry points for moisture and other contaminants, leading to rust.

4. Low-quality paint job: Low-quality paint jobs or repainting done by non-professional can also lead to rust, as the paint may not have been properly prepared or applied, leaving the metal exposed to the elements.

5. Leaks: Leaks from the car's undercarriage, the AC, windows or even the sunroof can cause water to accumulate and create a moist environment that can lead to rust formation.

6. Sea-side living: If you live near the sea, the salt in the air can also cause rust formation due to the salt corrosion.

To prevent rust from forming on a car's paint, it's important to keep the car clean and free of contaminants, to fix any damage to the paint as soon as possible, and to have regular paint maintenance such as polishing, waxing and sealing .Additionally, if you live in a location with cold winters or near the sea, it's especially important to wash your car frequently to remove any salt or other contaminants that can cause rust.

How the chemical process of rust formation works:

The chemical process of rust formation is a complex one, but it can be generally broken down into the following steps:

1. Oxidation: The first step in rust formation is the oxidation of the metal. Oxygen in the air reacts with the metal at the surface, creating a thin layer of metal oxide. This process is exothermic, meaning it releases heat.

2. Moisture: The next step is the presence of moisture, either in the form of water or humidity. When the metal oxide layer comes into contact with moisture, it creates an electrolyte solution that can facilitate the rusting process.

3. Electrolysis: With the metal oxide and moisture present, an electrolytic reaction occurs. The oxygen in the metal oxide and the hydrogen in the water combine to create hydroxide ions (OH-), while the metal ions are oxidized to form metal oxide. This process consumes the metal and creates rust (Iron oxide).

4. Iron oxide: Rust is made up of hydrated iron(III) oxide, which usually has the chemical formula Fe2O3·nH2O, where "n" can range from 0 to about 3. The color of rust can vary depending on the size of the rust particles and how they're arranged. The most common color is orange-red, but it can also be brown or black.

5. Spread: As the rust forms, it can spread to nearby metal surfaces through the electrolytic process, leading to further corrosion. This process can be slow or fast depending on the environment, the presence of other chemicals, and the metal properties.

It is worth mentioning that not all metals rust in the same way, for example, aluminum does not rust but it can corrode when exposed to certain types of environments or chemicals. Also, some other metals may rust differently due to the chemical composition.