Physical and Chemical Changes

Question 1

What is a particle model?

Answer 1

The particle model is a simplified representation of solids, liquids and gases and their properties.
The particle model states that all forms of matter (solids, liquids and gases) are made up of invisible, ball- like particles that are:
* hard, incompressible (not able to be squashed) and indivisible (unable to be split)
* attracted to each other
* constantly moving.

Question 2

Explain expansion and contraction as a physical change.

Answer 2

In solids, the particles vibrate in fixed positions. As the temperature increases, so do the vibrations—pushing the particles further apart and causing the solid to expand (get larger).
When the solid is cooled, the reverse happens. The particles vibrate less, allowing them to be packed more tightly and the solid contracts (shrinks).

Question 3

What is boiling?

Answer 3

a change of state where a liquid is heated and changes to a gas within the liquid (happens at 100 degrees Celsius).

Question 4

What is a chemical change

Answer 4

a change that results in a new substance being formed

Question 5

What is condensation?

Answer 5

a change of state where a gas is cooled and forms a liquid

Question 6

Contraction

Answer 6

a decrease in size

Question 7

What is a deposition?

Answer 7

a change of state
from gas to solid

Question 8

What is evaporation

Answer 8

the change of state where a liquid changes to a gas at the surface of the liquid

Question 9

What is expansion

Answer 9

an increase in size

Question 10

What is freezing

Answer 10

the change of state
from liquid to solid

Question 11

What is melting

Answer 11

the change of state
when a solid is heated and
forms a liquid

Question 12

What is physical change

Answer 12

a change
that does not result in a new
substance being produced

Question 13

Precipitate means…

Answer 13

a solid formed during a
chemical change

Question 14

Solidification or freezing means

Answer 14

the change
of state from liquid to solid

Question 15

Solution is

Answer 15

a mixture where the
substances are mixed on the
atomic level

Question 16

Sublimation is

Answer 16

a change of state from
solid to gas

Question 17

Diffusion is

Answer 17

a process where two liquids
or two gases mix due to the motion
of their particles (perfume gas with air)

Question 18

Particle model in simple terms is

Answer 18

a simplified
representation of solids, liquids
and gase

Question 19

Chemical reaction is

Answer 19

when atoms rearrange to form new
substances

Question 20

Molecule is a

Answer 20

a cluster of atoms

Question 21

List four examples of physical changes

Answer 21

Physical Changes
Melting Ice: Ice melting into water.
Boiling Water: Water boiling and turning into steam.
Tearing Paper: Cutting or tearing a piece of paper into smaller pieces.
Dissolving Sugar: Dissolving sugar in water.
Chemical Changes
Rusting Iron: Iron reacting with oxygen to form rust.
Burning Wood: Wood burning in a fireplace to produce ash and smoke.
Baking a Cake: Ingredients like flour, sugar, and eggs combining and reacting to form a cake.
Digesting Food: Enzymes breaking down food in the stomach to produce energy and nutrients.

Question 22

List four examples of chemical changes

Answer 22

Chemical Changes
Rusting Iron: Iron reacting with oxygen to form rust.
Burning Wood: Wood burning in a fireplace to produce ash and smoke.
Baking a Cake: Ingredients like flour, sugar, and eggs combining and reacting to form a cake.
Digesting Food: Enzymes breaking down food in the stomach to produce energy and nutrients.

Question 23

List four things that indicate a chemical change has
occurred

Answer 23

1. Color Change: A substance changes color, such as when iron rusts or leaves turn brown in autumn.
2. Formation of a Gas: Bubbles or fumes are produced, like when vinegar reacts with baking soda.
3. Temperature Change: The temperature of a substance increases or decreases without external heating or cooling, such as when a hand warmer pack is activated.
4. Formation of a Precipitate: A solid forms from a solution during a chemical reaction, like when mixing solutions of silver nitrate and sodium chloride to form silver chloride.

Question 24

List the changes of state that occur as you heat a
solid to a gas and cool a gas to a solid.

Answer 24

Heating a Solid to a Gas
Melting: Solid to liquid (e.g., ice melting to water).
Boiling (or Evaporation): Liquid to gas (e.g., water boiling to steam).

