What does constant variable mean in science?

Zoe is investigating the composition of substance a, an unknown substance. using chemical processes, she analyzes substance a and determines it is composed of sodium, oxygen, and hydrogen atoms in a ratio of 1 : 1 : 1. what is substance a? a. a compound b. an element c. a heterogeneous mixture d. a homogeneous mixture

Significant figures are digits read directly from the measuring instrument plus one more digit, which is __ by the observer.

What is the ph of a solution that has a hydrogen ion concentration of 1.0 * 10 -9 m?