What are the main points of chemistry inference questions in junior high school?

It's good to master all the textbooks Chemistry in the third year of junior high school is very simple.

A must have dilute hydrochloric acid, (only dilute hydrochloric acid is acidic among the four substances) B must have soda ash and may have caustic soda (there can be no dilute hydrochloric acid and barium chloride with soda ash, if there is, there will be gas or precipitation, and it is impossible to coexist).

BaCl2 goes out of which soda ash.

Phenolphthalein does not change color.

(1) dilute hydrochloric acid (because if it does not contain dilute hydrochloric acid, the solution will not be acidic: only sodium chloride may be present in the solution - neutral, caustic soda, sodium carbonate or sodium bicarbonate are also alkaline; In summary, it can only be dilute hydrochloric acid).

2) Group B inferred that there must be soda ash (sodium carbonate), and there may be caustic soda (sodium hydroxide) Na2CO3 + 2HCl = 2NaCl + H2O + CO2 Because there is sodium carbonate and barium ions, precipitation will occur.

ba2+ +co32- =baco3

Add excess barium chloride, BaCl2 + Na2CO3 = BaCO3 + 2NaCl, remove soda ash (or carbonate ions, see requirements).

The phenomenon is that phenolphthalein does not change its red color, indicating that there is no caustic soda.

Caustic soda is NaOH and soda ash is Na2CO3

Group A inferred that there must be dilute hydrochloric acid.

Group B inferred that there must have been pure barium, possibly barium chloride, and excessive CaCl2 was added to NaOH to remove caustic soda.

Colorless (Note: There is a problem with this question, if there is HCL in the waste liquid, it is impossible to have soda ash and caustic soda).

pH<7 Acidic There must be acid HCl

If there are bubbles, it means that there is gas formation Only NaCO3 in the waste liquid will react with HCl to form CO2 gas. So there must be naco3

There may be BACL2

Add (Na)2SO4 to remove BaCl2

If it doesn't change color, there must be no NAOH

group A is dilute hydrochloric acid; Group B must have soda ash, possibly barium chloride and caustic soda.

Add excessive salts containing sulfate but cannot react with sodium hydroxide, such as sodium sulfate, potassium sulfate, etc.

Remove the barium chloride from it.

The phenomenon is that the solution is colorless, indicating that there must be no such substance.

pH 7, then group A inferred that there must be a solute——— dilute hydrochloric acid, group B added hydrochloric acid dropwise to the waste liquid and found that there were bubbles, then group B inferred that there must be ——— soda ash in the waste liquid.

There may be ——— caustic soda.

In order to prove the possible ingredients, students in Group B are asked to complete the following blanks.

Take a small amount of waste liquid and add excess ——— barium chloride to remove the ——— soda ash and then add phenolphthalein to the waste liquid, the phenomenon is ——— colorless, indicating that there must be no such substance.

From the experiment of group A, it can be obtained: the pH of the waste liquid is <7, so there must be dilute hydrochloric acid (HCl) in it.

From the experiment of group B, it can be obtained: soda ash reacts with dilute hydrochloric acid and produces bubbles, so there must be CaCO3, and because the metathesis reaction of barium chloride and sodium carbonate will form precipitate, it cannot coexist stably in large quantities, so there may be caustic soda (NaOH).

Sodium carbonate can be removed by first adding excess barium chloride.

The phenomenon is still colorless.

Solution, group A deduced that there must be HCL and NaCl, and group B inferred that there must be Na2CO3, NaCl, and may have NaOH in the waste liquidA small amount of waste liquid was added to excess barium chloride to remove Na2CO3The phenomenon is not discolored.

pH<7 indicates that the solution is acidic, and there must be HCL in group A, and sodium carbonate in group B with hydrochloric acid.

Add excess calcium chloride to make the sodium carbonate completely turn into a precipitate, and then add phenolphthalein if it is red, indicating that there is sodium hydroxide.

Group A inferred that there must be dilute hydrochloric acid, because only dilute hydrochloric acid pH 7 Group B inferred that there must be soda ash (sodium carbonate), Na2CO3 + 2HCl = 2NaCl + H2O + CO2

There may be caustic soda (sodium hydroxide).

Add excess barium chloride, BaCl2 + Na2CO3 = BaCO3 + 2NaCl, remove soda ash (or carbonate ions, see requirements).

The phenomenon is that phenolphthalein does not change its red color, indicating that there is no caustic soda.

Today's homework.

I don't know if it's right.

A must have HCL NACL

B must have Na2CO3 NaCl and may have NaOH plus BaCl2 to remove Na2CO3

The solution does not turn red.

Because NaCl is formed as long as there is a reaction during the experiment, NaCl must be present in both groups.

The rest is the same as everyone else would have inferred.

(1) HCl is acidic.

2) Soda ash Caustic soda hydrochloric acid reacts with soda ash.

3）h2so4;Barium chloride Barium sulfate is precipitated (4) colorless; No caustic soda phenolphthalein does not change color.

There must be sodium carbonate and there may be barium chloride and sodium hydroxide.

A: dilute hydrochloric acid B: There must be soda ash and there may be caustic soda.

1 BaCl2 soda ash.

2 Phenolphthalein does not change color.

pH 7 solution is definitely acidic and must not be alkaline. There must be hydrochloric acid.

B has air bubbles, indicating that there is sodium carbonate, and there may be sodium bicarbonate.

HCL (ask about solutes, not dilute hydrochloric acid); soda ash, caustic soda; calcium chloride, soda ash (or carbonate ions); There was no significant change in the solution.