Question 1

What does pH actually measure?

Accepted Answer

**pH** measures the **concentration of hydrogen ions** in a solution, on a logarithmic scale — and both parts of that matter for interpreting the number correctly.

**The definition.** pH is the **negative logarithm (base 10) of the hydrogen ion concentration** in moles per litre. The negative sign exists so that acidic solutions, which have high hydrogen ion concentrations, get low numbers.

**Why the scale is logarithmic, and what that implies.** Each whole number represents a **tenfold** change. pH 4 is ten times more acidic than pH 5, and **a hundred times** more acidic than pH 6. This is the most frequently missed point, and it makes small-sounding pH shifts substantial: ocean acidification of a few tenths of a unit represents a large proportional change in hydrogen ion concentration.

**The scale in practice.** Pure water at 25°C sits at **7** and is neutral — not because 7 is a magic number, but because water self-ionises slightly and, at that temperature, produces equal and specific concentrations of hydrogen and hydroxide ions. **Below 7 is acidic; above 7 is alkaline (basic).**

**The scale is not capped at 0 and 14.** Those are conventional limits reflecting common solutions. Concentrated acids can have negative pH and concentrated alkalis can exceed 14.

**Neutral is temperature-dependent.** Because self-ionisation increases with temperature, pure water at higher temperatures has a neutral pH below 7 — while remaining genuinely neutral, since the two ion concentrations are still equal.

**Common misconceptions:**

**pH does not measure strength of an acid**, it measures the resulting concentration. A dilute strong acid can have a higher pH than a concentrated weak one.

**Alkaline diets and "alkaline water"** rest on a misunderstanding: blood pH is tightly regulated within a narrow range by buffering systems, and what you drink does not move it. Stomach acid is strongly acidic by design, and anything swallowed encounters that first.

**Buffers** resist pH change, which is why biological systems stay stable.

Question 2

What is the difference between an atom, a molecule, an element and a compound?

Accepted Answer

Two of these describe particles; two describe substances. Mixing the two categories is the source of nearly all the confusion. The particle terms: Atom — the smallest unit of an element that retains

Question 3

Why is the ocean salty?

Accepted Answer

Because rivers have been carrying dissolved minerals into the sea for billions of years, and the salt has nowhere to go. The main source is rock weathering. Rain is slightly acidic — atmospheric

Question 4

What are the SI units actually based on now?

Accepted Answer

Since a redefinition that took effect in 2019, all seven base units are defined by fixing the numerical values of physical constants — not by physical objects, and not by properties of materials. It

Question 5

What causes earthquakes, and how is magnitude measured?

Accepted Answer

The sudden release of elastic strain accumulated in rock, and magnitude is measured on a logarithmic scale — which is why the numbers behave in ways people consistently misjudge. The mechanism:

Question 6

What does a p-value actually mean?

Accepted Answer

The probability of observing results at least as extreme as yours, assuming the null hypothesis is true. It is a statement about the data given a hypothesis — not about the hypothesis given the data,

Question 7

Why does the sea look blue when water is colourless?

Accepted Answer

Two effects combine, and one of them surprises people: water is not actually colourless. Water genuinely absorbs red light. Water molecules absorb light at the red end of the visible spectrum through

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