Key Points

Matter is defined as anything that has mass and occupies space. All physical objects around us, such as books, water, and air, are examples of matter.

A mixture consists of two or more substances that are physically combined but not chemically bonded. Each substance in a mixture retains its own individual properties.

Mixtures can be uniform (homogeneous), where components are evenly distributed and indistinguishable, like saltwater. They can also be non-uniform (heterogeneous), where components are unevenly distributed and visible, like a salad.

A pure substance is made of only one type of particle (atom or molecule) and has a constant composition and distinct properties. It cannot be separated into simpler substances by physical methods.

Pure substances are classified into two categories: elements and compounds. Elements are the fundamental building blocks of matter, while compounds are formed from the chemical combination of elements.

An element is a pure substance that cannot be broken down into simpler substances by any chemical reaction. It is composed of only one type of atom. Examples include Iron ($Fe$), Oxygen ($O_2$), and Carbon ($C$).

A compound is a pure substance formed when two or more elements are chemically combined in a fixed ratio. The resulting compound has properties that are different from its constituent elements.

The elements in a compound are always combined in a fixed ratio by mass. For instance, water ($H_2O$) is always composed of two hydrogen atoms for every one oxygen atom.

A compound's properties are unique and different from its constituent elements. For example, Sodium ($Na$) is a reactive metal and Chlorine ($Cl_2$) is a toxic gas, but they form Sodium Chloride ($NaCl$), which is harmless table salt.

Components of a mixture can be separated by physical means (e.g., magnetism, filtration). In contrast, the elements in a compound can only be separated by chemical reactions, such as the electrolysis of water to produce hydrogen and oxygen.

Alloys are uniform mixtures, typically of two or more metals. Examples include brass (a mixture of copper and zinc) and bronze (a mixture of copper and tin).

Air is a uniform mixture of various gases. The primary components are Nitrogen ($N_2$, approx. 78%) and Oxygen ($O_2$, approx. 21%), along with smaller amounts of other gases like Carbon Dioxide ($CO_2$).

The formation of a compound from its elements is a chemical change. For example, heating a mixture of iron ($Fe$) and sulfur ($S$) creates the compound iron sulfide ($FeS$), a new substance with different properties.

The presence of carbon dioxide ($CO_2$) can be confirmed using lime water (calcium hydroxide solution). When $CO_2$ is passed through it, the lime water turns milky due to the formation of insoluble calcium carbonate ($CaCO_3$).

Minerals are naturally occurring solid substances with a definite chemical composition. Most minerals are compounds (e.g., quartz, $SiO_2$), but some, known as native minerals, are pure elements (e.g., gold, $Au$).