Chemical Analysis

Question 1

What is a pure substance in chemistry?

Answer 1

A single element or compound, not mixed with anything else.

Question 2

How can melting and boiling points be used to determine purity?

Answer 2

Pure substances melt and boil at specific temperatures, while mixtures melt/boil over a range of temperatures.

Question 3

What is an example of a pure substance in everyday language?

Answer 3

A substance with nothing added, like pure milk.

Question 4

What is a formulation?

Answer 4

A carefully designed mixture where each component has a specific purpose.

Question 5

Why are components in formulations mixed in precise quantities?

Answer 5

To ensure the product has the required properties.

Question 6

Give three examples of formulations.

Answer 6

Medicines, fuels, cleaning agents.

Question 7

Why are alloys considered formulations?

Answer 7

They are mixtures of metals, designed for specific properties like strength or resistance to corrosion.

Question 8

What is chromatography used for?

Answer 8

Separating mixtures and helping identify substances.

Question 9

What are the two phases in chromatography?

Answer 9

Stationary phase (does not move) and mobile phase (moves through the stationary phase).

Question 10

How does chromatography separate substances?

Answer 10

Based on how substances distribute between the stationary and mobile phases.

Question 11

What is the Rf value and how is it calculated?

Answer 11

Rf = distance moved by substance / distance moved by solvent.

Question 12

Why are Rf values useful?

Answer 12

Different compounds have different Rf values in different solvents, which helps in identification.

Question 13

How can you tell if a substance is pure using chromatography?

Answer 13

A pure substance produces a single spot in all solvents.

Question 14

Why might a mixture produce multiple spots on a chromatogram?

Answer 14

It contains different compounds, which separate based on solubility and attraction to the paper.

Question 15

What is paper chromatography?

Answer 15

An analytical technique that separates substances by their relative speed in a solvent moving through paper.

Question 16

What determines how far a substance moves in paper chromatography?

Answer 16

Solubility – more soluble substances travel further up the paper.

Question 17

What kind of substances can be separated using paper chromatography?

Answer 17

Pigments, inks, dyes, and food colorings.

Question 18

What is a pigment?

Answer 18

A solid, coloured substance that can be separated using chromatography.

Question 19

How do you test for hydrogen gas?

Answer 19

Hold a burning splint at the open end of a test tube containing the gas.

Question 19

What is the positive result for hydrogen?

Answer 19

A ‘squeaky pop’ sound (hydrogen burns rapidly).

Question 20

How do you test for oxygen gas?

Answer 20

Insert a glowing splint into a test tube containing the gas.

Question 21

What is the positive result for oxygen?

Answer 21

The splint relights.

Question 22

How do you test for carbon dioxide gas?

Answer 22

Bubble the gas through limewater (calcium hydroxide solution).

Question 23

What is the positive result for carbon dioxide?

Answer 23

The limewater turns milky/cloudy.

Question 24

How do you test for chlorine gas?

Answer 24

Hold damp litmus paper in the gas.

Question 25

What is the positive result for chlorine?

Answer 25

The litmus paper is bleached white.

Question 26

What is a flame test used for?

Answer 26

Identifying metal ions by their characteristic flame colours.

Question 27

What are the flame colours for different metal ions?

Answer 27

Lithium (Li⁺) → Crimson Sodium (Na⁺) → Yellow Potassium (K⁺) → Lilac Calcium (Ca²⁺) → Orange-Red Copper (Cu²⁺) → Green

Question 28

Why might a flame test be unreliable with mixtures?

Answer 28

Some flame colours can mask others, making it difficult to identify ions.

Question 29

How do metal ions react with sodium hydroxide (NaOH)?

Answer 29

They form insoluble hydroxide precipitates.

Question 30

Which metal ions form white precipitates?

Answer 30

Aluminium (Al³⁺), Calcium (Ca²⁺), Magnesium (Mg²⁺)

Question 31

How can you distinguish aluminium from calcium and magnesium?

Answer 31

Aluminium hydroxide dissolves in excess NaOH, but calcium and magnesium do not.

Question 32

What are the colours of other metal hydroxide precipitates?

Answer 32

Copper(II) (Cu²⁺) → Blue Iron(II) (Fe²⁺) → Green Iron(III) (Fe³⁺) → Brown

Question 33

Write a balanced equation for copper(II) hydroxide formation.

Answer 33

Cu²⁺ + 2OH⁻ → Cu(OH)₂

Question 34

How can you test for carbonate (CO₃²⁻) ions?

Answer 34

Add dilute acid to the sample and bubble the gas produced through limewater.

Question 35

What is the positive result for a carbonate?

Answer 35

Limewater turns milky (cloudy), indicating the presence of carbon dioxide (CO₂).

Question 36

How do you test for halide ions (Cl⁻, Br⁻, I⁻)?

Answer 36

Add dilute nitric acid, followed by silver nitrate solution.

Question 37

What are the precipitate colours for halide ions?

Answer 37

Chloride (Cl⁻) → White Bromide (Br⁻) → Cream Iodide (I⁻) → Yellow

Question 38

What is a mnemonic to remember halide precipitate colours?

Answer 38

"Cats With Brains Can Ideally Yodel" (Chloride = White, Bromide = Cream, Iodide = Yellow).

Question 39

How do you test for sulfate (SO₄²⁻) ions?

Answer 39

Add dilute hydrochloric acid (HCl), then barium chloride solution (BaCl₂).

Question 40

What is the positive result for sulfate ions?

Answer 40

A white precipitate of barium sulfate (BaSO₄) forms.

Question 41

Why are instrumental methods preferred over chemical tests?

Answer 41

They are accurate, sensitive, and rapid.

Question 42

What is flame emission spectroscopy used for?

Answer 42

Identifying metal ions in solutions and measuring their concentrations.

Question 43

How does flame emission spectroscopy work?

Answer 43

A sample is placed in a flame. The emitted light is passed through a spectroscope. A line spectrum is produced, which identifies metal ions based on unique wavelengths.