Mixing materials - Separating materials from Rocks

A geologist has a mixture of crushed rock and iron filings. The total mass is $50\text{ g}$. If the rock mass is $42\text{ g}$, what is the mass of the iron filings, and how can they be separated?

$Mass_{iron} = 50\text{ g} - 42\text{ g} = 8\text{ g}$

To separate the iron filings from the rock, we use a magnet. The magnet will attract the iron but not the rock. This works because iron is a magnetic material.

You have a mixture of small pebbles and fine sand. The pebbles have a diameter of $10\text{ mm}$ and the sand grains are $0.5\text{ mm}$. What tool should you use to separate them?

A sieve with holes sized between $1\text{ mm}$ and $5\text{ mm}$.

Since the sand is smaller than the holes ($0.5\text{ mm} < 1\text{ mm}$), it will fall through. The pebbles are larger than the holes ($10\text{ mm} > 5\text{ mm}$), so they will stay on top of the sieve.

A student finds a rock that contains salt. They crush the rock and add it to a beaker of water. After stirring, they see some solid pieces at the bottom and the water tastes salty. How can they get the pure salt back?

Step 1: Filtering; Step 2: Evaporation.

First, the student should filter the mixture to remove the solid rock pieces. Then, they should heat the salty water (filtrate) so the water evaporates, leaving the solid salt crystals behind.