The constitution declares india as union of states although it does not use the word federation, the india union is based on the principles of federalism how can this be substantiated

Indeed, the word "Federation" is not used in the constitution which creates India's government. It's Federalism, however, can be still be proven because:

It has a trilayered structure of government comprising the Local, State, and Central Government. According to the laws of India, each level of its government has powers to pass laws. However, the central government has more powers over the other tiers of its legislature. The legislative arms within India's tiers of government according to the hierarchy (descending order) are Union, State and Concurrent. The most powerful tier of India's government, which is also regarded as central, is the Union Government.

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