"The Composition of Water" Water in Nature PPT Courseware 7

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"The Composition of Water" Water in Nature PPT Courseware 7

Physical properties of water:

Pure water is a colorless, odorless and transparent liquid.

At 101KPa, the freezing point of water is 0°C.

The boiling point is 100℃,

The density is maximum at 4°C, which is 1 g/ml.

When water freezes, its volume expands, so the density of ice is less than that of water and it floats on the water.

hydrogen

1) Physical properties

Colorless, odorless, poorly soluble gas with the smallest density.

2) Chemical properties: H2 is flammable (can be used as fuel)

⑴Ignite pure H2

Phenomenon: A light blue flame is emitted, and there are water droplets on the inner wall of the beaker.

⑵ Ignite impure H2

Phenomenon: Explosion H2+O2--H2O

1. What does water consist of?

H2 purity verification method:

① Collect the gas in a test tube using the drainage gas collection method;

② Block the mouth of the pipe with your thumb and move closer to the flame;

③ Light it and listen to the sound. If there is a "pop" sound, the H2 is pure; if there is a sharp popping sound, the H2 is impure.

Note: (please check the purity before lighting, and fire is strictly prohibited where they exist)

Any flammable gas or combustible dust may explode if mixed with air. Such as H2, coal powder, flour, petroleum gas.

Electrolysis water experiment:

1. Main devices: water electrolyzer and DC power supply

2. Experimental phenomena:

① After energization, gas is generated on both electrodes, and the volume ratio of the gas generated on the positive electrode to the gas generated on the negative electrode is approximately 1:2.

② The gas generated on the positive electrode can re-ignite the wood strips with sparks, and the gas generated on the negative electrode can burn, and the flame will be light blue.

3. Conclusion: Hydrogen and oxygen are generated after electrolysis of water, that is, water is composed of two elements: hydrogen and oxygen.

4. Text expression: water, electricity, hydrogen + oxygen

5. Symbolic expression: H2O-- H2 + O2

Experimental notes:

1. The purpose of adding a small amount of sodium hydroxide or dilute sulfuric acid to the water when electrolyzing water: to enhance the conductivity of water

2. The power supply uses "DC power supply".

3. The volume ratio of hydrogen and oxygen produced by the electric decomposition of water should be 2:1, but in the experiment the volume ratio of hydrogen and oxygen was greater than 2:1. The main reasons are:

①Due to the different solubility abilities of hydrogen and oxygen in water, the solubility of oxygen in water is greater than that of hydrogen;

② During the electrolysis process, the nascent oxygen undergoes an oxidation reaction with the electrode before escaping from the electrode, consuming oxygen.

Thinking about the problem

1. Is the electrolysis of water a chemical change or a physical change? What about the formation of icebergs?

The former is a chemical change and the latter is a physical change.

2. Are oxygen and oxygen, hydrogen and hydrogen the same?

no the same.

3. Can we say "because electrolysis of water generates oxygen and hydrogen, so water is composed of oxygen and hydrogen"?

No, it only means that water is composed of hydrogen and oxygen.

Class exercises

1. Regarding the water electrolysis experiment, which of the following statements is incorrect ( )

A. This experiment proved that water is composed of hydrogen and oxygen

B. This experiment proves the important property that "water can produce hydrogen and oxygen under the condition of direct current"

C. In order to enhance the conductivity of water, a small amount of sodium hydroxide or sulfuric acid is often added to the water.

D. The volume ratio of oxygen and hydrogen collected in the experiment is 2:1

2. Which of the following changes is a chemical change ( )

A. Water turns into steam when heated

B． Water dissolves many substances

C. Water can be electrolyzed under the action of direct current

D． The density of water is maximum at 101kPa and 4℃

3. When electrolyzing water, dilute sulfuric acid or sodium hydroxide solution is generally added because ( )

A. It is a catalyst for water splitting reaction

B． Can increase the conductivity of water

C. Can produce more hydrogen and oxygen

D． Can remove harmful substances from water

4. Among the following properties of hydrogen gas, which are chemical properties ( )

A. Colorless, odorless

B． able to burn

C. Hardly soluble in water

D． density is very small

3. Elements and compounds

Oxygen (O2), hydrogen (H2), iron (Fe), nitrogen (N2), sulfur (S), etc.

[Elementary substance] A pure substance composed of the same elements is called an elemental substance.

Water (H2O), carbon dioxide (CO2), potassium permanganate (KMnO4), hydrogen peroxide (H2O2)

[Compound] A pure substance composed of different elements is called a compound.

The basis for dividing substances into pure substances and mixtures: the types of constituent substances

The basis for dividing pure substances into elements and compounds: the types of constituent elements

Differences and connections between elements and compounds:

1. Pure substances can be divided into elements and compounds according to the number of constituent elements.

2. Pure substances composed of the same kind of elements are called simple substances. The concept contains two elements: pure substances and the same kind of elements, which are recorded as "one thing and one element". For example: oxygen, phosphorus, copper, etc.

3. Pure substances composed of different elements are called compounds. There are also two elements: pure matter and different elements, which are recorded as "one thing with many elements". For example: water, potassium permanganate, etc.

Class exercises

1. Please read the following descriptions about water before answering: The one that describes the physical properties is ( ), the one that describes the chemical properties is ( ), and the one that describes its uses is ( )

A. Pure water is a colorless, odorless, transparent liquid

B． Water decomposes when exposed to electricity

C. Water can be heated and boil into water vapor

D． Water vapor passing through the red-hot carbon layer produces carbon monoxide and hydrogen

E. Industrial production of hydrogen from water

F. Water is a common fire extinguishing agent in life

2. It is best to use ( ) to identify the four gases H2, O2, N2 and air.

A. Observe its color and status

B． smell its scent

C. Use wooden sticks with sparks to extend into the gas respectively.

D． Insert burning sticks into the gas

3. Which of the following substances is a compound ( )

A. air b. River water c. ice d. Red phosphorus

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Update Time: 2024-09-03

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