Capital gain on purchase of jewellery (exchange of old jewellery)

Mere exchange wont attract Capital Gain since for capital gain ownershipo and possesion of Asset shall be transferred.

Yes, you may show Gold bar as gift from Mother

agreed with ruchika somani

Upto Gift from mother it's OK.Sale/exchange of gold bar will be taxable under capital gain/loss as per holding period by the mother.

1) Yes gold bar shall be treated as gift received from your mother.
If you want you can make a gift deed for a safe side.

2) In case of exchange yes it amounts to transfer, capital gains will be attracted.

3) Capital Gains shall be attracted based on holding period.
Assuming your mother purchased the gold bar before 2000-2001, base index shall be 100 and for cost of shall be multiplied with cost of indexation for current FY. 2018-2019.
Cost you can assume of the year 2000-2001 as the base index is 100.