Will the result of a LINQ query always be guaranteed to be in the correct order?

will be helpful for those in need , You're looking for .ThenBy():

.OrderBy(o => o.OrderDate).ThenBy(o => o.OrderID).

will help you Both sources are sort of right: on one hand, due to the implementation details of the Interface Builder, the order in which you add items to IBOutletCollection will be maintained on retrieval; on the other hand, you should avoid making use of that ordering in your code, because there is no way to check this order.
Imagine taking over someone else's project. If you see a loop over an IBOutletCollection, observe that the order of iteration matters, and decide to check what that order is or force the new order, you would have to remove and re-add the controls to your outlet collection. That is why you should treat your IBOutletCollection elements as unordered. If it is necessary to maintain a specific order, copy the elements into an NSArray, sort them on some known property, and then iterate the copied collection.

fixed the issue. Will look into that further LINQ to Entities is trying to create a SQL query out of your condition, but is not able to do it for the expression you specified.
One way to "fix" the problem would be to do the filtering in code instead of in SQL, but this will impact performance, because all of the records will be retrieved to the client and filtered there. This is how you could do it (notice the added ToList()):

var chemicals = db.ChemicalItem.ToList().Where(c => c.CategoryNumber.ToCharArray().Intersect(catekey.ToCharArray()).Count()>0);