Does heating or Cooling affect the pH of the substances?

Answer 1

This can be hard to get your head around...especially if you're as awful with pH as I am. Still, let's just give this a think through.

What is pH? Well it's the amount of H+ you have floating around in a solution. The lower the pH, the more H+ there is floating around. Seems odd, but just remember that.

Specifically, it's the concentration of H+ floating around...and the pH can (very roughly) be said to indicate the position of the decimal point on this concentration. I'll give an example.

Say something is 0.1 mol dm-3 H+ ...this would be pH 1. At pH 4 it is 0.0001 mol dm-3 H+.

Water is the best example. We say neutral water is pH 7. This means the H+ concentration is 0.0000001 mol dm-3 in water...at room temperature!

Because what happens when we add heat? The H2O gains energy and dissociates more. H2O becomes H+ and OH-...in equal amounts. So we have more H+...and the pH value lowers.

That makes intuitive sense. We're adding energy and it's breaking apart. More H, lower pH at higher temperatures. What is important to realise, though, is that the water is still neutral. We have more H+, but we have more OH-. The water is NOT more acidic per se....the pH has just changed, that's all.

You get a lower pH, but you also get a higher pOH. pOH isn't measured like pH is, it's just 14-pH. But I think it's a nice way to consider things where water is concerned. Because with water alone, you don't get H+ without OH-, really.

pH + pOH always = 14.

So...that may be too much information So I'll try to sum up. pH is tricky and just sitting around and thinking about it for a few minutes can do you a lot of good. The answer is YES, pH is temperature-dependant. It's not often a big enough change to be relevant, but it's still fundamentally related to temperature.

However this doesn't always relate directly to a change in neutrality of the solution...which is not only an important thing to keep in mind, but also helps you understand this kind of thing as a concept.