KS3 · Chemistry

Acids, bases and alkalis

Lemon, fizzy drink, toothpaste, a drain cleaner with a corrosive warning: which are acids and which are alkalis? You need a test, and sometimes its answer is: nothing happens.

Household solutions

Acid or alkali? Let the litmus decide

Each of these was tested with one strip of red litmus paper and one strip of blue. Read what the two strips did, then decide: acid or alkali?

Still to sort

Acid (0)

Turns blue litmus red.

Alkali (0)

Turns red litmus blue.

6 of 6 still to sort.

The litmus rules

Build the rule, one box at a time

You've just used these rules. Now build them yourself. Tick every result you'd see in an acid, and every result you'd see in an alkali.

Acid
Alkali

Spot the slip

Where did this write-up go wrong?

Test solution A with litmus paper and say whether it is an acid or an alkali.

A student's write-up — which line goes wrong?

Bases and alkalis

Is every base an alkali?

A base is any substance that reacts with acids to form a compound called a salt. An alkali is a base that dissolves in water, forming an alkaline solution. Put each substance in the right circle: base, alkali, both, or neither.

  • A Base (reacts with acids to form a salt)
  • B Alkali (a base that dissolves in water)
  1. Copper oxide
  2. Sodium hydroxide
  3. Caustic soda (drain cleaner)
  4. An acid from the laboratory

Safe or dangerous?

Which of these is dangerous?

On the kitchen table: a lemon and a can of fizzy drink, both acids. Under the sink: a bottle of drain cleaner called caustic soda, an alkali, with a corrosive hazard symbol on the label.

Which idea about acids, alkalis and safety is closest to what you think right now?
How sure are you?

WHAT YOU'VE LEARNED

A quick recap of today's lesson.

Two strips of litmus paper can sort them out, even when one strip doesn't change at all.

What you need to know

  • How red and blue litmus paper tell an acid from an alkali.
  • Why a strip of litmus paper that doesn't change colour still counts as a result.
  • How to test a solution with litmus paper and record what you see.
  • The difference between a base and an alkali.
  • Which acids and alkalis are safe, and which are corrosive.

The big picture

Litmus paper comes in two colours, red and blue. An acid turns blue litmus red and leaves red litmus red. An alkali turns red litmus blue and leaves blue litmus blue. So you need both strips, and a strip that stays the same is still a result. A base is any substance that reacts with acids to form a salt. An alkali is a base that dissolves in water. Some acids and some alkalis are corrosive, but the edible acids in food are safe.

Key points

1Read both litmus strips together: each acid or alkali gives one 'turn' and one 'stay'.
2Acid: blue turns red, red stays red. Alkali: red turns blue, blue stays blue.
3'Stays red' and 'stays blue' are results, not failed tests.
4Every alkali is a base, but not every base is an alkali. Copper oxide is a base that doesn't dissolve, so it isn't an alkali.
5Sodium hydroxide is a base that dissolves in water, so it is an alkali. Its solution is also called caustic soda.
6Some acids and some alkalis are corrosive and can cause chemical burns. Edible acids are safe, but laboratory acids must never be eaten or drunk.
7Lemon juice, vinegar, milk and a fizzy drink tested as acids; toothpaste and a household cleaner tested as alkalis.

Worked example

Problem

Two unlabelled solutions, P and Q, are tested with red and blue litmus paper. The results table says: P: red litmus stayed red, blue litmus turned red. Q: red litmus turned blue, blue litmus stayed blue. Identify each solution.

⚠ Watch out

Thinking 'base' and 'alkali' mean the same thing. An alkali is a base that dissolves in water. Copper oxide reacts with acids to form salts, so it's a base, but it doesn't dissolve in water, so it isn't an alkali.

🧠

Memory hook

Follow the strip that changed. Blue turning red means acid. Red turning blue means alkali. The strip that stays put is the other half of the same answer.

✓

Check yourself

A friend picks up a bottle and says, "It's an alkali, so it must be safe." What would you tell them, and what should they look for on the label?

Flashcards

(14)
What do edible acids taste like, and where can you find them?
Sour. Fizzy drinks, fizzy sweets and fruits such as lemons contain edible acids, which are safe to eat or drink.
Can you eat or drink an acid from a science laboratory?
No. Not all acids are edible. Laboratory acids are not safe and should never be eaten or drunk.
What is a base?
Any substance that reacts with acids to form a compound called a salt.
What is an alkali?
A soluble base: a base that dissolves in water to form an alkaline solution.
Why is copper oxide a base but not an alkali?
It reacts with acids to form salts (so it's a base), but it does not dissolve in water (so it isn't an alkali).
Why is sodium hydroxide an alkali?
It reacts with acids to form a salt, so it's a base, and it dissolves in water to form an alkaline solution.
What does the corrosive hazard symbol warn you about?
That the substance is corrosive and can cause chemical burns. Some acids and some alkalis carry it.
What is caustic soda?
Another name for sodium hydroxide solution. It's used as a drain cleaner and can carry the corrosive hazard symbol.
Do edible acids cause chemical burns?
No. The acids in food are safe.
In a test, blue litmus turns red and red litmus stays red. Acid or alkali?
Acid. The blue strip changing to red is the acid result, and red staying red agrees with it.
What does an alkali do to red and blue litmus paper?
Red litmus turns blue. Blue litmus stays blue.
Can 'no colour change' be a positive litmus result?
Yes. 'Stays red' and 'stays blue' are results, which is why both colours of litmus paper are needed to identify a solution.
How do you test a solution with litmus paper?
Pipette three or four drops into a well of a spotting tile, put the end of a red and a blue strip in it, then observe and record each colour in a results table.
Which household solutions tested as acids, and which as alkalis?
Acids: lemon juice, vinegar, milk and a fizzy drink (blue litmus turned red). Alkalis: a cleaner and toothpaste (red litmus turned blue).

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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