When you buy Bitcoin and sell it for a profit you are subject to capital-gains taxes. This is the case whether you buy Bitcoin and sell it for dollars or whether you exchange it for other cryptocurrency for a profit. This is also the case if you buy Bitcoin, it goes up in value and then you exchange the Bitcoin for goods or services.
The IRS has given guidance twice on cryptocurrency tax issues in IRS Revenue Ruling 2014-21 and 2019-24. The critical determination by the IRS in 2014 was that cryptocurrency is property, not currency, for federal tax purposes. This critical determination meant that crypto-trading profits will be treated similar to stock-trading profits, as stock and crypto are both considered property for tax purposes. Treating crypto profits like stock seems straight forward enough, but the rules are not so clear for Bitcoin and other cryptocurrency, since crypto can be purchased with dollars, go up or down in value and then be exchanged to buy a Tesla, pizza or even withdrawn for cash at a Bitcoin ATM. These exchange outcomes are not common in stock trading, and as a result the rules for crypto taxation are confounding.