Pure Substances�and Mixtures

Grade 7 Science

Separating Mechanical Mixtures

Lesson 5

Mechanical Mixture

• RECALL: A mechanical mixture is a mixture in which you can see the different parts. They are also called heterogeneous mixtures.

PIZZA

CEREAL

LEGO

JELLY BEANS

ALL OF THE ABOVE ARE MECHANICAL MIXTURES

Separating Mechanical Mixtures

• Often, we need to separate mixtures into their component parts.
• There are several different ways that we can extract the separate parts of mechanical mixtures.

1) Sorting

• Sorting is the simplest way to separate mechanical mixtures.
• Sorting involves physically separating the pieces of a mechanical mixture so that similar pieces are together
• e.g. Picking the mushrooms off a piece of pizza, sorting laundry

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2) Floating and Settling

• Works well if the different parts of a mixture have different buoyancy.

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BUOYANCY is how well something FLOATS

Floats – More Buoyant

Settles – Less Buoyant

Floating

• Floating is a separation technique in which a more buoyant component of mixture rises to the top
• Once the more buoyant material has risen to the top, it can be skimmed or poured off
• e.g. The melted fat in chicken broth rises to the top and can be skimmed off with a spoon

Settling

• Settling is a separation technique in which a less buoyant component of a mixture sinks to the bottom
• Once the material has settled, the top substance can be poured off
• e.g. During the gold rush, miners often panned for gold, a technique that relied on the fact that gold particles sink faster than rock particles in liquid

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3) Magnetism

• Works if the solution contains some components that are magnetic and some that are not.
• e.g. If separating a mixture of sand and iron filings, using a magnet allows you to pull the magnetic iron filings out and leave the non-magnetic sand behind

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4) Sieves and Filters

• Works when the mixture contains particles that are different sizes
• A sieve contains visible holes and is used when a�mixture has solid and liquid parts
• e.g. You use a sieve to strain off your�pasta water
• A filter contains smaller holes�that are not visible
• e.g. You use a coffee filter when�you make coffee

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5) Dissolving Soluble Components

• Works when one part of a mixture will dissolve and the other will not
• e.g. If you were separating a mixture of salt and pepper, you could dissolve the salt in water, filter off the pepper and then use evaporation to remove the water from the salt

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