If any of your friends has had an accident, you'll see the hurt arm covered in plaster. Why do doctors do this? Chemistry tells us why.
How a cast is made
When you have a fracture, the doctor will first clean up the wounds, reset the bone and apply medicine. He'll then wrap some bandages to protect the wound. Now watch the doctor's attendant carefully. He'll take some packets of gypsum and mix them with water. He'll dip bandages in them and wrap them around the fracture.
Does it feel warm? That's because the gypsum in the plaster (which is really calcium sulfate) reacts with water to make an insoluble form. This is a sticky form that settles like cement. Over time, the plaster will become hard and dry, and make a firm case for your fracture. Now your body can take over the healing process.
Here's a video that shows you how to make a cast:
If you've unfortunately had an accident and got your hand in a cast, call your friends home. They can put their autographs on the cast. That is a way for them to show they care for you. When your fracture is healed, you can keep the cast as a token of friendship!
Why do casts work?
The best reason is that they are so hard, that the bone cannot go out of alignment again. Also, they can be made to suit the shape of your arm, so they fit really snug and are not uncomfortable. And they are fairly cheap and quick to make.
There are some problems too. They are not waterproof, so you have some trouble while bathing or washing hands. If water got in, it would erode the cast. Also, they can be quite heavy. And as your wound heals, your arm needs more flexibility. Then you have to break the old cast and make it again.
Nowadays doctors suggest you use a fibreglass cast instead. It's much lighter, waterproof and flexible. But it is still expensive, and you cannot autograph it!