Physical and Chemical Changes
A Physical change is a change in which the chemical identity or composition of a substance remains the same. There is a change in one or more of the properties of the substance, but no new substances are formed. Examples include wax melting and water freezing.
A Chemical change is a change in which at least one new substance, with entirely different properties, is produced. For example, when a candle burns, carbon dioxide and water vapour are produced.
Evidences of a chemical reaction:
1. A colour change occurs, which is not simply a blending of the initial colours.
2. A gas is produced, which does not result from the boiling of a liquid.
3. A solid (a precipitate) is formed when two liquids are mixed together.
4. Energy (usually in the form of heat or light) is released or absorbed.
These observations are evidence that a chemical change has taken place but they are not conclusive proof. The proof that a chemical change has occurred is that at least one new substance has been formed.
Chemical changes occur all around us in industry, in our homes, our bodies, in the automobile engine, and in the environment. Some of the changes that occur in cooking and baking are chemical changes. When milk goes sour, bleaching of clothes, the burning of gasoline, the digestion of food, and the formation of acids that cause acid rain are examples of chemical change. In each case at least one new substance is produced.
Describe the following as physical or chemical changes.
a) the disappearance of mothballs
b) the formation of clouds
c) the baking of a cake
d) the burning of leaves
e) the formation of ice cubes
f) the explosion of dynamite
Physical and Chemical Properties
Physical properties can be determined without changing the chemical composition of the material. Colour, odour, electrical conductivity, density, and melting / boiling point are physical properties.
Chemical properties of matter describe how matter behaves in the presence of other substances or when subjected to heat, light, or electricity. For example, a chemical property of baking soda is that when it is heated, it produces carbon dioxide. A chemical property of hydrochloric acid is that it reacts with magnesium to produce hydrogen gas. A chemical change must take place in order for the chemical properties of a substance to be studied.
Identify each to the following as physical or chemical properties of sodium chloride (table salt).
a) it is soluble in water
b) it melts at 801 ēC
c) it reacts with sulphuric acid to produce HCl gas
d) it forms cubic crystals
e) it possesses a distinctive salty taste
f) it has a density of 2.2 g/cm3
g) electric current decomposes into sodium metal and chlorine gas when melted
The Classification of Matter
Classify each of the following as an element, compound, solution, or mechanical mixture.
b) clear tea
c) polluted air
d) tomato juice
e) sulfur dioxide
g) windshield washer fluid
i) lawn fertilizer