As shown in figure 1, a device designed by a classmate himself: cut off the bottom of the bottle with a large plastic bottle, leaving a bottle mouth about 8 cm ~ 10 cm, and plug it with a rubber stopper from the inside out. (1) DC is used in the water electrolysis experiment, and the gas generated at the A terminal of electricity is less oxygen, so the A terminal is connected to the positive pole of power supply;

(2) The A-end of electricity produces less gas, which indicates that oxygen can be tested with wooden strips with Mars, and if it is rekindled, it proves the existence of oxygen;

(3) Basis: The ratio of positive oxygen to negative hydrogen is one to two, and the volume of oxygen can be determined;

(4) According to the fact that oxygen is more soluble in water than hydrogen, the volume ratio of hydrogen to oxygen is often greater than 2:1;

(5) According to the schematic diagram of electrolyzed water particles, it can be seen that water molecules, hydrogen molecules and oxygen molecules are all composed of atoms, that is, molecules are composed of atoms; According to this, we can also judge the macro composition of water. Through the comparison before and after the change, it can be found that the types and quantities of hydrogen atoms and oxygen atoms that constitute water molecules are unchanged before and after the reaction, that is, the types and quantities of atoms before and after the reaction are unchanged. Comparing the whole process, we can also find that in the process of change, molecules are decomposed into atoms, but atoms are not further decomposed but recombined into new molecules, that is, molecules can be subdivided into atoms in the process of chemical change, but atoms cannot be subdivided;

So the answer is (1) DC positive? (2) Oxygen bars with Mars? (3)4 ml

(4) At the beginning, a part of oxygen is dissolved in water (or oxygen may react with the electrode material).

(5)① Water decomposes to produce oxygen and hydrogen when charged? Water is composed of hydrogen and oxygen. (3) In a chemical reaction, molecules can be split into atoms, but atoms cannot be split (or the types and quantities of atoms have not changed before and after the chemical reaction; Or other reasonable)