When an iron object is left exposed to moist air for a long time, a brownish flaky film forms on its surface. In basic chemistry this layer is called what?

Introduction / Context:
Many metal objects made of iron or steel, such as gates, bridges, tools, and railings, slowly develop a brownish or reddish brown surface layer when they are left outside in air and moisture. This common phenomenon is an example of corrosion. The question asks for the correct name of this brownish film on iron, which is a very familiar term in school level chemistry and general science.

Given Data / Assumptions:

• The metal object mentioned in the question is made primarily of iron.
• The object is left in the open, so it is exposed to air and moisture from the environment.
• A brownish or reddish brown film forms on the surface after some time.
• We are asked to recall the standard chemistry term for this corroded surface layer.

Concept / Approach:
When iron reacts slowly with oxygen and water from the atmosphere, it forms hydrated iron oxides. This process is known as rusting, which is a specific type of corrosion that affects iron and steel. The brownish flaky film that appears is what we commonly call rust. It weakens the metal structure by converting the strong metallic iron into brittle oxides and hydroxides that can flake away. Therefore the correct term for the film is rust, not generic dust or tools like shovels and spades.

Step-by-Step Solution:
Step 1: Identify the metal in the question as iron, which is known to rust when exposed to moisture and air. Step 2: Recall that the brownish flaky deposit formed on iron over time is called rust and consists mainly of hydrated iron oxides. Step 3: Compare this with dust, which is simply fine particles from many sources and is not chemically produced by corrosion. Step 4: Recognise that shovel and spade are names of tools, not surface deposits or chemical products. Step 5: Conclude that the correct term for the brownish film on iron is rust.

Verification / Alternative check:
Everyday observations support this answer. Iron railings, old cars, and steel bridges often show brown flakes and patches that are widely called rust. Basic chemistry experiments, such as leaving an iron nail in water or humid air, demonstrate the formation of a similar brown coating. Textbooks define rust as hydrated iron(III) oxide formed when iron slowly reacts with oxygen and water. No scientific source refers to this deposit as dust, shovel, spade, or scale in the sense used here, so rust is the only appropriate choice.

Why Other Options Are Wrong:
- Dust: This refers to loose fine particles that settle on surfaces, not to the chemical corrosion product formed by reaction of iron with air and water.
- Shovel: This is a hand tool used for digging or moving materials and has nothing to do with corrosion layers on metal surfaces.
- Spade: Another type of digging tool, again unrelated to the brown film formed by corrosion of iron.
- Scale: In some contexts scale can refer to hard deposits, for example in boilers, but the specific brown film on rusting iron is known as rust, not scale, in school level chemistry.

Common Pitfalls:
A possible confusion is to think of any brown dirt on metal as dust, especially if the object is outdoors. However dust can be wiped away easily and is not chemically bonded to the metal. Rust, on the other hand, grows from the metal itself and often continues underneath the surface. Another mistake is to overlook that the question is from chemistry, not from everyday tool vocabulary, so tool names like shovel and spade are clearly distractors rather than serious options.

Final Answer:
The brownish film formed on iron when it is left in the open is called Rust.