Chemistry: Elements and Substances

Study Notes

Non-metals are elements that are not attracted to magnets, examples include hydrogen, oxygen, iodine, carbon, bromine, and chlorine.
Metalloids are elements that have intermediate properties between metals and non-metals, examples include boron, silicon, and germanium.

Mixtures are formed when elements or compounds combine without forming a new compound, and have a variable composition.
Compounds are formed when elements react to form a new substance with a fixed composition.
Mixtures show the properties of their constituent substances, while compounds have totally different properties.
The constituents of a mixture can be separated fairly easily by physical methods, while the constituents of a compound can only be separated by chemical means.

A substance is considered pure if it has a fixed composition and characteristic properties.
Examples of pure substances include sodium chloride (salt) and sugar.
Mixtures contain more than one substance, such as soft drinks and soil.
The word "pure" on packaging means that the product has no adulteration, but scientifically, it means that the substance is not a mixture.

Physical changes occur without a change in composition, such as melting, boiling, and interconversion of states.
Chemical changes occur with a change in composition, such as rusting, burning, and dissolving.
Examples of physical changes include cutting of trees, melting of butter, boiling of water, and passing of electric current.
Examples of chemical changes include rusting of almirah, boiling of water to form steam, and burning of paper and wood.

A homogeneous mixture has a uniform composition, while a heterogeneous mixture has a non-uniform composition and physically distinct parts.
Examples of heterogeneous mixtures: mixtures of sodium chloride and iron filings, salt and sulphur, and oil and water.

A solution is a homogeneous mixture of two or more substances.
Examples of solutions: lemonade, soda water, alloys (e.g. brass), and gaseous solutions (e.g. air).
In a solution, there is homogeneity at the particle level, meaning the particles of the substances are evenly distributed.
A solution has a solvent and a solute as its components.
The solvent is the component that dissolves the other component, usually present in larger amount.
The solute is the component that is dissolved in the solvent, usually present in lesser quantity.

Sugar in water: sugar is the solute and water is the solvent.
Iodine in alcohol (tincture of iodine): iodine is the solute and alcohol is the solvent.
Aerated drinks like soda water: carbon dioxide is the solute and water is the solvent.
Air: a homogeneous mixture of gases, with oxygen (21%) and nitrogen (78%) as its main constituents.