【PEP】Junior Chemistry Grade 9 Volume 2: Chemical Chameleons — Color Change Rules of Acid-Base Indicators

In the world of chemistry, there is a group of magical "color-changing chameleons" —acid-base indicatorsThese are substances that chemically react with acidic or basic solutions and display different colors. Through their color changes, we can qualitatively distinguish colorless liquids that are otherwise indistinguishable to the naked eye (such as dilute hydrochloric acid and sodium hydroxide), serving as an essential bridge connecting microscopic properties with macroscopic phenomena.

Core Color Change Rules

Acidic Environment: Acids turn purple litmus solution red but do not change the color of colorless phenolphthalein (remains colorless).
Basic Environment: Bases turn purple litmus solution blue and colorless phenolphthalein solution red.
Who Changes Color: It is the indicator itself that changes color, not the acid or base. Just as Boyle observed, it was the pigment in violet petals that "changed outfits" after reacting with hydrochloric acid.

Evidence & Logic

Experiments often use spot plates for comparative observation (see Figure 10-2). By comparing white vinegar and apple juice (acidic) with limewater and sodium hydroxide (basic), we can conclude the scientific logic of indicators as "substance classification detectors."

QUESTION 1

[Short Answer] [Reading Context: During an experiment, British scientist Robert Boyle accidentally splashed concentrated hydrochloric acid onto a bouquet of violet petals. Being fond of flowers, he immediately rinsed them, only to find the purple petals had turned red. ... Later, people extracted blue powdered litmus pigment from lichen plants and developed acid-base indicators.] After reading the "Reference Card — The Discovery of Acid-Base Indicators," what insights did you gain?

QUESTION 2

[Short Answer] In chemistry lab inquiry activities, students discovered that many plant petals or fruit juices display different colors when exposed to acidic or basic solutions. A certain red cabbage juice turns red in acid and yellow in base. If this red cabbage juice is added to soapy water (which is basic), what experimental phenomenon is expected? What function of indicators does this process demonstrate?

QUESTION 3

[Short Answer] Briefly describe how to use an acid-base indicator to distinguish between two unlabeled colorless liquids: dilute hydrochloric acid and saltwater.

QUESTION 4

[Multiple Choice] (1) Toilet cleaner contains hydrochloric acid. If accidentally spilled on a marble floor (whose main component is calcium carbonate), it produces a fizzing sound and generates gas. This gas is ( ).

Oxygen ($O_2$)

Hydrogen ($H_2$)

Carbon dioxide ($CO_2$)

Sulfur dioxide ($SO_2$)

QUESTION 5

[Short Answer] Think about it: When smelling an unknown gas or liquid in the laboratory, what method should be used?