Grade 7 Chapter 14: Elements, Compounds and Mixtures

Introduction to Matter

Everything around us, whether it's the air we breathe, the water we drink, or the ground we walk on, is made up of matter. Matter can exist in different forms, and in this chapter, we will explore its fundamental classifications: elements, compounds, and mixtures. Understanding these distinctions is crucial to comprehending the world of chemistry.

14.1 Elements

What are Elements?

An element is a pure substance that cannot be broken down into simpler substances by ordinary chemical means. Think of elements as the basic building blocks of all matter. Each element is made up of only one kind of atom.

• Examples: Oxygen (O), Hydrogen (H), Gold (Au), Iron (Fe).
• Symbols of Elements: Each element is represented by a unique symbol, usually one or two letters. For example, 'H' for Hydrogen, 'O' for Oxygen, 'Fe' for Iron (from its Latin name Ferrum), and 'Au' for Gold (from its Latin name Aurum).

Metals and Non-metals

Elements can be broadly classified into metals and non-metals based on their properties:

• Metals:
  • Lustrous: They have a shiny appearance.
  • Malleable: They can be hammered into thin sheets (e.g., aluminum foil).
  • Ductile: They can be drawn into thin wires (e.g., copper wires).
  • Good Conductors: They are excellent conductors of heat and electricity.
  • State: Most metals are solid at room temperature, with the notable exception of Mercury (Hg), which is a liquid.
  • Examples: Gold, Silver, Copper, Iron.
• Non-metals:
  • Dull: They generally have a dull appearance (not shiny).
  • Brittle: They are brittle and break easily when hammered (not malleable or ductile).
  • Poor Conductors: They are generally poor conductors of heat and electricity (insulators), though graphite (a form of carbon) is an exception.
  • State: They can exist as solids (e.g., Carbon, Sulfur), liquids (e.g., Bromine), or gases (e.g., Oxygen, Hydrogen) at room temperature.
  • Examples: Oxygen, Carbon, Sulfur.

14.2 Compounds

What are Compounds?

A compound is a pure substance formed when two or more different elements chemically combine in a fixed ratio by mass. When elements combine to form a compound, they lose their individual properties, and the new compound has entirely different properties.

• Formation: Compounds are formed through chemical reactions, where atoms of different elements bond together.
• Fixed Ratio: The elements in a compound are always present in a specific, fixed proportion. For example, water (H₂O) always has two hydrogen atoms for every one oxygen atom.
• Separation: Compounds can only be broken down into their constituent elements by chemical means (e.g., electrolysis). Physical methods cannot separate them.
• Examples:
  • Water (H₂O): Formed from Hydrogen and Oxygen. Unlike hydrogen (a flammable gas) and oxygen (a gas that supports combustion), water is a liquid that extinguishes fire.
  • Carbon Dioxide (CO₂): Formed from Carbon and Oxygen.
  • Salt (Sodium Chloride, NaCl): Formed from Sodium and Chlorine.

14.3 Mixtures

What are Mixtures?

A mixture is an impure substance formed when two or more substances (elements, compounds, or both) are physically mixed together. Unlike compounds, the substances in a mixture do not chemically combine, and they retain their individual properties.

• No Fixed Ratio: The components of a mixture can be mixed in any proportion.
• Properties: Each component in a mixture retains its original properties.
• Separation: Mixtures can be separated into their components by physical methods.

Types of Mixtures

Mixtures can be classified into two main types:

• Homogeneous Mixtures:
  • Have a uniform composition throughout. This means that the components are evenly distributed, and you cannot see the individual components.
  • Often called solutions.
  • Examples: Salt solution (salt dissolved in water), Air (a mixture of gases like nitrogen, oxygen, etc.).
• Heterogeneous Mixtures:
  • Have a non-uniform composition. The components are not evenly distributed, and you can often see the individual components with the naked eye.
  • Examples: Sand and water, Oil and water, a mixture of iron filings and sulfur powder.

14.4 Methods of Separation of Mixtures

Since components of a mixture retain their individual properties and are not chemically bonded, various physical methods can be used to separate them. Here are some common methods:

• Filtration:
  • Principle: Used to separate insoluble solids from liquids. The mixture is passed through a filter medium (like filter paper) that allows the liquid to pass through but retains the solid particles.
  • Example: Separating sand from water, or tea leaves from brewed tea.
• Evaporation:
  • Principle: Used to separate a soluble solid from a liquid in a solution. The liquid is heated and allowed to evaporate, leaving the solid behind.
  • Example: Obtaining salt from saltwater.
• Decantation:
  • Principle: Used to separate an immiscible liquid from another liquid, or a liquid from a settled solid. The heavier component settles at the bottom, and the lighter liquid is carefully poured off.
  • Example: Separating oil from water, or water from settled mud.
• Sieving:
  • Principle: Used to separate solid particles of different sizes. The mixture is passed through a sieve (a mesh screen) with specific pore sizes.
  • Example: Separating flour from bran, or pebbles from sand.
• Magnetic Separation:
  • Principle: Used to separate magnetic substances from non-magnetic substances in a mixture. A magnet is used to attract the magnetic component.
  • Example: Separating iron filings from sulfur powder or sand.
• Distillation:
  • Principle: Used to separate components of a homogeneous liquid-liquid mixture with different boiling points, or to separate a liquid from a dissolved solid. The liquid with the lower boiling point evaporates first, is then condensed back into a liquid, and collected.
  • Example: Separating alcohol from water, or obtaining pure water from saltwater.
• Chromatography:
  • Principle: A sophisticated technique used to separate components of a mixture based on their differential movement through a stationary phase (e.g., filter paper) by a mobile phase (e.g., a solvent). Components that are more soluble in the mobile phase or less adsorbed by the stationary phase travel faster.
  • Example: Separating different colored dyes in ink.