Here's how we get there:

1. Move the decimal point to the right until you have a number between 1 and 10. In this case, we move it 11 places to the right, resulting in 2.5.

2. Count the number of places you moved the decimal. This number becomes the exponent of 10. Since we moved the decimal 11 places to the right, the exponent is -11.

Therefore, 0.000000000025 is written as 2.5 x 10^-11 in scientific notation.