Physical & Chemical Properties - Review
1. Mass is the amount of matter in an object. Volume is the amount of space an object occupies. The density (click on the linked word to see density problems and solutions) of an object is the mass per unit volume of an object (D = m/V).
a) The mass of water in a cup is much larger than the amount of water in a pool. The density of water in a cup of water taken from a fountain is the same as the density of water taken from a pool (ignoring the chlorine in it).
b) The mass of water in a cup is much smaller than the mass of a wood from a maple tree.
The density of maple wood (D = 760 Kg/m3) is less than the density of water (1000Kg/m3).
2. Mechanical mixtures can be heterogeneous or homogeneous. Homogeneous mixtures show only one phase whereas heterogeneous mixtures show two or more phases. A phase is a distinct portion of matter. Examples: oil + water = heterogeneous, sand + charcoal powder = heterogeneous, sugar + water = homogeneous. Solutions are Homogeneous Mixtures.
3. A qualitative property is a property of an object that can be described using your five senses (colour, odour, taste...)
A quantitative property describes matter in terms of measurements. (mass, volume, density, temperature).
4. A Colloid is a mechanical mixture in which the solute particles are so tiny that cannot be separated by gravity and remain suspended in the solvent. A suspension is a mechanical mixture in which the particles of the solute are temporarily suspended in the solvent but can eventually settle out due to gravity. Colloids show the Tyndall Effect.
5. A solution is a homogeneous mixture made up of two parts solvent and solute. The solvent is the part of the solution that is doing the dissolving. The solute is the part of the solution being dissolved. Dissolving is a physical change that can be reversed. The reversal of dissolving is precipitation or dehydration which can be obtained by evaporation (remember Copper sulfate solution when dried on an evaporating dish will form crystals of solid copper sulfate, sea salt is obtained by salt sea water by evaporation).
A mixture of two solids (example: sand and salt ) is a mechanical mixture. When two metals are liquefied (melted) and are mixed together, the resulting mixture is called an alloy. For example brass = copper + tin.
Pure gold for example is not commercially practical in most cases because it is too soft and expensive. Alloys of gold contain different amounts of either copper or nickel to make it more durable. Gold purity is measured in karats. 24-karat gold is 100% pure. 50% pure gold is 12-karat.
6. An element is the smallest portion of matter of a specific substance. The element Oxygen is the smallest portion of oxygen.
An element cannot be broken down any further by chemical change. A compound is a substance made up of two or more elements. Example: Water = 2 parts hydrogen + one part Oxygen. Sodium Chloride (table salt) = 1 part sodium + 1 part chlorine.
7. An inference is an interpretation of facts and data gathered during many observations. A theory is a possible explanation of events or phenomena explained after many observations and supported by many experiments.
8. A chemical change is usually irreversible. This means that once a chemical change has occurred it forms a completely new substance with new chemical and physical properties. Example: table salt (a food product) is made up of two elements which chlorine (a poisonous gas) and sodium (a very reactive metal). When a pure substance undergoes a physical change its chemical properties are usually not changed.
9. Properties of pure water as either qualitative or quantitative:
- A very poor conductor of electricity
- Water is transparent when taken in small depths
- Water is odourless
- The volume of a sample of water is 26.8 m
- LThe density of water is 1000 Kg/m3
- The boiling point of water is 100 oC
10. Some physical properties cannot be used to identify substances because they are so common among many different substance. Example -- both alcohol and water are colourless, both hydrogen and oxygen are transparent, both aluminum and cadmium are silvery shiny metals, both copper and gold are good conductors of electricity. Physical properties that are unique for different substances are called characteristic physical properties and can be used to identify substances that may look otherwise alike. Example: alcohol has a boiling point of 650C, water has a boiling point of 1000C.
11. A chemical property is a property that describes how a substance will react and/or combine with another substance.
How to tell if a change is chemical or physical:
- Irreversible (generally)
- New substance forms (gas, solid precipitate, change in colour..)
- Energy is visibly given off (change in temperature -- heat, sound , light..)
- A new compound forms
Examples: Frying an egg, Toasting bread
- A change of state or form
- Reversible (usually)
- No new substance forms
- Energy change may not be noticed
Examples: dissolving salt or sugar in water