Kiến thức lý hóa, khoa học: Lý thuyết Môn hóa học lớp 7-8

Elements are made up of one kind of atom which cannot be broken into simpler substances
Compounds are made up of two or more kinds of atoms(elements), chemically combined in a fixed proportion.

Mixtures are made up of two or more kinds of atoms(elements) or compounds or both mechanically mixed in any proportion.
Mixtures are of two kinds : homogeneous mixtures and heterogeneous mixtures.

In homogenous mixture composition is uniform and properties are same throughout the mixture.
In Heterogeneous Mixture the composition is non-uniform throughout and properties vary throughout the mixture.

Sedimentation :The process of deposition of a solid material from a state of suspension or solution in a fluid. e.g. obtaining clear water from muddy water.Â
Decantation : DecantationÂ is a process for theÂ separation of mixtures, by removing a top layer of liquid from which a precipitate has settled.
Filtration: Filtration is the process of removing one substance or material from another by mechanical means. e.g. tea is filtered using a strainer.
Evaporation: Soluble solid constituent is separated from its liquid constituent. Solid component remains behind in evaporating dish, liquid component is lost.
Distillation: Soluble solid constituent from its liquid constituent. Solid component remains behind in distillation flask and liquid component is collected after condensation

Air is the invisible gaseous substance surrounding the earth, a mixture mainly of oxygen and nitrogen.
It consists of 78% nitrogen, 21% oxygen and 1% of other gases, dust, impurities, water vapor.
The Physical properties of air are:

This module also deals with preparation, uses and properties of oxygen and carbon dioxide.

Chemistry is a part of our everyday life. Most of the materials we use are developed through the application of chemistry like : metals , medicines, textiles, soaps.
Metals are lustrous, malleable, ductile and are good conductors of heat and electricity. They are solids at room temperature, except mercury which is a liquid.
Non-metals have properties opposite to that of metals. They are neither malleable nor ductile. They are bad conductors of heat and electricity, except for graphite, which conducts electricity.

We'll learn about some useful compounds like common salt, hydrochloric acid, fertilizers, plastics, medicines in this module.

Do you know what makes lemon and vinegar sour in taste ? Its Acids.
Acids taste sour while Bases taste bitter and feels soapy to touch.
Litmus is an indicator as it can be used to test whether a substance is acidic or basic.
Acid turns blue litmus red. Bases turn red litmus blue.
Substances are called neutral when they not acidic or basic.
When you combine acid and a base they neutralize each other to form salt. The resulting salt could be acidic, basic, or neutral.