Form 1040 does not show long-term capital gain separately from ordinary income. To see the amount of long-term capital gain you have to look at Schedule D. The tax calculation takes into account the different types of income and applies the appropriate rates. Since you have long-term capital gain your tax is probably calculated using the Qualified Dividends and Capital Gain Tax Worksheet, or possibly the Schedule D Tax Worksheet. The worksheet will show the tax rate that is being applied to various amounts of income.
Even if the long-term capital gain is taxed at 15%, the total tax on your tax return might increase by more than 15% of the gain. That can happen because adding income to a tax return can have side effects besides the direct tax on the added income. The increase in income could reduce or eliminate various deductions or credits that are not related to the income that was added, but that have income-based limits or phase-outs. If you received Social Security benefits it could make more of your Social Security taxable. You have to compare your entire Form 1040 line by line before and after adding the interest to see what else is changing besides the capital gain. If there's a change in an amount that comes from another form or schedule, you have to look at that form or schedule to see what's happening.
