Properties of substances
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Lesson notes
What is a Substance?
- A **substance** is a unique form of matter with constant chemical composition and characteristic properties.
- Substances can be **elements** (e.g., diamond, a form of carbon) or **compounds** (e.g., water H₂O, table salt NaCl).
- If two or more substances mix without reacting, they form a **mixture** (e.g., air, salt water).
- A **pure substance** is a single element or compound, not mixed with any other substance.
Physical Properties
- **Physical properties** can be observed or measured without changing the substance's identity.
- Examples: melting point, boiling point, density, color, hardness, and electrical conductivity.
- Pure water boils at **100 °C** (212 °F) at standard pressure – a characteristic physical property.
- Substances can change **state** (solid, liquid, gas) without altering their chemical composition.
Chemical Properties
- **Chemical properties** describe how a substance reacts with other substances to form new substances.
- Examples: flammability, reactivity with acid, ability to rust or tarnish.
- A chemical reaction produces one or more **new substances** with different properties.
- **Inert** substances (e.g., noble gases) do not readily undergo chemical reactions.
Using Properties to Identify Substances
- Every pure substance has a unique set of characteristic properties that can be used to identify it.
- Measuring **melting point** or **boiling point** is a common way to test purity – pure substances melt/boil at sharp temperatures.
- If a sample has a different boiling point than expected, it may be impure or a different substance.
- Property data can also show whether a chemical reaction has occurred (new properties appear).
Law of Constant Composition
- The **law of constant composition** states that a given compound always contains the same elements in the same proportion by mass.
- For water, the mass ratio of hydrogen to oxygen is always 1:8 (or 2:16).
- This law was established by Joseph Proust in the late 1700s.
- It distinguishes compounds from mixtures, where proportions can vary.
States of Matter
- Substances can exist as **solids**, **liquids**, or **gases** (and plasma).
- In a **solid**, particles are tightly packed in a fixed arrangement; they vibrate but do not move past each other.
- In a **liquid**, particles are close but can slide past each other; the substance takes the shape of its container.
- In a **gas**, particles are far apart and move freely; the substance fills the entire container.
Pure Substances vs. Mixtures
- A **pure substance** has a fixed composition and distinct properties (e.g., distilled water).
- A **mixture** contains two or more substances physically combined; its composition can vary.
- Mixtures can be **homogeneous** (uniform, like salt water) or **heterogeneous** (not uniform, like sand and water).
- Mixtures can be separated by physical means (filtration, distillation) without chemical reactions.
Examples of Substances
- **Water (H₂O)** – two hydrogen atoms bonded to one oxygen atom; boils at 100 °C.
- **Table salt (NaCl)** – ionic compound; high melting point, dissolves in water.
- **Diamond** – pure carbon; extremely hard, high melting point.
- **Refined sugar (C₁₂H₂₂O₁₁)** – organic compound; sweet taste, decomposes on heating.
Particle arrangement in solid, liquid, and gas states.
Bohr model of a sodium atom, showing electron shells.
Slides
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Practice questions
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1.What is a chemical substance?
Easy- AA unique form of matter with constant chemical composition and characteristic properties
- BAny material that can be found on Earth
- CA mixture of two or more elements
- DA substance that can only exist as a solid
2.Which of the following is an example of a chemical substance?
Easy- APure water (H2O)
- BAir
- CSeawater
- DGranite
3.The law of constant composition states that:
Easy- AAll samples of a compound have the same proportions by mass of the elements present
- BAll mixtures have variable composition
- CElements always combine in the same physical state
- DThe composition of a compound changes with temperature
4.Which of the following is a characteristic property of pure water?
Medium- AIt boils at 100°C at standard pressure
- BIt is always a liquid
- CIt conducts electricity well
- DIt has a variable boiling point
5.What is the atomic ratio of hydrogen to oxygen in every molecule of pure water?
Medium- A2:1
- B1:2
- C1:1
- D2:2
6.Which of the following is NOT a chemical substance?
Medium- ASeawater
- BDiamond
- CTable salt (NaCl)
- DRefined sugar (C12H22O11)
7.Why are isomers considered distinct chemical substances even though they have the same composition?
Hard- AThey have different arrangements of atoms, leading to different properties
- BThey have different numbers of atoms
- CThey are mixtures of different elements
- DThey exist only in different phases
8.In geology, a mineral is defined as:
Medium- AAn inorganic solid substance of uniform composition
- BA mixture of two or more rocks
- CAny naturally occurring liquid
- DAn organic compound found underground
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