Explanation:
Sand and iron fillings have the same size of particles.
It would be difficult to use a mesh to separate them. Also, sometimes , the iron filing clings well to the surface of the sand particles.
The best way to separate them is to use a bar magnet. The magnetic properties of the iron fillings will get it attracted to the bar magnet leaving the sand particles behind.
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Answer: the following are essential factors to be considered when choosing a solvent for crystallization
1. The crystallization solvent should not dissolve the substance to be purified at room temperature, but it should dissolve it well at the solvent’s boiling point
2. The solvent should dissolve soluble impurities well at room temperature.
3. The solvent should not dissolve insoluble impurities even at the solvent’s boiling
point.
4. The solvent must not react with the substance to be purified .
From the ideal gas equation,

where n is number of moles, R is Universal gas constant, P is pressure, V is volume, and T is temperature of the gas.
The pressure and volume are inversely proportional to each other at constant temperature and number of moles.
Hence, on decreasing the pressure, the volume will increase.
As the hiker reaches a height of a mountain, the pressure would decrease which results in the reestablishment of equilibrium between gas molecules thus resulting in pushing of bag outwards.
Hence, the bag will expand as the hiker reaches the top of the mountain.
Answer: Convection Currents
Explanation: The mantle rock is solid and denser because it is cooler. This means it will sink closer to lower mantle as warmer, lense dense rock/magma moves upwards.