Materials that dissolve into water. Like sand, soft rock etc. If the material does not dissolve into the water or any type of liquid then that won't need filter paper. You can just use a sieve just...
Yes filter paper blocks sand so it cant get through and therefore stays on top of the filter while the rest of the mixture goes through... Hope this helps
You don't change the filter on a sand filter, you back wash it. Unless you mean how to you change the sand in a sand filter. This is done by removing the multi port control from the top of the unit...