Chemistry: Elements and Substances

Non-Metals and Metalloids

• 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 and Compounds

• 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.

Purity of Substances

• 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.

Types of Mixtures

• Mixtures can be classified based on the nature of their components.

Physical and Chemical Changes

• 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.

Homogeneous and Heterogeneous Mixtures

• 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.

Solutions

• 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.

Examples of Solutions

• 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.

Properties of a Solution

• A solution is a homogeneous mixture.