When is tax considered for public purpose

Tax must be for a public purpose. How can the public-purpose nature of a tax be determined? Here are four (4) examples:

[1] If the tax is for the welfare of the nation;
[2] If the tax is for greater portion of the population;
[3] If the tax affects an area as a community rather than as individuals; and
[4] If the tax aims to support the services of the government for some of its recognized functions.