Chemical changes

Question 1

What are the differences between mixtures and compounds?

Answer 1

[Separation]
- The components of a mixture can be separated by physical methods (filtration, distillation, chromatography).
- The elements in a compound can only be separated by chemical reactions or by using electricity.

[Properties]
- The chemical properties of a mixture are the same as
those of its components.
- The physical and chemical properties of a compound are different from those of the elements in the compound.

[Energy change]
- No chemical reaction takes place when a mixture is formed, usually there is little or no energy change.
- A chemical reaction takes
place when a compound is formed usually there is an energy change.

[Composition (proportion by mass)]
- The components of a mixture can be mixed in any proportion.
- The elements in a compound are always combined in a
fixed proportion (by mass).

Question 2

What are the factors of physical changes?

Answer 2

• No new substances are formed.
• No or little heat is taken in or given out.
• Properties of substance remain the same.
• Reversible.

Question 3

What are the factors of chemical changes?

Answer 3

• New substances are formed
  (products).
• Heat and/or light energy may be taken in or given out.
• Properties of products are different from starting reactants
  mostly not reversible.

Question 4

What are the different types of chemical changes?

Answer 4

(CCCPOFTPEP)
- Combination
- Combustion
- Cellular respiration
- Oxidation
- Film photography
- Thermal decomposition
- Photosynthesis
- Electrolysis
- Precipitation reaction

Question 5

Describe combustion.

Answer 5

It is a chemical reaction in which a substance is heated in the presence of oxygen to form one or more new substances. Heat and light are usually given out during combustion.

Question 6

What is an example of combustion?

Answer 6

• An example of combustion is the burning of fuels such as coal, natural gas, petrol and diesel.

Question 7

Explain complete combustion and provide an example.

Answer 7

A substance containing carbon burns in sufficient amount of oxygen and carbon dioxide is produced.

methane + oxygen = carbon dioxide + water

Question 8

Explain incomplete combustion and provide an example.

Answer 8

A substance containing carbon burns in an insufficient amount of oxygen and carbon monoxide or soot is produced.

methane + oxygen = carbon monoxide + water

Question 9

Describe thermal decomposition.

Answer 9

Thermal decomposition is a process in which a substance is broken down into two or more simpler substances upon heating.

Question 10

State an example of thermal decomposition.

Answer 10

Sugar breaks down into carbon and water vapour when heated for too long.

Question 11

Explain photosynthesis.

Answer 11

The chlorophyll in the green leaves absorbs light energy to convert carbon dioxide and water into glucose.

carbon dioxide + water — (sunlight, chlorophyll) —> glucose + oxygen

Question 12

Describe oxidation.

Answer 12

Oxidation refers to a chemical change in which a substance gains oxygen.

Question 13

What are the two other processes where oxidation occurs?

Answer 13

• Combustion
• Cellular respiration

Question 14

Describe film photography.

Answer 14

Silver bromide decomposes to form silver and bromine when it is exposed to light. Silver bromide is used to coat X-ray films or black and white photogenic films. When taking photos or X-rays, exposure of the film to light decomposes the silver bromide to silver, forming the dark areas on the film.

silver bromide = silver + bromine

Question 15

Describe precipitation reaction and provide an example.

Answer 15

When certain solutions are mixed, an insoluble solid, called a precipitate, is produced.

When sodium hydroxide solution is added to copper sulfate solution, a precipitate of copper hydroxide is formed.

copper sulfate + sodium hydroxide = copper hydroxide + sodium sulfate

Question 16

What are some organic acids (weak acids)?

Answer 16

• Tartaric acid (grapes)
• Citric acid (oranges)
• Malic acid (Apple)

Question 17

What are some mineral acids (strong acids)?

Answer 17

• Hydrochloric acid
• Nitric acid
• Sulfuric acid

Question 18

What are the properties of acids?

Answer 18

• Acids have sour taste
• Acids turn blue litmus paper red
• Acids has a pH value less than 7
• Acids are corrosive
• Acids dissolve in water to form solutions which can conduct electricity (electrolytes)
• Acids react with metals, carbonates and bases/alkalis

Question 19

What are the properties of alkali?

Answer 19

• They have a bitter taste
• They have a slippery or soapy feel
• They turn red litmus paper blue
• They are corrosive
• They can conduct electricity
• They react with acid to produce salt and water (neutralisation)

Question 20

What is neutralisation?

Answer 20

When alkali react with acids, a chemical reaction called neutralisation occurs. Water is always formed.

Question 21

What is produced when acids react with metals?

Answer 21

Dilute acid + metal = salt + hydrogen

Question 22

How do we test for hydrogen gas produced in a reaction?

Answer 22

A lighted splint is used for testing hydrogen gas. When a ‘pop’ sound is produced and the lighted splint extinguishes, the presence of hydrogen is confirmed.

Question 23

What is produced when acids react with carbonates?

Answer 23

Dilute acid + carbonate = salt + carbon dioxide + water

Question 24

How do we test for carbon dioxide in a reaction?

Answer 24

Limewater (calcium hydroxide) is used to test for the carbon dioxide produced from the reaction. White precipitate is formed and limewater turns chalky if carbon dioxide is present.

