Topic 1 Changes and Properties of Matter

Unit 1 Into the World of Chemistry

Topic 1

Changes and properties of matter

As large as stars in the universe, as small as particles invisible to the naked eye, they constitute the material world in various forms. There are multiple interactions between various substances, and they are constantly changing. We live in such an ever-changing material world every day. Therefore, understanding matter and its changes is crucial to understanding natural phenomena and laws, and is also closely related to our daily life.

1. Chemical change and physical change

We know that water can turn into water vapor or ice under certain conditions, that iron and steel products will rust in wet places, that coal, wood and firewood can burn in the air and emit light and heat, and so on. From a chemical point of view, what is the essential difference between these changes in matter? Let's do a few experiments first.

Experiment 1-1

（1）Clamp the test tube containing a small amount of water obliquely on the iron stand (such as I-1) "heat carefully at the bottom of the test tube until the water boils. Move a clean glass piece (or a small beaker filled with cold water) close to the test tube mouth and observe and record what happened"

method guide

When doing chemical experiments, you should focus on observing the changes in the color, state, and smell of reagents (such as water, alum, and limestone) before and after the experiment, and think about why the changes occurred.

(2) Take a small amount of copper sulfate crystals (commonly known as bile alum or blue alum) in a mortar, and grind the alum with a pestle. Observe and record the changes in gallbladder:

(3) Put a small amount of bile alum before and after grinding into two test tubes, add a small amount of water, and shake to obtain a clear copper sulfate solution. Add sodium hydroxide solution dropwise to the two test tubes respectively, observe and record the phenomena occurring in the test tubes.

(4) As shown in Figure 1-3, add an appropriate amount of dilute hydrochloric acid into a test tube containing a small amount of limestone (or marble). Observe and record the changes that occur in the test tube and beaker.

In experiment 1-1(1) and experiment 1-1(2), the liquid water turns into water vapor after boiling, and turns into liquid water after cooling; the blocky bile alum turns into powder after grinding For bile alum, although water and bile alum have undergone a change in form, no other substances have been produced. This change without the formation of other substances is called a physical change. The volatilization of gasoline, the casting of molten iron into pots, and the melting of candles are all physical changes. In experiment 1-1(3) and experiment 1-1(4), bile alum and limestone (or marble) all produced other substances in the change. This change to produce other substances is called a chemical change, also known as a chemical reaction. The burning of firewood and the rusting of iron are all chemical changes.

The basic feature of chemical change is the formation of other substances, often manifested as color change, gas evolution, and precipitation. Chemical changes not only generate other substances, but are also accompanied by changes in energy, which often manifest as heat absorption, heat release, and luminescence. The above observable phenomena can often help us judge whether a substance has undergone a chemical change.

During a chemical change of matter, a physical change occurs simultaneously. For example, when lighting a candle, the melting of paraffin is a physical change, while the burning of paraffin to produce water and carbon dioxide is a chemical change.

2. Chemical and physical properties

We call the properties of substances shown in chemical changes as chemical properties. For example, iron can rust in moist air, copper can produce patina in moist air, carbon can burn in air to generate carbon dioxide and emit light, heat, etc.; another example, copper sulfate solution can react with sodium hydroxide solution A blue precipitate of copper hydroxide is generated, and limestone can react with hydrochloric acid to generate carbon dioxide gas, etc.

The properties that a substance exhibits without chemical changes are called physical properties. The color, state, odor, hardness, melting point, boiling point, density, etc. of a substance all belong to its physical properties. For example, under normal conditions, oxygen is a colorless and odorless gas, water is a colorless and transparent liquid, bile alum is a blue solid. Understanding the physical properties of substances is also very important for studying their composition, structure and changes.

When the external conditions change, the properties of the material will also change. Therefore, the conditions are often indicated when describing the properties of the material. For example, when the temperature rises, solid ice turns into liquid water, and when water is heated to a certain temperature, water boils. Experiments have proved that the boiling point of a liquid will change with the change of atmospheric pressure. For example, in a place where the atmosphere is thin, the atmospheric pressure becomes smaller, and the boiling point of water will decrease at this time. Since the atmospheric pressure is not constant, people set 101 kPa as the standard atmospheric pressure.

Combining your own life experience and knowledge, describe the properties of oxygen and carbon dioxide as much as possible, try to judge which are physical properties and which are chemical properties, and which methods can be used to distinguish them, and communicate your views with your classmates.

We have learned many facts in life, such as water and carbon dioxide can be used to extinguish fire, ethanol (commonly known as alcohol) can be used as fuel, graphite can be used to make pencil lead, and so on. These uses of substances are determined by their properties. The study of the properties of matter and the methods of inquiry will definitely give you a new understanding of the material world around you.

You should know after finishing this topic

1.Chemistry is a natural science that studies the properties, composition, structure, and changes of substances at the molecular and atomic levels, and is closely related to human progress and social development.

2.Changes that do not produce other substances are called physical changes; changes that produce other substances are called chemical changes, also known as chemical reactions.

3.The properties that a substance exhibits during chemical changes are called chemical properties; the properties that do not require chemical changes are called physical properties.