chemical changes

Question 1

what is a physical change?

Answer 1

a physical change is a change in which no new substances are formed.

Question 2

what is a chemical change?

Answer 2

a chemical change is a change in which new substances (in the form of elements and/or compounds) are formed. when a chemical change occurs, it means that a chemical reaction has taken place.

Question 3

what are reactants?

Answer 3

they are substances that react with each other (found on the left side of a chemical equation)

Question 4

what are products?

Answer 4

they are the new substances formed by the reactants (found on the right side of a chemical equation)

Question 5

what are the two groups of chemical reactions?

Answer 5

1. a reaction in which two or more substances chemically combine to form one or more new substances.
2. reactions in which a substance breaks down to form two or more new substances.

Question 6

what are some characteristics of chemical changes?

Answer 6

1. new substances (elements/compounds are formed)
2. chemical changes are usually not easily reversed. the reactants cannot be obtained easily from the products. it usually possible to do so only by using large amounts of energy.
3. energy is sometimes taken in or given out during a chemical reaction.

Question 7

what are some characteristics of a physical change ?

Answer 7

1. no new substances are formed

2. most physical changes are reversible.

Question 8

explain the rearrangement of atoms in a chemical change

Answer 8

during a chemical reaction, atoms of the reactants rearrange themselves and combine chemically together to form products. the total number of atoms before and after a chemical reaction is balanced. this means the number of each type of atom remains the same during the reaction. atoms of different reactants combine together in different ways to form products, but no atoms are lost or formed in a reaction. thus, the mass of the reactants is equal to the mass of the products. we say that mass is conserved during a chemical reaction.

Question 9

what is combustion?

Answer 9

combustion is the 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 give out during combustion.

Question 10

what are some examples of combustion?

Answer 10

• sparklers release a lot of heat and light when they burn.
• charcoal burns in a barbecue pit in oxygen to form carbon dioxide gas.
• when petrol undergoes combustion in the engine of a moving car, chemical potential energy is converted to kinetic energy.

Question 11

what is thermal decomposition?

Answer 11

thermal decomposition is the process in which a substance breaks down into two or more simpler substances upon heating.

Question 12

what is an example of thermal decomposition?

Answer 12

when calcium carbonate is heated, it breaks down to form calcium oxide and carbon dioxide.

Question 13

what are some examples of oxidation?

Answer 13

1. rusting. this process occurs when iron reacts with water and oxygen to form rust, a reddish-brown flaky solid.
2. cellular respiration. food molecules, such as glucose, are broken down in the presence of oxygen to provide cells with energy to survive and reproduce. during respiration, water and carbon dioxide are produced.

Question 14

what is oxidation?

Answer 14

matter is able to interact with oxygen. when a substance gains oxygen, it undergoes a process called oxidation.

Question 15

what are the characteristics of acids?

Answer 15

• have a sour taste
• may be dangerous as some acids can burn the skin (corrosive)
• turns moist blue litmus paper red
• reacts with alkalis, metals and carbonates

Question 16

what are some characteristics of alkalis?

Answer 16

• have a bitter taste
• has a soapy feel
• may be dangerous as some alkalis can burn the skin (corrosive)
• turns moist red litmus paper blue
• reacts with acids in neutralisation reactions

Question 17

what are the three common chemical reactions that occur between acids and other substances? + their products

Answer 17

1. an acid reacts with an alkali in a neutralisation reaction to form a salt and water.
2. an acid can react with a metal to form a salt and hydrogen gas
3. an acid can react with a carbonate to form a salt, carbon dioxide gas and water.

Question 18

how do we test for hydrogen gas in a chemical reaction?

Answer 18

we can use a lighted splint. it will extinguish with a ‘pop’ sound in the presence of hydrogen gas.

Question 19

how do we test for carbon dioxide gas produced in a chemical reaction?

Answer 19

we can use limewater. white precipitate will form in the limewater and it will turn cloudy or chalky if carbon dioxide gas is bubbled through it.

Question 20

describe the pH scale

Answer 20

we can use the pH scale to measure how acidic or alkaline a solution is. the values of the pH scale range from 0 to 14. solutions with pH values less than 7 are acidic. the lower the pH value, the more acidic the solution. solution with pH values greater than 7 are alkaline. the greater the pH value, the more alkaline the solution is. if a solution is neither acidic or alkaline, we say that it has a neutral pH or a pH value of 7.

Question 21

what takes place during neutralisation?

