Elements, compounds and mixtures Revision Notes

An element is a pure substance made up of only one type of atom. This means all the atoms in an element have the same number of protons in their nuclei, which defines the element. For example, oxygen gas (O₂) consists entirely of oxygen atoms bonded together. Elements cannot be broken down into simpler substances by chemical reactions because they are the simplest form of matter in terms of atomic composition.

Elements can exist as single atoms, like noble gases (e.g., helium, He), or as molecules made of the same atoms chemically bonded, such as nitrogen gas (N₂) or oxygen gas (O₂). Each element has unique chemical and physical properties, such as melting point, density, and reactivity, which distinguish it from other elements.

• Consist of only one type of atom.
• Cannot be broken down into simpler substances by chemical means.
• Examples: Oxygen (O₂), Iron (Fe), Helium (He).

A compound is a pure substance formed when atoms of two or more different elements chemically bond together in fixed proportions. The atoms in a compound are held together by chemical bonds, such as covalent or ionic bonds, which give compounds distinct properties different from their constituent elements.

For example, water (H₂O) is a compound made from hydrogen and oxygen atoms chemically bonded in a 2:1 ratio. Unlike mixtures, compounds have a fixed composition and cannot be separated into their elements by physical methods. Instead, chemical reactions are required to break the bonds and separate the elements.

Compounds have unique properties; for instance, sodium chloride (NaCl) is a white crystalline solid that dissolves in water, whereas sodium metal and chlorine gas, its elements, have very different properties.

• Formed from two or more different elements chemically bonded.
• Have fixed proportions of atoms.
• Can only be separated into elements by chemical reactions.
• Examples: Water (H₂O), Carbon dioxide (CO₂), Sodium chloride (NaCl).

A mixture consists of two or more substances physically combined, not chemically bonded. The substances in a mixture retain their individual properties and can be present in any proportion. Because there are no chemical bonds between the components, mixtures can be separated by physical methods such as filtration, distillation, or chromatography.

For example, air is a mixture of gases including nitrogen, oxygen, and small amounts of other gases. Another example is salt dissolved in water, which forms a solution where salt and water coexist without chemical bonding.

Mixtures can be homogeneous (uniform composition, like saltwater) or heterogeneous (non-uniform, like sand mixed with iron filings).

• Consist of two or more substances physically combined.
• Components retain their own properties.
• Can be separated by physical methods.
• Examples: Air, salt dissolved in water, sand and iron filings.

The key difference between elements, compounds, and mixtures lies in their composition and how they can be separated. Elements contain only one type of atom and cannot be broken down further chemically. Compounds contain atoms of different elements chemically bonded in fixed ratios and require chemical reactions to separate them into elements.

Mixtures contain substances physically combined without fixed ratios and can be separated by physical means. For example, to separate a mixture of sand and salt, you could dissolve the salt in water and filter out the sand, then evaporate the water to recover the salt.

Understanding these differences is essential for identifying substances and choosing appropriate separation techniques in practical chemistry.

• Elements: one type of atom, cannot be separated chemically.
• Compounds: chemically bonded atoms of different elements, separated chemically.
• Mixtures: physically combined substances, separated physically.
• Separation examples: filtration, evaporation, chemical decomposition.

When classifying substances, look for clues about composition and bonding. If the substance contains only one type of atom, it is an element. If it contains atoms of different elements chemically bonded in fixed ratios, it is a compound. If it is a physical combination of substances that can be separated by physical methods, it is a mixture.

In exams, be precise with terminology: use 'chemical bonds' for compounds and 'physical combination' for mixtures. When asked to explain differences, focus on atomic composition, bonding, and separation methods.

For example, explain that water is a compound because it contains hydrogen and oxygen atoms chemically bonded, whereas air is a mixture because it contains different gases physically mixed without bonding.

• Use atomic composition and bonding to classify substances.
• Explain separation methods linked to bonding type.
• Use precise scientific terms in answers.
• Provide clear examples to support explanations.