How do we identify a chemical change
Physical changes:
- No new substances are formed
- The changes can usually be reversed. We can get the constituents back using separation
techniques
ni
Chemical changes:
- One or more substances are formed
hi
- Substances that react with each other are known as reactors
- Substances that are formed are products
- The changes are usually not easily reversed. We cannot get the reactants back
ut
Representing chemical reactions :
sr
sh
Rearrangement of atoms in a chemical reaction :
e
- Atoms are neither created nor destroyed in a chemical reaction. They rearrange themselves
- The total number of atoms before and after a chemical reaction is balanced
an
- No atoms are lost or formed in a chemical reaction
What are the different types of chemical changes
Combustion : A substance is heated in the presence of oxygen to form one or more new
llig
substances. Heat and light are usually given out during combustion. The condition is oxygen
and not heat
Thermal Decomposition : A substance breaks down into two or more simpler substances upon
heating. For example, sugar when heated melts first (physical change). Afterwards it turns black
de
(chemical change) which is carbon + water vapour
Oxidation : When a substance gains oxygen, it undergoes a chemical change called oxidation
E.g Rusting —> Iron / Steel reacts with oxygen and water (water vapour in the air). Iron +
Oxygen —> Iron (III) oxide
, Cellular respiration —> Food molecules such as glucose are broken down in the presence of
oxygen to provide cells with energy to survive and reproduce. Carbon dioxide and water are
produced. Energy is released thus is not a product. Glucose and oxygen are the reactors
Neutralisation :
- When an acid is mixed with alkali a neutralisation reaction occurs
ni
- Acids
● Sour in taste
● Corrosive
hi
● Turns blue litmus paper red
● React with alkalis, metals and carbonates
● Citric acid in citrus fruits and hydrochloric acid
- Alkalis
ut
● Bitter in taste
● Corrosive
● Soapy feel
sr
● Turns red litmus paper blue
● Reacts with acids in neutralisation reactions
● Sodium hydroxide and baking soda
- Reactions of Acids
sh
● (Neutralisation) Acid + Alkali —> salt + water where only water remains the same and
others can differ
➢ Different acids react with different alkalis to form different salts
➢ However in all neutralisation reactions water is always formed as a product
e
● Acid + Metal —> salt + hydrogen where only hydrogen remains the same and the others
can differ
an
➢ Gold tested for by using acid test
➢ Not all matters react with an acid in the same manner
➢ Some metals like copper, silver, gold do not react with acids
➢ To test for hydrogen, we can use a lighted splint. If there is hydrogen, it will extinguish
with a pop sound
llig
● Acid + Carbonate —> salt + carbon dioxide + water where carbon dioxide and water
remain the same and the others can differ
➢ We can use limewater to test for Carbon dioxide.
➢ Limewater will cause white precipitate (cannot say chalky or cloudy) ..to form if Carbon
de
dioxide is present
➢ Sodium is salt
- Acidity or Alkalinity of a Solution
● pH scales
➢ Scale range is from 0-14
Physical changes:
- No new substances are formed
- The changes can usually be reversed. We can get the constituents back using separation
techniques
ni
Chemical changes:
- One or more substances are formed
hi
- Substances that react with each other are known as reactors
- Substances that are formed are products
- The changes are usually not easily reversed. We cannot get the reactants back
ut
Representing chemical reactions :
sr
sh
Rearrangement of atoms in a chemical reaction :
e
- Atoms are neither created nor destroyed in a chemical reaction. They rearrange themselves
- The total number of atoms before and after a chemical reaction is balanced
an
- No atoms are lost or formed in a chemical reaction
What are the different types of chemical changes
Combustion : A substance is heated in the presence of oxygen to form one or more new
llig
substances. Heat and light are usually given out during combustion. The condition is oxygen
and not heat
Thermal Decomposition : A substance breaks down into two or more simpler substances upon
heating. For example, sugar when heated melts first (physical change). Afterwards it turns black
de
(chemical change) which is carbon + water vapour
Oxidation : When a substance gains oxygen, it undergoes a chemical change called oxidation
E.g Rusting —> Iron / Steel reacts with oxygen and water (water vapour in the air). Iron +
Oxygen —> Iron (III) oxide
, Cellular respiration —> Food molecules such as glucose are broken down in the presence of
oxygen to provide cells with energy to survive and reproduce. Carbon dioxide and water are
produced. Energy is released thus is not a product. Glucose and oxygen are the reactors
Neutralisation :
- When an acid is mixed with alkali a neutralisation reaction occurs
ni
- Acids
● Sour in taste
● Corrosive
hi
● Turns blue litmus paper red
● React with alkalis, metals and carbonates
● Citric acid in citrus fruits and hydrochloric acid
- Alkalis
ut
● Bitter in taste
● Corrosive
● Soapy feel
sr
● Turns red litmus paper blue
● Reacts with acids in neutralisation reactions
● Sodium hydroxide and baking soda
- Reactions of Acids
sh
● (Neutralisation) Acid + Alkali —> salt + water where only water remains the same and
others can differ
➢ Different acids react with different alkalis to form different salts
➢ However in all neutralisation reactions water is always formed as a product
e
● Acid + Metal —> salt + hydrogen where only hydrogen remains the same and the others
can differ
an
➢ Gold tested for by using acid test
➢ Not all matters react with an acid in the same manner
➢ Some metals like copper, silver, gold do not react with acids
➢ To test for hydrogen, we can use a lighted splint. If there is hydrogen, it will extinguish
with a pop sound
llig
● Acid + Carbonate —> salt + carbon dioxide + water where carbon dioxide and water
remain the same and the others can differ
➢ We can use limewater to test for Carbon dioxide.
➢ Limewater will cause white precipitate (cannot say chalky or cloudy) ..to form if Carbon
de
dioxide is present
➢ Sodium is salt
- Acidity or Alkalinity of a Solution
● pH scales
➢ Scale range is from 0-14
