A few questions about chemistry are urgently needed

2) Blue.

nacl,hcl

3) Unscientific, NaOH solids are dissolved in water and exothermic.

24 [Conjectures and assumptions] pH = 7 pH greater than 7 design and experiment] phenolphthalein test solution.

Evaluation] Among the three students, the operation specification is (A) students take the pH test paper and immerse it directly in the sodium chloride solution, but it is not possible.

Conclusion and Analysis】 Correct Soda ash is an alkaline salt.

30 Ca(OH)2+Na2CO3=2NaOH+CaCO3Na2CO3+2HCl=2NaCl+H2O+CO232 1 (purple litmus solution).

2 (phenolphthalein solution).

3 (carbon dioxide).

4 (Iron powder).

5 (sodium carbonate).

6 (calcium carbonate).

Calculated by chemical equations, this belongs to junior high school chemistry.

2) Blue.

nacl,hcl

3) Unscientific, NaOH solids are dissolved in water and exothermic.

24 [Conjectures and assumptions] pH = 7 pH greater than 7 design and experiment] phenolphthalein test solution.

Evaluation] Among the three students, the operation specification is (A) students take the pH test paper and immerse it directly in the sodium chloride solution, but it is not possible.

Conclusion and Analysis】 Correct Soda ash is an alkaline salt.

30 Ca(OH)2+Na2CO3=2NaOH+CaCO3Na2CO3+2HCl=2NaCl+H2O+CO232 1 (purple litmus solution).

2 (phenolphthalein solution).

3 (carbon dioxide).

4 (Iron powder).

5 (sodium carbonate solution).

6 (calcium carbonate solution).

7 (all metals with activity greater than hydrogen, e.g., zinc powder, magnesium strips) 8 (all soluble carbonates, e.g., potassium carbonate solution).

You will have the answer when you go to Lenovo New 100 School.

1.It is also to prevent overboiling, because chemicals also have impure factors.

2.Mercury vapor is toxic, but when it reacts with sulfur to form mercury sulfide, it will not become mercury vapor.

3.Because phosphorus burns are caused by the production of a large number of acidic substances, copper sulfate is alkaline, is a strong acid and weak alkali salt, neutralizing effect.

4.Because they are all flammable medicines, prevent combustion.

The first is because at high temperature, water is in a gas state, so there is no need for the second to analyze: the first, second, and third times each time 5g of hydrochloric acid is added, the solid mass is reduced, which is proportional, and to the fourth time, it is only reduced, indicating that the calcium carbonate has completely reacted at this time, and the hydrochloric acid has been excessive. Then we can look back and find that 1-3 times of hydrochloric acid is insufficient, and 5g of hydrochloric acid can be reacted off.

caco3 --2hcl

x x= means that there is hydrochloric acid in 5g, so the mass fraction is x100%=

Because the water is in a vapor state in this reaction, it is not marked with a gas symbol.

1 The function of the sand is to prevent the combustion of phosphorus, exothermic and break the bottom of the Erlenmeyer flask, and wait until it cools to room temperature because it wants to exclude the influence of temperature on the experiment.

2 This is due to ions in solution. The divalent Fe ions are green in solution and Cu ions are blue in solution.

3 C + O2 = CO2 2C + O2 = 2CO It is easy to see that 1 mol of carbon and 1 mol of oxygen form carbon dioxide, and with oxygen produces carbon monoxide. Therefore, when charcoal is burned, there is enough oxygen to produce carbon dioxide, and insufficient oxygen to produce carbon monoxide.

4 Conservation of mass 2mol of hydrogen reacts with 1mol of oxygen to produce 2mol of water.

2 hydrogen molecules and 1 oxygen molecule make 2 water molecules (you can get a lot of this as I remember it, because I haven't been exposed to chemistry for a long time, I can't remember all of them).

5 Must be based on objective facts. Follow the law of conservation of mass.

is to prevent **; Because of the heat rises and colds; Different chemicals are of course different; CO is produced when oxygen is insufficient or not ventilated; Water is made up of two H's and one O; Water is made up of the elements H and O; Balancing and valence.

Let's add to the fourth point on the first floor.

2 hydrogen molecules and 1 oxygen molecule react to form 2 water molecules.

Every 4 parts of hydrogen reacts completely with 32 parts of oxygen, resulting in 36 parts of water.

Mass ratio between substances: 4:32:36

1.This is because according to the law of conservation of mass, the number of atoms of the reactant and the product is equal. Oxygen has two atoms, so there should be 2 before water, so 2 should be added before hydrogen. The coefficient can represent the number of individual molecules and represent the stoichiometric number.

2。The total mass of the reactants is equal to the total mass of the product.

3.The mass of oxygen in the carbon dioxide is multiplied by the mass fraction of oxygen in the carbon dioxide in grams.

Mass of oxygen in water: multiplied by the mass fraction of oxygen in water as grams.

The mass of oxygen in the product should be grams.

There are grams of oxygen in the reactants, so the original substance does not contain a ridge cavity or oxygen element.

So choose D4. The formula in question 1 should be a chemical equation. Among them, H2, O2, and H2O are called chemical formulas.

1: The preceding coefficient is to level the chemical equation, which means that the table of the group is not"One unit of water is obtained after the reaction with two units of hydrogen and one unit of oxygen.

2: After knowing the mass of the reactants, compare the relative molecular mass reputation or sedans of the corresponding products: for example.

Knowing that water is produced 18g to find the mass of H2.

Solution: Let the mass of hydrogen be xg

2h2 + o2 ==h2o

x 18g has the following relationship: [(1+1)*2] xg==(1+1+16) 18g

Solution x=4g

3: According to the atomic conservation theorem, if CO2 and H2O are generated in the product, then there must be C and H in the reactant

According to the data, it is obtained that the CO2 must have carbon atoms, and then compared the product with the oxygen consumption, it is found that the oxygen consumption is just conserved, so the answer D is obtained.

4: The chemical equation is to use the chemical formula of the corresponding substance to write the reaction. The two cannot be compared.

1.For example, 2h2+o2=2h2o, why should the coefficient be added before h2? What does this coefficient represent?

According to the number of molecules, the branch width ratio is 2:1.

2.According to the law of conservation of mass, how to know the mass of reactants to find the mass of the product?

The ratio of the column proportional formula to the relative molecular mass.

3.When a substance is completely burned in grams of oxygen, grams of carbon dioxide and grams of water are generated, then the substance (d).

A must contain carbons, hydroxides, and b, may contain hydrocarbons.

c must contain hydrocarbons, but Mengyan Liang can contain oxygen d only contains hydrocarbons, not oxygen.

After the reaction, the oxygen element in the water is as if there is a total of oxygen element, so there is no oxygen element in the original compound.

Answers and reasons).

4 Is there a difference between a chemical equation and a general chemical formula? What's the difference?

To trim.

The coefficient represents the number of molecules, and the mass of reactants = the mass of the product, c,

2h2+o2=2h2o What should be added before h2? What does this coefficient represent?

Representatives: The reaction of two parts of hydrogen and one part of oxygen produces two parts of water to widen;