3: The Ocean

Question 1

What is the relationship between solutes, solvents, and solutions?

Answer 1

solute + solvent = solution

Question 2

What does it mean if a substance is soluble in a solvent and vice versa?

Answer 2

If a substance can dissolve in a given solvent, it is soluble in that solvent. If it cannot dissolve, it is insoluble in that solvent.

Question 3

What is the difference between a dilute solution, a concentrated solution, and a saturated solution?

Answer 3

A dilute solution contains a small amount of a solute in a given volume of solution. It is not saturated.
A concentrated solution contains a large amount of solute in a given volume of the solution.
If in a concentrated solution no more solute dissolved in the solution at that temperature, it is a saturated solution.
saturated solution = solution with maximum possible concentration at that temperature.

Question 4

How is concentration calculated?

Answer 4

It is the amount of solute dissolved in a stated volume of the solution. It can be obtained by dividing the mass or amount of the solute by the volume of the solution. Its unit is g dm^-3 or mol dm^-3.

Question 5

What is decantation?

Answer 5

Decantation is a quick method for separating a much denser, insoluble solid from a liquid. In decantation, pour out the liquid and use a glass rod to guide the liquid flow until the container has no liquid left.

Question 6

What is filtration and its working principle?

Answer 6

Filtration is a technique to remove suspended, insoluble solids from a liquid. A filter paper has tiny, microscopic holes. When you filter a mixture of a liquid and an insoluble suspended solid, particles of the liquid (and other particles that dissolve into the liquid) are small enough to pass through the filter paper, but the larger particles of insoluble suspended solids cannot. The solid stays behind on the filter paper as the residue while the liquid passes through as the filtrate.

Question 7

Give two items that utilise the principle of filtration.

Answer 7

1. face masks: they separate dust fom air
2. vacuum cleaners

Question 8

What is evaporation?

Answer 8

Evaporation is the conversion of a liquid to vapour at any temperature below the boiling point of the liquid.
It can be done gently in the lab by heating the liquid on a boiling water bath.

Question 9

Give one scenario that utilises the principle of evaporation.

Answer 9

In hot countries, sea water is led into shallow ponds. The water evaporates in the sun and leaves the common salt behind.

Question 10

What are two methods of crystallisation?

Answer 10

1. by cooling a hot saturated solution: since the solubility of most solids decreases as temperature decreases, when the temperature drops, the solution can hold less solute and some solute crystallises out.
   steps:
   a. heat the solution until it becomes saturated.
   b. leave the saturated solution to cool and wait for crystals to form.
   c. filter the crystals.
2. by slow evaporation of a solution at room temperature: as the solvent evaporates, the remaining solution becomes more concentrated and gradually crystals start forming.
   steps:
   a. put a piece of filter paper with holes onto a beaker of the solution.
   b. wait for the solution to evaporate and from crystals.

Question 11

What is the disadvantage of evaporation and crystallisation?

Answer 11

The solvent is lost during the process.

Question 12

What are the requirements and advantages of distillation?

Answer 12

Distillation is a method to retain both the solute and the solvent of a solution. It relies on the solvent having a much lower boiling point than that of solute so that when the solution is heated, the solvent boils but the solute does not.

Question 13

Why should we add anti-bumping granules to a distillation set-up?

Answer 13

Sometimes, violent boiling may throw the liquid out of the container while it is being heated. This is called bumping. Adding anti-bumping granules ensure smooth boiling and cuts the risk of bumping.

Question 14

What is the definition of miscible liquids?

Answer 14

Miscible liquids mix together completely, forming only one liquid layer when mixed. An example of a pair of miscible liquids are ethanol and water.

Question 15

What is the procedure of flame test?

Answer 15

1. Clean a nichrome wire by dipping it into concentrated hydrochloric acid and holding it in a non-luminous flame.
2. Moisten the clean wire by dopping it into the acid again. Then dip it into the solid sample and let some solids stick to it.
3. Hold the wire in the hottest part of a non-luminous flame and observe the colour of the flame.

Question 16

What is the flame colour of a potassium compound?

Answer 16

Lilac

Question 17

What is the flame colour of a sodium compound?

Answer 17

Golden yellow

Question 18

What is the flame colour of a calcium compound?

Answer 18

Brick-red

Question 19

What is the flame colour of a copper compound?

Answer 19

Bluish green

Question 20

List two tests for the presence of water.

Answer 20

1. Water turns cobalt(II) chloride paper from blue to pink.
2. Water turns anhydrous copper(II) sulphate from white to blue.

Question 21

What is the word equation for the electrolysis of sea water?

Answer 21

sea water ^{direct current} -> chlorine gas + hydrogen gas + sodium hydroxide solution

Question 22

Where can the products after sea water is electrolysed be found?

Answer 22

Chlorine gas is formed at the electrode connected to the positive terminal of the d.c. power supply while hydrogen gas is formed at the other electrode. Eventually, the solution in the set-up becomes sodium hydroxide solution.

Question 23

List 3 usages of hydrogen.

Answer 23

1. As a rocket fuel
2. For making ammonia
3. For making hydrochloric acid

Question 24

List 5 usages of chlorine.

Answer 24

1. Making PVC (a plastic)
2. Sterilising swimming pool water
3. Sterilising drinking water
4. Making hydrochloric acid
5. Making bleach

Question 25

List 3 usages of sodium hydroxide solution.

Answer 25

1. Making soaps and detergents
2. Making paper
3. Making bleach

Question 26

What is the definition of a physical change?

Answer 26

A physical change is a change in which no new substance is formed.

Question 27

What is the definition of a chemical change?

Answer 27

A chemical change is a change in which at least one new substance is formed.

Question 28

What is the definition of physical property?

Answer 28

Physical properties of a substance are properties that can be observed without changing the chemical composition of the substance.

Question 29

What is the definition of chemical property?

Answer 29

Chemical properties of a sibstance are properties that can be observed or measured only when the substance undergoes a chemical change to form new substance(s).