GCSE · Chemistry · Edexcel · Spec 1CH0 · Higher

Hydrogen-ion concentration and pH

A pH 2 solution has a thousand times more hydrogen ions than a pH 5 one. Three small steps on the scale, a thousandfold jump in ions. Here's why.

Crack the pH code

Where does each solution sit on the pH scale?

Every solution below is described by its hydrogen ions (H⁺). Here's the code: an H⁺ concentration of 1 × 10⁻⁷ mol/dm³ is pH 7. The pH is the power of ten, without the minus sign. Drag each solution to its pH, then check.

A — H⁺ concentration of 1 × 10⁻⁴ mol/dm³B — H⁺ concentration of 1 × 10⁻¹³ mol/dm³C — Equal concentrations of H⁺ and OH⁻ ionsD — H⁺ concentration of 1 × 10⁻¹ mol/dm³

Which way does it go?

More hydrogen ions: bigger pH or smaller pH?

Solution X has a pH of 2. Solution Y has a pH of 7.

Which solution has more hydrogen ions (the higher H⁺ concentration)? Pick the idea closest to what you think right now.

Y, because 7 is a bigger number than 2, so it must have more H⁺.They have the same amount, because they're both solutions.X, because the lower the pH, the higher the H⁺ concentration.

How sure are you?

I'm sureI think soI'm guessing

Y, because 7 is a bigger number than 2, so it must have more H⁺.

This is a common misconception

That's the most natural guess, and it's backwards. pH runs the other way: the higher the hydrogen ion concentration, the LOWER the pH. Y's pH of 7 is the bigger number, so Y has fewer H⁺ ions. X, at pH 2, has more.

They have the same amount, because they're both solutions.

This is a common misconception

Being a solution doesn't fix how many H⁺ ions there are. These two have different pH values, and pH tracks the hydrogen ion concentration, so their H⁺ concentrations are different. The lower pH, pH 2, goes with the higher H⁺ concentration: X has more.

X, because the lower the pH, the higher the H⁺ concentration.

This matches the evidence

Exactly right. More H⁺ ions means a lower pH, so X at pH 2 has more hydrogen ions than Y at pH 7. Keep that direction locked in: it's the idea every pH question is built on.

Lower pH, more H⁺. A solution of pH 2 has more hydrogen ions than a solution of pH 7.

Alkalis and pH

Reason it through

Why does adding an alkali to an acidic solution make its pH go up?

Link 1 of 4

First link · your turn

An alkali dissolves in the water. What does it release into the solution?

Locked — reveal the link above first

One step down the scale = ten times the H⁺

pH: 5. H⁺ concentration (× the pH 7 value): 100

Drag the point to pH 7, then slowly back towards pH 3. Stop at each whole number and read the value: 1, 10, 100, 1000, 10,000.

Watch out: One pH unit is not a small, even step. Going from pH 7 to pH 6 multiplies the H⁺ concentration by 10. Going all the way from pH 7 to pH 3 multiplies it by 10,000.

Your turn to finish it

Comparing two solutions

Solution P has a hydrogen ion concentration of 1 × 10⁻³ mol/dm³. Solution Q has a pH of 6. What is the pH of P, and how many times more hydrogen ions does P have than Q?

P: H⁺ concentration = 1 × 10⁻³ mol/dm³. The power of ten is −3.Always write down what you're given first.
missing step

Which line is step 2?

So the pH of P is 3.So the pH of P is −3.So the pH of P is 0.001.

WHAT YOU'VE LEARNED

A quick recap of today's lesson.

Lower pH, more H⁺. One step down the scale means ten times as many.

What you need to know

Acids release hydrogen ions (H⁺) when they dissolve in water, and the concentration of those ions decides how acidic the solution is.
pH measures the hydrogen ion concentration on a scale from 0 to 14: below 7 is acidic, 7 is neutral, above 7 is alkaline.
More H⁺ means a lower pH, and each pH unit is a tenfold change in H⁺ concentration.
Alkalis release hydroxide ions (OH⁻), which react with H⁺ ions to form water, so adding an alkali raises the pH.

