Hi, your site is really helpful. In the sentence, ' chemicals and other materials that/which aren't good for the environment must be properly disposed of to prevent contamination ' is it gramatically correct to use that or which?
It's something I was wondering about, too. Are they EXACTLY the same? I mean, no two words can be exactly the same in my opinion.
It depends. If you're a traditionalist, you're probably going to follow the traditional rules (See mykwyner's post for those rules). But according to descriptive linguistics (i.e., how speakers actually use language), speakers and writers do in fact use 'which' to begin a defining (i.e., restrictive) clause.
For such writers, it's the comma and not the choice of relative pronoun that signals whether the clause is restrictive or non-restrictive.
Originally Posted by Wikipedia
Some writers follow a normative rule that that should be used only in restrictive clauses and which should be used only in non-restrictive clauses. However, many writers do not adhere to this rule, and in particular, which is widely used for both types of clauses
Some people . . . insist that should be used only in restrictive clauses, and which should be used only in nonrestrictive clauses. By this thinking, you should avoid using which in sentences such as I need a book which will tell me all about city gardening, where the restrictive clause which will tell me all about city gardening describes what sort of book is needed. But this use of which with restrictive clauses is very common, even in edited prose. If you fail to follow the rule, you have plenty of company.
I'm not sure why speakers use 'which' with restrictive clauses, but I think it may have something to do with its distribution elsewhere in the grammar.
EX: It is a philosophy in which ordinary people may find solace and which many have found reason to praise.
EX: We want to assign only that book which will be most helpful.
In both examples, 'which' heads a restrictive clause. There isn't a comma or a pause in speech to let the reader/listener know the clause is non-restrictive.
As for which relative pronoun is best on a school assignment, school is a 'traditional' institution, so you could follow "the rules", but if not, be prepared to defend your reasons for using 'which'.
According to its distribution within the grammar, 'which' can indeed function either restrictively (i.e., without a comma) or non-restrictively (i.e., with a comma). But consistency is important, so if the majority of people aren't using 'which' with restrictive clauses, then that'd be a good rule of thumb to follow.