【PEP Edition】Junior High Chemistry, Grade 9, Volume 2: Chemistry in the Kitchen

Welcome to your home "lab" — the kitchen. When we mention "salt," most people first think of table salt ($NaCl$) used for seasoning. But in the world of chemistry,salt is a vast family of compounds composed of metal ions (or ammonium ions) bonded with acid radical ions.

₂bubbles, which rise due to buoyancy CaCO₃ + 2CH₃COOH → ...

Salt Family Members in the Kitchen

Sodium chloride ($NaCl$): commonly known as table salt, an essential electrolyte for the human body.
Sodium carbonate ($Na_2CO_3$): commonly known as soda or washing soda, used in dough processing and glass production.
Sodium bicarbonate ($NaHCO_3$): commonly known as baking soda. When heated, it decomposes to produce $CO_2$, creating many small pores in the dough, making it light and fluffy.
Calcium carbonate ($CaCO_3$): the main component of limestone and eggshells, also an important calcium supplement.

Life & Safety

Our food (as shown inFigure 12-1) is rich ininorganic salts (minerals)such as iron, calcium, and potassium, which are essential for life activities. However, chemicals must be used with caution:Industrial salt (sodium nitrite $NaNO_2$) looks very similar to table salt but is highly toxic and must never be ingested by mistake.

Common Mistake Alert

Never consume industrial salt (sodium nitrite $NaNO_2$) as table salt. It is highly toxic, and excessive intake can lead to poisoning or even death.

QUESTION 1

When steaming buns, adding baking soda (sodium bicarbonate) makes the dough soft and porous. The main reason is:

Baking soda has a sweet taste that improves flavor

Heating decomposes it to produce carbon dioxide gas, forming small pores

Baking soda can kill bacteria in the dough

Sodium bicarbonate increases the protein content of the dough

QUESTION 2

Mixing $Na_2CO_3$ solution with $CaCl_2$ solution in the lab — which observation and chemical equation are correct?

White precipitate forms, $Na_2CO_3 + CaCl_2 = CaCO_3 \downarrow + 2NaCl$

Lots of bubbles appear, $Na_2CO_3 + CaCl_2 = NaCl + CO_2 \uparrow$

No visible change — the two salts do not react

QUESTION 3

Which of the following fertilizers can be easily distinguished from the other three by appearance alone?

Potassium sulfate

Ammonium chloride

Phosphate rock powder

Sodium nitrate

QUESTION 4

Which statement about sodium nitrite ($NaNO_2$) is incorrect?

It has a salty taste and can be used as a substitute for table salt in cooking

It is a type of industrial salt

Ingestion can cause poisoning

It looks very similar to table salt

QUESTION 5

Which of the following belongs to the category of "salt"?

Clear limewater [Ca(OH)₂ solution]

Soda ash (Na₂CO₃)

Hydrochloric acid (HCl)

Distilled water (H₂O)