Cooling a Gas to a Solid
Condensation: Gas to liquid (e.g., steam condensing to water).
Freezing (or Solidification): Liquid to solid (e.g., water freezing to ice).

For a direct change from solid to gas and vice versa, the terms are:
Sublimation: Solid directly to gas (e.g., dry ice sublimating to carbon dioxide gas).
Deposition: Gas directly to solid (e.g., frost forming from water vapor).

Question 25

Discuss whether a kilogram of feathers has the same
mass as a kilogram of rocks.

Answer 25

A kilogram of feathers and a kilogram of rocks both have the same mass: 1 kilogram. Mass is a measure of the amount of matter in an object, and it does not change regardless of the material. Therefore, a kilogram of feathers and a kilogram of rocks each have a mass of 1 kilogram.

Question 26

Use the particle model to describe what happens to
the particles of a solid as it is heated to form a liquid
and then heated further to form a gas

Answer 26

Solid to Liquid (Melting)
1. Particle Arrangement: In a solid, particles are closely packed in a fixed, orderly arrangement. They vibrate in place but do not move around.
2. Heating Effect: As the solid is heated, the particles gain energy. This added energy increases the vibrations of the particles.
3. Breaking Bonds: When the temperature reaches the melting point, the energy is sufficient to overcome some of the intermolecular forces holding the particles in their fixed positions.
4. Particle Movement: The particles begin to move more freely, sliding past each other, which causes the solid to turn into a liquid. The liquid has a definite volume but takes the shape of its container.

Liquid to Gas (Boiling or Evaporation)
1. Particle Arrangement: In a liquid, particles are still close together but are not in a fixed arrangement. They can move around each other, allowing the liquid to flow.
2. Heating Effect: As the liquid is heated further, the particles gain even more energy.
3. Overcoming Intermolecular Forces: The increased energy allows the particles to overcome the intermolecular forces that are keeping them in the liquid state.
4. Particle Movement: When the temperature reaches the boiling point, the particles have enough energy to break free from the liquid’s surface and escape into the air as a gas. In the gas phase, the particles are far apart, move rapidly in all directions, and occupy the entire volume of their container.

In summary, as a solid is heated:
- Melting: Particles gain enough energy to move from a fixed position in a solid to moving more freely in a liquid.
- Boiling: Particles gain even more energy to move from the liquid state to a gas, where they spread out and move independently.

Question 27

Describe exothermic reactions in terms of energy balance

Answer 27

Exothermic Reactions
Energy Release: In an exothermic reaction, energy is released to the surroundings. This means that the total energy of the products is lower than the total energy of the reactants.
Energy Balance: The energy released during the formation of products is greater than the energy required to break the bonds in the reactants. This excess energy is released as heat, light, or other forms of energy.
Example: Combustion of a fuel (like burning wood or gasoline) is an exothermic reaction. For instance, when methane (CH₄) burns in the presence of oxygen (O₂), it releases energy in the form of heat and light:

Question 28

Describe endothermic reactions in terms of energy balance

Answer 28

Endothermic Reactions
Energy Absorption: In an endothermic reaction, energy is absorbed from the surroundings. This means that the total energy of the products is higher than the total energy of the reactants.
Energy Balance: The energy required to break the bonds in the reactants is greater than the energy released during the formation of the products. Therefore, additional energy is absorbed from the surroundings to make up the difference.
Example: Photosynthesis in plants is an endothermic reaction. For instance, when carbon dioxide (CO₂) and water (H₂O) react in the presence of sunlight to form glucose (C₆H₁₂O₆) and oxygen (O₂), energy is absorbed from sunlight:

Question 29

Key difference between exothermic and endothermic reactions:

Answer 29

Exothermic Reactions: Release energy to the surroundings, resulting in products with lower energy than the reactants.
Endothermic Reactions: Absorb energy from the surroundings, resulting in products with higher energy than the reactants.
The key difference between the two lies in the direction of energy flow: exothermic reactions release energy, while endothermic reactions absorb energy.

Question 30

Give common examples of gas producing chemical reactions using
simple word equations.

Answer 30

Vinegar and Baking Soda Reaction:

Equation: Vinegar + Baking Soda → Carbon Dioxide + Water + Sodium Acetate

Example: Acetic Acid + Sodium Bicarbonate → Carbon Dioxide + Water + Sodium Acetate