Final answer:
Iron filings mixed with either air, salt, or water can be separated by magnetism, as iron is ferromagnetic and responds to a magnetic field, allowing for easy separation from non-magnetic substances.
Step-by-step explanation:
The question asks about two pure substances that can be mixed and then separated using magnetism. From the given options, D. iron filings and any of the other substances (air, salt, or water) can form a mixture where the iron filings can later be magnetically separated due to iron's magnetic properties.
Here's how it would work with each substance: If mixed with air, the iron filings would simply rest amidst the air and can be picked up by a magnet. When mixed with salt, a magnet would attract the iron filings, leaving the salt behind. In a water and iron filings mixture, the iron could be drawn out of the water by a magnet. This separation is possible because iron filings have magnetic properties, thereby allowing them to be attracted to a magnet.
To address your second question, the property that allows for this separation is magnetism. This is because iron is a ferromagnetic material, which means it is attracted to magnets and can be magnetized itself. This allows the separation of iron filings from other non-magnetic substances in a mixture.