Chemistry topic for junior high school, a chemistry topic for junior high school

The measured volume fraction of oxygen will be on the low side. Because the amount of red phosphorus is insufficient, the oxygen in the bottle is not consumed after combustion. Oxygen accounts for about 1 5 of the volume of air, and now that the oxygen is not consumed, the water entering the bottle is less than 1 5, so the measured volume fraction of oxygen will be smaller.

The reaction utensils can use a bell jar, put red phosphorus in a burning spoon, quickly reach into the bell after ignition, put some water in the sink, and put the bell upside down. Later, when the amount of red phosphorus is insufficient, and the oxygen is not completely consumed, the water entering the bell will be less than 1 5The measured volume fraction of oxygen will be smaller.

On the small side. This is because if the amount of red phosphorus is insufficient, oxygen cannot be consumed.

Rubber stopper with 2 holes, burning spoon, red phosphorus, catheter, container for water, gas collection cylinder, water stopper.

First, assemble the instrument and connect the burning spoon and the conduit to the rubber stopper.

Ignite the red phosphorus and put the rubber stopper on the gas cylinder filled with air, and open the water stop clamp after the red phosphorus is extinguished, and the water will be sucked into the gas cylinder, about 1 5.

There are many ways to do this).

On the small side. There was too little red phosphorus, and the oxygen in the cylinder was not depleted, so the measurement results were small.

Let the mass of the solute in the remaining solution be x, then it will be determined by x 70=(x-4) (70-4-10) x=20

Because the solution is saturated, it is 70 x = 10 4, and the answer is 28g

(1) Take a small amount of sample and add hydrochloric acid, if there are colorless bubbles, it proves that it has not completely deteriorated; On the contrary, it is completely deteriorated.

2) Magnetite adsorption iron powder (b) all Fe2O3 (c) both Fe(Oh)3 and Fe2O3

3) B4) take a small amount of brown powder in the test tube, add an appropriate amount of hydrochloric acid to dissolve the powder, if the solution is yellow, add a sufficient amount of NaOH solution to produce a reddish-brown precipitate, which proves that the precipitate is Fe(OH)3, so as to prove that the brown powder contains iron.

5) Select CH4 by igniting the gas and covering a small beaker with clarified lime water on the inner wall above the flame, if the lime water becomes turbid, it proves that CH4 contains carbon elements.

CH4 is selected by igniting the gas and covering a small beaker with dry cold above the flame, if there is a mist of water on the inner wall of the beaker, it proves that CH4 contains hydrogen.

NaCl solution is selected by adding nitric acid and silver nitrate solution, if there is a white precipitate generated, it proves that NaCl contains chlorine.

The last question I don't understand is do you want to know whether the valency of the atomic cluster is the valency of the atomic cluster or the valency of the metal cluster, which is the valency of sulfate-2 and the valence of hydroxide-1.

Turn left to start the spiritTurn right.

Among them, the mixtures are: seawater, fresh and clean air, copper sulfate dissolves Kaixin solution.

The pure substances are: red phosphorus, phosphorus pentoxide, liquid oxygen, potassium permanganate, ammonia, ice-water mixture, mercury pat wheel.

The elemental ones are: red phosphorus, liquid oxygen, mercury.

The compounds are: phosphorus pentoxide, potassium permanganate, ammonia, ice-water mixture.

It is a pure thing

It is elemental

Belonging to the compounds are:

Children, pure substances are all about elemental substances and compounds. Don't be misled by the pure air, ice-water mixture.