Question 25

What are some applications of neutralisation?

Answer 25

- Indigestion tablets are alkaline to neutralise excess acid in the stomach. - Toothpaste is alkaline to neutralise acids found in the mouth. - Pollutants from burning fossil fuels are often acidic gases and cause acid rain. This can be neutralised by adding lime (acid) to the affected water and soil. - Insect stings inject an alkali beneath our skin. The sting can be neutralised by applying mild acids. However, some stings can be acidic and it needs to be neutralised by a mild alkali.

Question 26

Describe the pH scale and state the pH of acids, neutrals and alkali.

Answer 26

The pH scale is numbered between 0 to 14. It measures how acidic or alkaline a solution is in water. Strong acids have a value of 1 while weak acids have a value of 6, while weak alkali have a value of 8 and strong alkali have a value of 14. Any substance on 7 is neutral.

Question 27

State some common examples of the substances of each pH value from 0 to 14.

Answer 27

0 - car battery acid (sulfuric acid) 1 - hydrochloric acid, Nitric acid, stomach juice 2 - lemon juice, vinegar 3 - grape juice, orange juice 4 - tomato juice, acid rain 5 - soft drinks, black coffee, tea 6 - saliva, piped water, fresh milk 7 - distilled water 8 - seawater 9 - baking soda 10 - milk of magnesia 11 - ammonia solution 12 - soapy water 13 - bleaches, oven cleaner 14 - liquid drain cleaner and detergent (sodium hydroxide)

Question 28

State the colours in acidic, neutral, or alkaline solutions when tested for pH values on litmus paper.

Answer 28

0-6: red 7: remains unchanged 8-14: blue

Question 29

State the colours in acidic, neutral, or alkaline solutions when tested for pH values on Universal Indicator.

Answer 29

0-2: red 3-4: orange 5-6: yellow 7: pale green 8-11: green-blue 12-14: violet

Question 30

State the colours in acidic, neutral, or alkaline solutions when tested for pH values on Methyl orange.

Answer 30

0-2.5: red 2.5-14: yellow (only identifies strong acids)

Question 31

State the colours in acidic, neutral, or alkaline solutions when tested for pH values on Phenolpthalein.

Answer 31

0-8.5: colourless 8.5-14: pink (only identifies strong alkaline)

Question 32

What is an indicator?

Answer 32

An indicator is a substance that changes colour depending on whether the solution being tested is acidic or alkaline.

Question 33

What happens when acid comes to contact with both litmus papers?

Answer 33

The blue litmus paper turns red; the red litmus paper remains red.

Question 34

What happens when alkali comes to contact with both litmus papers?

Answer 34

The blue litmus paper remains blue; the red litmus paper turns blue.

Question 35

What happens when neutral solution comes to contact with both litmus papers?

Answer 35

The blue litmus paper remains blue; the red litmus paper remains red.

Question 36

What is cellular respiration?

Answer 36

Cellular respiration is the process in which living cells of plants and animals take in oxygen to release the energy stored in glucose. The energy is used by cells to carry out different activities. glucose + oxygen = carbon dioxide + water + energy

Question 37

What is electrolysis?

Answer 37

Electrolysis is the chemical decomposition of substances with the passage of an electric current. In electrolysis, an electric current is passed through a liquid which can conduct electricity, known as electrolyte. The electrolyte will decompose into simpler substances.

Question 38

Describe electroplating and how and why it is carried out.

Answer 38

Electroplating is the process in which a substance is coated with metal with the passage of an electric current. The metal which coats the other object is placed at the positive terminal of the power supply whereas the object to be coated with a layer of a metal is placed at the negative terminal of the power supply. It is carried out to - Prevent objects made of iron and steel from rusting - To enhance the appearance of an object such a as making it more shiny

Question 39

Describe the extraction of pure metal from impurities using electrolysis.

Answer 39

The sample of impure metal is placed at the positive terminal of the power supply. A sample of pure metal is placed at the negative terminal of the power supply. The metal from the impure sample will be separated from the impurities and coated onto the pure sample of the metal.

Question 40

What are the benefits of chemical change?

Answer 40

- Cooking food - The decomposition of dead animals breaks down unwanted matter and releases nutrients back to the Earth - Manufacturing various useful products such as plastics, iron, and glass from a variety of raw materials in production industries - Household detergents for cleaning and fertilisers for increasing crop yield - Obtaining metals and alloys from the earth by chemical reactions, and used as structure support in buildings, bridges and vehicles - Production of medicines to treat medical conditions, and insecticides to kill mosquitoes that cause dengue fever - Fuel combustion in engines produces energy to move vehicles

Question 41

What are the harmful effects of chemical change?

Answer 41

- Burning. Burning and combustion in cars and factories release air pollutants, such as soot and toxic gases, in the atmosphere. Gases such as sulfur dioxide in the atmosphere dissolve in the falling rain and lead to acid rain. Acid rain corrodes buildings and other structures. Polluted air leads to difficulty in breathing and increases the risk of respiratory illnesses. Acid rain destroys plants and kills fishes and other aquatic life. - Decomposition Decomposition leads to wastage of food supply and resources.