No. They are NOT interchangeable.
One cup of all purpose flour with 2 tablespoons of cornflour added.
It is; a better substitution is all purpose flour with 2 tbl cornflour (cornstarch) added per cup of flour used. Also, some cake flours have leavening agents in - you may need to add these to the all-purpose flour.
yes. They are the same thing. Plain flour is an Australian term where all-purpose is the American.
not same
There is no difference between plain flour and all-purpose flour. They are one and the same. All-purpose (plain) flour does not contain the salt and baking soda that self-rising flour has.
Not in all recipes; for most baking recipes substituting plain flour for cornflour will not work, since cornflour has no gluten (which is what makes dough springy) and it requires far more hydration. Adding some cornflour to the flour in baking will result in lighter baked goods, but only until the total flour is 5% cornflour; after that the baked good gets progressively more dense and inedible. Cornflour is useful for thickening custards, which plain flour is not so good at. But you cannot bake normally with cornflour. Both cornflour and wheat flour have a similar calorie content, wheatflour is lower in the glycemic load (GL) index compared to cornflour, and whereas cornflour is considered "highly inflammitory" (i.e likely to cause a reaction), wheatflour is only considered "inflammitory."
It should smell exactly the same as all-purpose flour. After all, cake flour is just a more refined version of all-purpose flour.
No. Some cake flours contain corn starch. Pastry flour, or all-purpose flour, does not.
All-purpose flour is the same thing as plain flour. The terms are simply different for different parts of the world. In Australia and the United Kingdom, it is known as plain flour while in the United States and Canada it is known as All-Purpose Flour. This type of flour has a lower amount of gluten protein than bread flour, but more than baking flour so it is balanced and can be used for a wide range of purposes.
Yes. All-purpose flour and unbleached flour are usually the same thing. Just be sure that the package doesn't say something like 'self rising', 'bread flour', or 'cake flour' - those ARE NOT all-purpose flour.
Yes.
You can sometimes substitute all-purpose flour for whole wheat flour in a recipe. To do this, you would use the same amount. However, not all recipes will turn out if you do this.