8th grade chemistry, 8th grade science chemistry questions

This is a chemical change;

Chemical changes are prevalent in production and life. Such as the rust of iron, the fireworks of festivals, the neutralization of acid and alkali, and so on. On a macroscopic scale, it can be seen that various chemical changes produce new substances, which is characteristic of chemical changes.

The essence of chemical change can be understood from a microscopic perspective: the type and number of atoms before and after the chemical reaction do not change, but only the way the atoms bond with each other changes. For example, in the case of a substance made up of molecules, it is a molecule in which atoms are recombined into a new substance.

The chemical properties of a substance can only be expressed through chemical changes in the substance, so the chemical properties of the substance can be studied by the method of making the substance react chemically and a new substance can be prepared. Chemical changes are often accompanied by light, heat, gas, precipitation or color and odor changes, and these phenomena can be referred to to determine whether chemical reactions occur. But be aware of the difference with physical change.

Physical changes are also often accompanied by luminescence (electric lights), heat release (friction), gas release (opening the cap of soda bottles), color changes (oxygen becomes liquid oxygen) and other phenomena, but no new substances are generated, which is the fundamental difference between physical changes and chemical changes. According to the different types of reactants and products, chemical reactions can be divided into four basic types: chemical combination, decomposition, displacement and metathesis. Chemical reactions can also be classified from other perspectives, such as redox reactions and non-redox reactions; endothermic reaction vs. exothermic reaction and so on.

The properties that an object exhibits in a chemical change are chemical properties.

Chemical changes, of course! Because it produces new substance, it will become rancid! Remember

1. The main physical properties of oxygen are:

Colorless and odorless gas at room temperature; It is not easily soluble in water (so it can be collected by drainage); The density is lighter than air (so oxygen can also be collected by exhaust air); When liquefied, it is pale blue.

2. The main chemical properties of oxygen are: oxidation.

1) Reaction with sulfur: The chemical equation is that sulfur burns a flame in the air and burns in pure oxygen to produce a flame to produce a body with .

2) Reaction with iron: The chemical equation is that iron in oxygen is an experimental phenomenon of combustion, generating a chromosome. The reason for this experiment is to spread a layer of fine sand or fill a small amount of water on the bottom of the bottle before doing this experiment.

From this, it can be seen that oxygen is a chemically reactive gas.

3 The reaction of a substance with oxygen is called an oxidation reaction. In oxidation reactions, oxygen is oxidizing.

4 The following changes are slowly oxidized: ABCD; The most complex and the most step-by-step is d.

aIron rusts; b. Rust of copper; c. Aging of rubber;

d. respiration of living things; e natural heating of straw piles; fWhite phosphorus spontaneous combustion.

5 Respiration is a complex slow oxidation, taking glucose as an example, the chemical equation of respiration is: decomposition to produce water and carbon dioxide.

6 Laboratory Preparation of Oxygen: (Write the chemical equation).

Pyrolysis of potassium permanganate: produces potassium manganate and oxygen;

Decomposition of hydrogen peroxide at room temperature: water and oxygen are generated;

In , mno2 only plays a role in changing the reaction speed, and its mass and volume remain unchanged before and after the reaction, and this substance is called a catalyst.

7. A large amount of oxygen is required in industry, which is commonly prepared by purification in air.

This process is a change is a physical change (physical change or chemical change).

8 The sequence of operations for heating potassium permanganate in the laboratory to produce oxygen is: .

Inspect the device for air tightness; solidified medicines; The mouth of the test tube is stuffed with a ball of cotton;

Oxygen is collected when bubbles appear continuously; Remove the catheter and then extinguish the alcohol lamp.

9. Combustion refers to a kind of heating reaction between combustibles and oxygen.

Combustion can occur if it occurs at a very rapid rate in a confined space;

The heat generated by the slow oxidation process of a substance may cause combustion if it is not dissipated in time.

Temperature: As the temperature rises (or decreases), salt dissolution accelerates (or slows) the size of the crystal particles.

The smaller the table salt particles, the faster it dissolves.

Stirring can speed up the dissolution of salt.

Factor : Temperature!

Your**: Hot water dissolves quickly!

Factor : Degree of stirring!

Your**: Dissolves quickly when stirring!

Temperature, the higher the temperature the faster the particles, and the smaller the particles, the faster.

1.If it is not saturated after evaporation, the solid will not be evaporated, if it is saturated before evaporation, it should be evaporated 4 grams, after evaporation, it is 80 grams of dust solution, 18 grams of solute, and it is saturated, then the solubility is grams. (20 degrees).

g solution contains 100 g of solute, evaporate 40 g of water, should evaporate 40* g of pie early.

3.In the first question, if the remaining solute is tested, it is not a subtraction that elementary school students will do20-2 = 18 grams.

In the second question, if the remaining solute is tested, then the dust brother is 500-40 = 460 grams multiplied by 20% = 92 grams, which is very simple, he wants to find a rare test.

Solution: Let the mass of hydrogen be m.

The solution is m=