Answer 21

when an alkali is added to an acid, the pH value of the solution increases as the alkali neutralises the acid. conversely, when an acid is added to an alkali, the pH value of the solution decreases. upon complete neutralisation, the solution has a pH value of 7, and is neither acidic nor alkaline.

Question 22

how is neutralisation applied?

Answer 22

• antacids are common medications that contain alkaline compounds. they can help to neutralise excess acids produced by the stomach and thus relieve discomfort.
• bacteria present on our teeth produce acids which can cause tooth decay. toothpaste is weakly alkaline and can help to neutralise any acids that is present in the mouth.
• soil that is too acidic or alkaline may not allow some plants to grow well. this can be resolved by adding some substances to the soil for neutralisation to take place.

Question 23

how do chemical changes occur?

Answer 23

1. mixing. when two or more reactants are mixed together, they combine irreversibly to form one or more products, resulting in a chemical change.
2. heating. heat or an increase in temperature is needed for chemical reactions such as combustion and thermal decomposition to take place.
3. exposure to light. photosynthesis is a chemical reaction that occurs in green plants. in the presence of light, green plants use chlorophyll in their leaves, carbon dioxide and water to produce glucose and oxygen. colours in clothes can fade over time when they are exposed to sunlight. this is due to a chemical reaction caused by ultraviolet (UV) rays in sunlight.
4. interaction with oxygen. glucose oxidises to form energy.

Question 24

what happens during electroplating ?

Answer 24

during the electroplating of silver with gold, and electric current is first passed through a solution containing gold particles. the electric current flows through the solution from one point (i.e. the gold) to another point (i.e. the silver). this allows the gold particles to coat the silver layer by layer. as a result, a gold-plated object is obtained.

it is possible to coat objects with different metals by changing the type of solution used during electroplating.

Question 25

how does the knowledge of chemical reactions help us with cooking?

Answer 25

cooking causes chemical reactions in food. heat from cooking softens the food and improves its flavour. cooking also makes it easier for our body to absorb nutrients from food, although some nutrients may be destroyed during the process.

Question 26

what happens during respiration?

Answer 26

during respiration, glucose molecules reacts with oxygen to produce carbon dioxide and water. the energy released allows each cell in an organism to survive and reproduce. respiration is therefore an important process that releases energy.

Question 27

what is decay?

Answer 27

decay, or decomposition, is the process of breaking down organic matter into simpler substances. it allows nutrients to be returned to the environment.

Question 28

how do we slow down decay in food?

Answer 28

one method of slowing down the decay of food is vacuum packaging. it involves removing the air from a package prior to sealing. this reduces the amount of oxygen in the package and limits the growth of bacteria and other microorganisms.

Question 29

what are some ways to prevent rusting?

Answer 29

- painting - oiling - galvanising, which involves coating iron with a protective layer of zinc - drying these methods prevent iron or steel objects from coming into contact with water or oxygen. this prevents rust from forming.

Question 30

what are the sources and effects of some air pollutants? (combustion)

Answer 30

carbon monoxide - source: burning of fuels in factories or motor vehicles in the presence of insufficient oxygen effects: a colourless, odourless and highly toxic gas. it reduces the amount of oxygen taken up by red blood cells, thus leading to headaches and eventually, death due to suffocation sulfur dioxide - sources : 1. burning of sulfur-containing gases such as petroleum and coal in power station and factories. 2. volcanic eruptions effects: 1. can irritate the eyes, nose and lungs and worsen the symptoms of asthma, heart disease and respiratory problems, even at low levels. 2. can dissolve in rainwater to form acid rain, which harms aquatic life and damages buildings oxides of nitrogen - sources: 1. burning of fossil fuels at high temperatures in motor vehicles, factories and power stations. 2. reaction of nitrogen and oxygen in the air during lightning strikes effects: 1. can irritate eyes, nose and lungs and worsens symptoms of asthmas, heart disease and respiratory problems, even at low levels. 2. dissolves in rainwater to form acid rain, which harms aquatic life and damages building.

Question 31

what are some negative impacts of the slash-and-burn practice to clear forests?

Answer 31

the smoke give off from burning can cause air pollution and harmful health problems when inhaled. removing forests increases the amount of carbon dioxide in the atmosphere as fewer trees absorb carbon dioxide. the large amount of carbon dioxide released traps heat within the atmosphere, resulting in global warming. the carbon dioxide released also causes acidification of oceans.