chapter 18 chemical changes

Question 1

physical changes are _______ but chemical changes are usually _______

Answer 1

reversible

irreversible

Question 2

when does chemical changes happen?

Answer 2

• occurs when matter interacts with heat, light or electricity
• also occurs when two or more matter interacts

Question 3

what is produced when acids react with:

1. metals
2. carbonates

Answer 3

1. salt and hydrogen gas

2. salt, carbon dioxide gas and water

Question 4

what is thermal decomposition? and what does sugar break down to when heated for too long?

Answer 4

a process in which a substance is broken down into two or more simpler substances by the effect of heat
heat
sugar&raquo_space;»> carbon + water vapour

Question 5

what is oxidation? and when does it occur?

Answer 5

• a process where a substance reacts with oxygen

- occurs during combustion and cellular respiration

Question 6

what is combustion? and what involves combustion?

Answer 6

• the combination of substance with oxygen in the presence of heat.
• one or more substance may be produced
• combustion usually releases high amounts of energy
• burning of fuels such as coal, petrol, diesel and natural gas

Question 7

identify the three sections of the word equation.

carbon + oxygen ———-> carbon dioxide + water
______________heat

Answer 7

reactants: carbon and oxygen
conditions: heat
product: carbon dioxide and water

Question 8

give an example of thermal decomposition.

Answer 8

• when producing glass, the process of thermal decomposition of limestone is impt
• limestone is mainly made of calcium carbonate
• when heated, the calcium carbonate breaks down to calcium oxide and carbon dioxide

Question 9

what causes rust?

Answer 9

when an object made of steel or iron is reacts with oxygen and moisture or water
- rusting is an oxidation process

Question 10

what is cellular respiration and why?

Answer 10

• a process which living cells in plants and animals take in oxygen to release energy stored in glucose
• during cellular respiration, oxygen and glucose reacts in the cells to form carbon dioxide and water with the release of energy
• in order for our body to get energy from the food that we eat, our body cells need to carry out cellular respiration

Question 11

what is photosynthesis and what is the word equation for it?

Answer 11

• the process where green plants make food in the presence of light
  -during photosynthesis, green plants use the energy from sunlight to make food in the leaves
  carbon dioxide + water —————>glucose + oxygen
  ……………………………..sunlight& chlorophyll

Question 12

what is x ray and how does it work?

Answer 12

• x rays are similar to light rays
• x ray films consists chemicals that are coated on a thin plastic base
  >chemicals that contain bromide crystals that are converted to silver by x rays or light

reactant: silver bromide
condition: light/ x ray
products: silver and bromine

Question 13

what is sulfur oxide and where is it usually produced?

Answer 13

• sulfur oxide is burning sulfur in air
• colourless and pungent gas with a choking smell
• coal and oil naturally contain sulfur and are often burnt in factories and power stations > combustion of sulfur

Question 14

what are some examples of interactions between matter and electricity?

Answer 14

• electrolysis

- electroplating

Question 15

what is electrolysis?

Answer 15

-chemical decompositions of substances with the passage of an electrical current
>when an electrical current passes through substances, the substances are broken down to simpler substances

Question 16

What is electroplating and what is it used for?

Answer 16

• a process in which a substances is coated with metal through the passage of an electric current
• objects like coins and trophies are coated with a thin layer of metal to look more attractive
• the metal layer prevents coins and trophies’ from rusting as well

Question 17

what are some examples of acids?

Answer 17

• citric acid: in limes, lemons and oranges
• sulfuric acid: used in car batteries
• hydrochloric acid: used to remove rust on iron and steel

Question 18

what are the properties of alkalis and what is it found in?

Answer 18

-the presence of alkalis in solutions causes it to taste bitter and have a slippery feel, found in soaps and liquid detergents
sodium hydroxide: making soaps and detergent
ammonia solution: used for fertilizers

Question 19

What is neutralisation and what is the word equation for it?

Answer 19

-when alkalis react with acid, a chemical reaction, neutralisation occurs
acid+ alkali > salt and water

Question 20

what is an example of neutralisation?

Answer 20

• acids is formed when bacteria ‘feed’ on food particles present on our teeth > tooth decay
• toothpaste contains alkali
• alkali in toothpaste reacts with the acid in our teeth >produces harmless products, salts and water
• acids neutralised by alkali to prevent tooth decay