pH scale

What this tool shows

The pH scale measures how acidic or basic (alkaline) a solution is. It runs from 0 to 14: below 7 is acidic, 7 is neutral and above 7 is basic. Lemon juice and vinegar are acids, pure water is neutral, and soap, baking soda and bleach are bases. In this tool, students tap a substance to test it, then drag it onto the scale or tap a number to place it. Check marks each placement as right or wrong. The pH values used are typical ones; real products vary a little.

Two indicators are available. Litmus only shows whether something is an acid or a base: blue litmus turns red in acids and red litmus turns blue in bases. Universal indicator changes through a rainbow of colors, from red for strong acids through green at neutral to purple for strong bases, so it also shows how strong.

Why testing first helps

Guessing where milk goes is a coin toss. Testing it, then reasoning from the evidence, is how chemists actually work. Students also discover that litmus doesn't change clearly for substances close to neutral, which is exactly why universal indicator is useful.

Common misconceptions

• Not all acids are dangerous. Lemon juice and vinegar are acids we eat, while some bases, like bleach and drain cleaner, are among the most harmful chemicals in a home.
• A pH of 7 doesn't mean something is safe to drink. It only means it is neutral.
• The scale is not like a ruler. Each step is ten times more acidic or basic than the next, so pH 2 is ten times more acidic than pH 3.
• Bases and alkalis are not exactly the same. An alkali is a base that dissolves in water.

A 10-minute acids and bases activity

• Ask the student to sort the substances into sour, soapy and neither before testing anything.
• Test each one with litmus and update the groups.
• Use universal indicator and place every substance on the scale.
• Check, then discuss any surprises, such as milk being very slightly acidic.
• With an adult, make red cabbage indicator at home and test kitchen substances like vinegar and baking soda, never cleaners.

Where this fits in school

• US, grades 5 to 8: acids and bases usually appear in middle school physical science, and the pH scale returns in high school chemistry.
• England, Key Stage 3 (Years 7 to 9): students define acids and alkalis, use the pH scale and indicators, and learn about neutralization.
• England, GCSE (Years 10 and 11): students explain pH in terms of hydrogen ions and compare strong and weak acids.

Learning more

Science is part of Eurekami's skill map, with lessons that draw science diagrams live and narrate them in English or Spanish. When a question is tricky, the AI tutor asks guiding questions so the student does the reasoning. For older students, Exam Prep includes GCSE Combined Science practice papers.