The characteristic of chemical change is that there must be precipitated gases and water formation,

No, there are precipitation, gas and water generation that meet one of the conditions, which is the condition for whether the metathesis reaction can be carried out in junior high chemistry. This is not the case with the availability of other types of reactions.

What you describe is only part of the phenomenon of chemical change.

If AlCl3 is dissolved in water, no precipitation, gas and water formation are visible, but chemical changes are made.

In addition, there are precipitations, gases, and water formations, which are not necessarily chemical changes.

If the CS2 solution of S is dropped into water, the precipitation of S will occur, which is only a physical change, but there is a deep formation;

For example, if water is heated, it will produce no vapor, and it is obviously not a chemical change;

Cooling the air containing water vapor to get water is obviously not a chemical change either.

No, the essence of chemical changes is the elimination of old substances and the creation of new ones, for example, the combustion of iron in oxygen, the formation of ferric tetroxide, there is no precipitation, gas and water formation.

No, it is some phenomenon in chemical change, but there are these phenomena that do not necessarily cause chemical change, and the essential characteristic of chemical change is the generation of new substances.

Not necessarily, some chemical reactions will have a change in color.

Are you asking the first question of the chemistry choice of BT in the second model of Nankai District, Tianjin?

Don't worry, the answer is D, for example, CaO and H2O reflect the generation of Ca(OH)2 without precipitation, gas and water.

And as long as one of the precipitation, gas and water is formed, the metathesis reaction can occur.

The most fundamental theory is that the old chemical bonds are broken, forming new chemical bonds.

1. The scale in the kettle is caused by calcium carbonate deposition;

2. The rust in the iron pot is ferric oxide and ferric oxide deposits;

3. The dirt in the toilet bowl is calcium carbonate;

4. The combustion of materials such as paper or wood produces water and carbon dioxide;

5. Iron rust produces rust.

Lime water reacts with CO2: Ca(OH)2 + CO2 ==CaCO3 + H2O

Sulfuric acid and barium chloride solution reaction: H2SO4 + BACL2 ==BaSO4 +2HCl

Sodium hydroxide with copper sulfate: 2NaOH + CuSO4 ==Cu(OH)2 + Na2SO4

Sodium hydroxide with ferric chloride: 3NaOH + FeCl3 ==Fe(OH)3 + 3NaCl

Sodium hydroxide with magnesium chloride: 2NaOH + MgCl2 ==Mg(OH)2 + 2NaCl

Sodium hydroxide with copper chloride: 2NaOH + CuCl2 ==Cu(OH)2 + 2NaCl

Calcium hydroxide with sodium carbonate: Ca(OH)2 + Na2CO3 ==CaCO3 + 2NaOH

Sodium chloride solution and silver nitrate solution: NaCl + Agno3 ==AgCl + Nano3

Sodium sulfate and fiber-searched barium chloride: Na2SO4 + BACL2 ==BaSO4 leaky wax + 2NaCl

You don't have to know why, just like 1+1 doesn't need to know why it equals 2, of course, if you have to do it, it's just

The reaction in which two compounds exchange components with each other to form two other compounds is called metathesis. Its essence is: the two substances that undergo metathesis reaction exchange ions with each other in the aqueous solution, and combine into substances that are difficult to ionize--- precipitation, gas, and water, so that the ion concentration in the solution is reduced, and the chemical reaction is carried out in the direction of the reduction of ion concentration.

It can be abbreviated as ab cd=ad cbThe essence of the metathesis reaction is that the ions in the solution combine into ionized substances (such as water), insoluble substances or volatile gases, so that the metathesis reaction tends to be completed.

In metathesis reactions, there are roughly three ways in which the concentration of ions in the solution can be reduced

1) In the reaction, substances with very little solubility (precipitation) are generated;

2) There is gas released in the reaction;

3) Weak electrolytes (including water, weak acids, weak bases, etc.) are formed in the reaction. This is the condition under which the metathesis reaction can occur. As long as one of these conditions is met, the reaction can take place.

In chemical change, there are generally eight major precipitates. It is AGCL (silver chloride), BaSO4 (barium sulfate), CaCO3 (calcium carbonate), BaCO3 (barium carbonate), Al(OH)3 (aluminum hydroxide), MG(OH)2 (magnesium hydroxide), Cu(OH)2 (copper hydroxide), Fe(OH)3 (hydrocracking cover iron oxide), classified as follows:

1. White precipitate: silver chloride, barium sulfate, calcium carbonate, barium carbonate, silver carbonate, magnesium hydroxide;

2. Blue precipitation: copper hydroxide;

3. Reddish-brown precipitate: iron hydroxide.

1. The scale in the kettle is caused by calcium carbonate deposition; 2. The rust in the iron pot is ferric oxide and ferric oxide deposits; 3. The dirt in the toilet bowl is calcium carbonate; 4. The combustion of materials such as paper or wood produces water and carbon dioxide; 5. Iron rust produces rust buckets. Extended Information: The nine major precipitations of junior high school chemistry refer to iron hydroxide, copper hydroxide, magnesium hydroxide, aluminum hydroxide, barium carbonate, silver chloride, barium sulfate, calcium carbonate, and silver carbonate. Precipitate, which chemically refers to the process of precipitating a solid substance from a solution; It also refers to the solid material that is precipitated during the precipitation process.

In fact, the precipitate is mostly insoluble (Luxin has a < solubility at 20 °C. In chemical experiments and production, precipitation methods are widely used for the wandering separation of substances. The process of producing a separable solid phase from the liquid phase, or tangerine, is an insoluble substance precipitated from a supersaturated solution.

Precipitation indicates the process of formation of a new condensed phase, or the process of deposition due to the addition of a precipitant that makes certain ions insoluble compounds. The chemical reaction that produces the precipitation is called the precipitation reaction. References**:

Encyclopedia – Chemical Changes.

The key to the metathesis reaction is the exchange of ions.

To understand metathesis reactions, it is necessary to first understand the dissociation of acid-base salts in solution. For example, sodium chloride solution essentially contains chloride ions and sodium ions. There are four ions in the mixed solution of sodium chloride and potassium nitrate, which is exactly the same as the mixed solution of potassium chloride and sodium nitrate.

When two compounds are dissolved in water, they are dissociated into two ions, and if they can be exchanged to form substances (water, gas, precipitate) that are difficult to dissociate in water, then the metathesis reaction occurs.

If the reaction seen in junior high school is a reaction between alkali and salt, salt and salt, it generally does not produce water and gas, but only precipitate, so it is required that the reactants be dissolved in water first and become a solution. That is, the reactants are required to be soluble.

For example, sodium carbonate and barium sulfate do not react to form barium carbonate precipitate and sodium sulfate. You may try to write down the chemical equation, and you will find that the reverse reaction of the reaction can be carried out without considering the solubility of the reactants, and then it will change back to the original substance.

Of course, there are exceptions, alkali and ammonium salts can produce ammonia and water, which can react.

So I think that if the metathesis reaction can produce water and gas, the reaction can take place; Such as acid and alkaline oxide, acid and alkali reaction, etc.

If only precipitate is generated, the reactants must also be soluble.

The second answer above is worth reading, he just didn't make it clear. This is a special case. If two compounds have the same ions, they generally do not react. After exchanging ions, it is the same as the original substance. This has nothing to do with physical change at all.