Separatory funnel facts for kids

A separatory funnel is a special piece of laboratory glassware. It's used to separate different liquids that don't mix together, like oil and water. This process is called liquid-liquid extraction.

Imagine you have a mixture where some parts are dissolved in water and other parts are dissolved in an oily liquid. A separatory funnel helps you separate these two liquids. The liquids usually have different densities, meaning one is heavier than the other. The heavier liquid sinks to the bottom, and the lighter liquid stays on top. You can then drain out the bottom liquid through a special tap.

Two funnels: A - Cone, or Pear-shaped, B - cylindrical.

Pear-shaped separatory funnel

What a Separatory Funnel Looks Like

A separatory funnel usually looks like a cone with a round bottom. It has a stopper at the top to close it and a stopcock (which is like a tap or valve) at the very bottom.

Most funnels in labs are made from strong borosilicate glass, which can handle different chemicals. The tap can be made of glass or a plastic material called PTFE. These funnels come in various sizes, from small ones (about 30 milliliters) to large ones (up to 3 liters). The sloped sides help you clearly see the different layers of liquid.

How to Use a Separatory Funnel

Using a separatory funnel is like a science puzzle! Here's how it works:

• First, make sure the tap at the bottom is closed.
• Pour your mixture of liquids into the top of the funnel.
• Put the stopper firmly on top.
• Now, gently shake the funnel by turning it upside down a few times. Be careful not to shake too hard, or the liquids might form a cloudy mixture called an emulsion.
• After shaking, turn the funnel upside down again and carefully open the tap to release any built-up pressure. This is called 'venting' and is very important for safety!
• Place the funnel in a stand and let it sit quietly. The liquids will separate into clear layers. The heavier liquid will be at the bottom.
• Once the layers are clear, open the stopper at the top. This lets air in.
• Then, slowly open the tap at the bottom. The heavier liquid will drain out into a separate container.
• When the bottom layer is completely out, close the tap.
• Finally, pour the top layer out through the top of the funnel into another container. This keeps the two separated liquids clean.

Separating funnel in use. The organic liquid (yellow, upper layer) is lighter than the water-based liquid (lower layer). The water-based liquid is being drained into the beaker.

Why Liquids Separate: The Science Behind It

The way a separatory funnel works depends on a simple rule: "like dissolves like." This means that liquids that are similar to each other will mix, and liquids that are very different won't. For example, water (which is "polar") mixes well with other polar liquids, but not with oil (which is "non-polar").

When you shake the funnel, the different parts of your mixture move into the liquid they prefer to dissolve in. Since the two main liquids (like water and oil) don't mix, they form separate layers when they settle. The liquid with the lower density (the lighter one) floats on top of the liquid with the higher density (the heavier one).

This way, you can separate different substances from a mixture. For example, if you want to get a specific product out of a liquid, you can use a separatory funnel. Sometimes, after separating, you can evaporate the liquid to get the pure product left behind.

Dealing with Emulsions

Sometimes, when you shake the funnel too hard, the liquids don't separate cleanly. Instead, they form a cloudy mixture called an emulsion. This is like when tiny droplets of one liquid are spread throughout the other, like mayonnaise (which is an emulsion of oil and vinegar).

If an emulsion forms, it can take a long time for the layers to separate. Here are a few tricks to help:

• Gently swirl the funnel.
• Add a small amount of salty water (called "saline solution"). This can help the tiny droplets come together and separate.

Safety First!

The most important thing to remember when using a separatory funnel is to be careful about pressure building up inside. When you shake the funnel, gases can sometimes form or expand, creating pressure.

To prevent this, you must always 'vent' the funnel. This means turning the funnel upside down and carefully opening the tap to let the pressure out. Always point the tip of the funnel away from yourself and others when you do this!

Gallery

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  1. Upper liquid layer
  2. Lower liquid layer
• 

  The lighter liquid (yellow) is on top, and the heavier water-based liquid is at the bottom.

• 

  Separating liquids from fruits and vegetables. The bottom layer is the oily liquid.

See also

In Spanish: Embudo de decantación para niños

• Decantation is a simple way to pour off a top liquid layer from a bottom liquid or solid.
• Dropping funnels look similar and can sometimes be used like separatory funnels.