Science questions for junior 3, science topics for junior 3

The first question is very simple, first of all, C powder must exist, otherwise it is impossible to explain the formation of CO2 after roasting filter residue. Secondly, cuo also exists, otherwise it would not be possible to explain the problem of solids remaining after final roasting. However, if Fe is not excessive, then Cu2+ in the solution will not be completely replaced, so three can not be selected.

What you said is incorrect, because according to the information in the question, the acid is excessive, so there must be no iron in the filter residue, and the iron has entered the solution phase. If there is no CUO, then the filter residue is only C powder, and there is no solids left after burning.

There is c, this goes without saying. In the end, the solid residue of C may be unreacted Zn, Fe replaced by Zn, and of course, Cu may be replaced. So it's not certain that there must be cuo in the raw materials!

So there is no right alternative answer.

Note that you missed it, it says that there are bubbles produced when adding zinc, which means that there is HCL, so there will be no iron left.

Child, if you follow what you say, zinc can be used to replace iron, and then iron reacts with oxygen to produce solids. However, according to the table of metal activity sequences, zinc reacts with copper first, and only reacts with ferrous ions after all the copper is replaced.

Copper oxide must exist because there is a solid residue behind it, and the bubbles are obviously acidic after the first filtration, and if there is more dilute sulfuric acid, the iron must be consumed; There will be no iron in the solid residue, and there is still any surplus after oxygen combustion, so the filter residue must be C powder and copper powder, (copper is obtained in exchange for iron). So it turns out that there must be copper oxide.

(1) i1=. The voltmeter measures the thermistor (r1), and the voltage of the thermistor (u1) is ohms.

2) Because U1+UX, the voltage at both ends of the sliding rheostat UX=U-U1=And because I=I1=ix, the current (Rx) passing through the sliding rheostat is equal to PX=UXIX=

Buoyancy is equal to the gravitational force exerted by an object to expel liquid. The volume discharged during the salvage process remains the same, so the buoyancy remains the same. The pressure is equal to the pressure divided by the area, because the gravitational buoyancy is not the same, so the pressure is the same, and the pressure is also the same.

In river water, the density of river water is less than that of seawater, so it is less buoyant, and it takes more pulling force to balance the same weight. So choose C

If you choose A, the friction will be 0, so the object will accelerate faster and faster under the action of f1.

The velocity is increasing, and although the force is decreasing, the velocity is increasing all the time, so the kinetic energy is increasing.

2) dilute nitric acid, HNO3+NaOH==nano3+H2O (3) (2) The purpose of (2) is to exclude the interference of hydroxide, so as not to combine with silver ions to form a precipitate, resulting in wrong conclusions.

c I think c

If no reagent is used, it can only be proved that three substances (sodium carbonate, sulfuric acid, barium hydroxide) react with each other to form gas or precipitation, and it cannot be identified, so reagents are needed.

If acid is chosen, then sodium carbonate can be identified, and then barium hydroxide and sulfuric acid can be identified with sodium carbonate, and another reagent is needed to distinguish between sodium chloride and sodium hydroxide, and reagents precipitated in alkali can be used.

If alkali (CO3 2- precipitates, SO4 2- does not precipitate) sodium carbonate can be identified and then another reagent is required.

With a neutral acid-base as above, the steps are pretty much the same.

If litmus or phenolphthalein is used, more than two steps are also required.

Test strips are not that precise.

Dry ice is sublimation and endothermy, not decomposition, sublimation is a physical change, and decomposition is a chemical change.

29.(1) Received.

2) Yes, the Pearl River is the sea water of the vertical brother, Shenzhen is the fresh water, and the volume of the water opened in the Peiyin Shenzhen Pai Pei Fiber Hall is greater than the volume of the sea water of the Pearl River, so the boat can pass through the draft in Shenzhen.

3) This question is simple and will not be answered.

4) Analyze the state of motion first, it is a uniform motion, so the force is balanced.

That is, the traction force is equal to the drag.

f traction force = f drag = g *