The big picture

pH is a number from 0 to 14 that measures the hydrogen ion (H⁺) concentration of a solution. Below 7 is acidic, 7 is neutral and above 7 is alkaline. The scale runs backwards (more H⁺ means a LOWER pH), and it goes up in powers of ten: every step of one pH unit is a tenfold change in hydrogen ion concentration. Alkalis raise the pH because their hydroxide ions (OH⁻) react with H⁺ ions to form water.

Key points

1pH is the measure of the hydrogen ion (H⁺) concentration of a solution, on a scale from 0 to 14.

2pH less than 7 is acidic, pH 7 is neutral, pH greater than 7 is alkaline. Acidity increases from 7 down to 0; alkalinity increases from 7 up to 14.

3The higher the H⁺ concentration, the lower the pH. A solution of pH 2 has more hydrogen ions than a solution of pH 7.

4The pH scale is logarithmic: a change of one pH unit is a tenfold change in H⁺ concentration, two units is 100 times and three units is 1000 times.

5Reading the code: 1 × 10⁻¹ mol/dm³ of H⁺ gives pH 1, 1 × 10⁻⁷ mol/dm³ gives pH 7 and 1 × 10⁻¹³ mol/dm³ gives pH 13. It works backwards too: pH 12 means 1 × 10⁻¹² mol/dm³.

6Alkalis (soluble bases) release hydroxide ions, OH⁻, into water. These neutralise hydrogen ions, H⁺(aq) + OH⁻(aq) → H₂O(l), lowering the H⁺ concentration and raising the pH.

7A neutral solution has equal concentrations of H⁺ and OH⁻ ions: that is pH 7.

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Worked example

Problem

Solution R has a pH of 2. Solution S has one hundredth (1/100) of R's hydrogen ion concentration. What is the pH of S, and is S acidic, neutral or alkaline?

⚠ Watch out

Reading pH like an ordinary scale, where a bigger number means more acid and each step is small and even. Both are wrong: a LOWER pH means MORE hydrogen ions, and one unit is a tenfold change. pH 3 has ten times the H⁺ of pH 4.

🧠

Memory hook

Low pH, lots of H⁺. Every step down the scale is another ×10.

✓

Check yourself

Cover the page. A solution has 1 × 10⁻¹¹ mol/dm³ of H⁺. What is its pH, and is it acidic, neutral or alkaline? (Answer: pH 11, alkaline.)

Flashcards

(12)

What do acids release when they dissolve in water?

Hydrogen ions, H⁺.

What does pH measure?

The hydrogen ion (H⁺) concentration of a solution.

What range of numbers does the pH scale cover?

0 to 14.

Which pH values are acidic, neutral and alkaline?

Below 7 is acidic, exactly 7 is neutral, above 7 is alkaline.

Moving from pH 7 towards 0, what increases? Moving from 7 towards 14?

Towards 0, acidity increases. Towards 14, alkalinity increases.

A solution's H⁺ concentration goes up. What happens to its pH?

The pH goes down. The higher the H⁺ concentration, the lower the pH.

What is an alkali, and what ions does it release into water?

A soluble base. It releases hydroxide ions, OH⁻.

Adding OH⁻ ions to a solution: what happens to the H⁺ concentration and the pH?

The H⁺ concentration falls and the pH rises.

Write the ionic equation for hydroxide ions neutralising hydrogen ions.

H⁺(aq) + OH⁻(aq) → H₂O(l)

What is special about the ions in a neutral solution?

The concentrations of H⁺ and OH⁻ ions are equal, and the pH is 7.

What change in H⁺ concentration does one pH unit stand for?

A tenfold (×10) change. The pH scale is logarithmic.

How do you get the pH from an H⁺ concentration written as 1 × 10⁻ⁿ mol/dm³?

The pH is n, the power without its minus sign.

Tap any card to flip it, or use Study as deck to go through them one at a time. In the full lesson these run as a spaced-repetition deck — you rate each card Hard, Good or Easy and the tricky ones keep coming back until they stick.

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Where does each solution sit on the pH scale?

More hydrogen ions: bigger pH or smaller pH?

One step down the scale = ten times the H⁺

Comparing two solutions

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