What is the ph of a 8.4x10^-6

What does x represent in the formula for the compound xcl4?

Which of the following is an example of physical change not a chemical change? a) a log gives off heat and light as it burns. b) a tree stores energy from the sun in its fruit. c) a penny lost in the grass slowly changes color. d) a water pipe freezes and cracks on a cold night.

Na chemical reaction, activation energy increases the of the reactants. this outcome causes the particles to collide, which results in the of new products.