Chemistry - Chapter 1 & 2

Question 1

What are the apparatus used for measuring volume & their functions?

Answer 1

Volumetric flask: accurate fixed volumes that are large (vary in sizes)
Beaker: stores liquid (least accurate)
Conical flask: to stir solutions
Measuring cylinder: range of volumes to the nearest 0.5cm3 (more accurate than beaker but not that accurate)
Pipette: measure specific volumes (10, 25) Most accurate
Burette: long tube with tap; range of volumes. (precise - 2d.p.)

Question 2

What are the methods for collecting gases?

Answer 2

Water displacement, downward delivery and upward delivery.

Question 3

What is water displacement for collecting gases?

Answer 3

• for insoluble to slightly soluble in water
• e.g. hydrogen, oxygen, carbon dioxide
• delivery tube is inserted into water and gas travels up a gas jar and is trapped inside

Question 4

What is downward delivery for collecting gases?

Answer 4

• for gases denser than air
• e.g. chlorine, hydrogen chloride, sulfur dioxide
• gas is inserted into a gas jar through a delivery tube and remains at the bottom due to being denser than air

Question 5

What is upward displacement for collecting gases?

Answer 5

• for gases less dense than air
• e.g. ammonia (NH3)
• gas inserted into upside-down gas jar through delivery tube and floats on top of the air, contained in the jar

Question 6

What are the drying agents for drying/dehydrating gases?

Answer 6

Concentrated sulfuric acid, quicklime (calcium oxide), fused calcium chloride

Question 7

Concentrated sulfuric acid for drying gases

Answer 7

• can be used by most gases, e.g. chlorine, hydrogen chloride
• not for gases that react with sulfuric acid (ammonia)
• water is removed from the gas (view textbook page)

Question 8

Calcium oxide (base) for drying gases

Answer 8

• e.g. ammonia
• absorbs moisture and CO2 from the air; has to be freshly heated
• not for acids as it will neutralise the base

Question 9

Fused (melted together) calcium chloride for drying gases

Answer 9

• hydrogen, nitrogen, carbon dioxide
• freshly heated before use
• not for gases that react with it (e.g. ammonia)

Question 10

Using magnetic attraction to separate substances in mixtures

Answer 10

To separate magnetic solids from non-magnetic solids. e.g. iron, cobalt, nickel, some alloys.
e.g. recycling plants

Question 11

Using sieving to separate substances in mixtures

Answer 11

To separate solids with different particle sizes. e.g. archeology, baking.

Question 12

Using suitable solvents to separate substances in mixtures

Answer 12

A suitable solvent can separate solid-solid mixtures in which only one of them is soluble in the solvent. (temperature can affect rate of dissolving)

Question 13

Using sublimation to separate substances in mixtures

Answer 13

Can be used to separate a substance that changes from the solid to gaseous state directly. For a mixture of solids where one sublimes on heating. The substance that undergoes sublimation is called the sublimate.

Question 14

Using filtration to separate substances in mixtures

Answer 14

Can be used to separate insoluble solids from liquids, where the liquid that passes through the filter paper is the filtrate and the remaining solid is the residue.

Question 15

Using evaporation to dryness to separate substances in mixtures

Answer 15

Used to separate a dissolved solid from its solvent by heating the mixture until all solvent has vapourised.

Question 16

Using crystallisation to separate substances in mixtures

Answer 16

To obtain a pure solid from its saturated solution (where no more solute can be dissolved).
It is a gentler method than evaporation.
The solution is gently heated until most of the solvent has evaporated, until the solution is saturated. The solution is cooled until crystals appear. They are collected through filtration. They are then washed with cold distilled water to remove impurities, and dried between sheets of filter paper.

Question 17

Using simple distillation to separate substances in mixtures

Answer 17

To separate a pure solvent from a solution.

Question 18

Using separating funnel to separate substances in mixtures

Answer 18

To separate immiscible (cannot be mixed together) liquids.
A heterogeneous mixture is left to settle is separate layers and is removed component by component through a tap at the bottom and collecting them in separate flasks.

Question 19

Using chromatography to separate substances in mixtures

Answer 19

To separate a mixture of substances which have different solubilities in a given solvent (miscible liquids - homogeneous mixture)
The higher the substance travels, the more soluble it is.
The Rf factor is the retention factor. It is the ratio of the distance a substance moves compared to the solvent front.
The formula for the Rf value is: distance travelled by substance/distance travelled by solvent
To locate colourless substances, a locating agent is used, which are chemicals that react with it to form coloured spots, or ultraviolet light.

Question 20

Using fractional distillation to separate substances in mixtures

Answer 20

To separate miscible liquids with different boiling points. E.g. mixture of water and ethanol, where the water condenses (not at boiling point) in the fractionating column and falls back into the flask, whereas ethanol continues to rise as it does not condense once it reaches the boiling point unless cooled. To avoid contaminating the distillate with the other substance, the conical flask has to be removed once the thermometer reading rises above the boiling point of the initial distillate.

Question 21

How to determine the purity of a substance?

Answer 21

A pure substance has a specific melting and/or boiling point under fixed conditions.

Question 22

(KPT) What is sublimation?

Answer 22

Substances changing from solid to gas. Thermal energy from surroundings is transferred to the solid. The temperature remains constant until the substance is in the gaseous state.

Question 23

(KPT) What is vapour deposition?

Answer 23

The particles in the gas are cooled until they slow down and arrange themselves directly into the solid state. E.g. iodine vapour comes into contact with a cool surface and immediately solidifies.

Question 24

(KPT) What is diffusion?

Answer 24

Diffusion is the process by which particles move freely to fill up any available space. It is the process by which particles move from a region of high concentration to a region of low concentration.

Question 25

(KPT) What are conditions that affect the rate of diffusion?

Answer 25

Higher temperature –> faster rate of diffusion
Less mass –> faster rate of diffusion