Grade 8 S&T Practice Sheet (pages 191 to 201)

ANSWER KEY

1. What is the difference between evaporation and boiling?

• Evaporation takes place at all temperatures, while boiling occurs at a particular temperature (the boiling point).

• Evaporation takes place at the surface of a liquid, whereas the entire liquid boils.

• Boiling requires an external source of heat, evaporation does not.

• Evaporation produces cooling but boiling does not.

• Evaporation is a slow while boiling is fast.

2. Give an example for deposition.

• Frost forming on a window pane

• Soot forming in a fireplace

3. List the 4 changes of matter which occur when a substance is heated.

Melting, evaporation, boiling and sublimation

4. List the 3 changes of matter which occur when a substance is cooled.

Solidification, condensation, and deposition

5. Using some blue (cold – less energy) & some red (hot – more energy) reproduce figure 20. Include an example after each change of state on your diagram.

6. Refer to the graph on page 192.

a) What does this graph show?

A heating curve

b) What happens to the temperature during the inclines (A, C & E) of the graph?

It increases

c) What happens during the plateaux (B & D) of the graph?

The substance changes state (undergoes a phase change)

7. Are chemical changes and physical changes the same? Explain.

No, during a chemical change new substances are formed. During a physical change, no new substances are formed.

8. Are physical changes always reversible? Give an example.

No, if you break a glass or chop wood, you can’t put them back together.

9. List the 5 signs which show that a chemical change may have occurred.

1. Gas bubbles or odour produced.

2. Colour change

3. Formation of a precipitate (cloudiness)

4. (and 5) Energy (heat, light, electricity, sound) produced.

10. What does the Law of Conservation of Matter say?

Matter cannot be created or destroyed, but it can be transformed.

11. Name the scientist behind this law.

Antoine Laurent de Lavoisier

12. C + O₂ CO₂

21g ? 33g

The mass of oxygen will be 12 g.

13. Matter separates into 2 large categories: they are pure substances& mixtures.

14. A pure substance, namely, an elementor a compoundare both made of one type of particle.

15. Actually we can say that compounds are made of 1 type of particle& elements are made of 1 type of atom.

16. There are 2 types of mixtures; homogeneousmixtures and heterogeneousmixtures.

17. You are looking at a mixture. How do you know if it is a homogeneous or heterogeneous mixture? Include an example of each type of mixture in your explanation.

• Homogeneous mixtures appear to be only one substance (Their particles are uniformly distributed)

• At least two different substances can be seen in heterogeneous mixtures. Their particles are not uniformly distributed throughout the mixture.

18. Define the following terms in your words. Include an example in your explanation.

a) Dissolution

When two or more substances mix to form a solution.

b) Solvent: The substance that dissolves the solute.

c) Soluble: A substance that can completely dissolve in a solvent.

d) Solute: The substance that dissolves in the solvent.

19. Are the following substances pure substances or homogeneous or heterogeneous mixtures?

a) Apple Juice Homogeneous

b) Platinum Pure (element)

c) C₂H₆ Pure (compound)

d) Muddy Water Heterogeneous

e) Taboulé salad Heterogeneous

f) Tea Homogeneous

c) Whole Grain Bread Heterogeneous

d) Air Homogeneous

e) Copper Pure (element)

f) Smog Heterogeneous

g) Sugar Water Homogeneous

h) Orange juice with pulp Heterogeneous

i) Maple Syrup Homogeneous

j) Steel Homogeneous (alloy of iron and carbon)

k) 14K Gold Homogeneous (alloy of gold and other metals)

20. Why do we separate mixtures? Give several reasons.

• To obtain specific types of fuels (octane, kerosene, paraffin) from petroleum.

• To extract metals (ex. iron, aluminum, copper, tin, nickel) from rocks (ores).

• To purify lake water to make it drinkable

21. Can we separate pure substances (either elementsor compounds) with the methods described on pages 198 to 201? Explain why or why not.

No, because the forces (bonds) holding the particles together are too strong to be broken by physical means. Compounds can only be separated by heat or electricity. Elements can only be separated by nuclear reactions.

22. You want to separate salty water. You can either boil or distill your substance. When would you use one method over the other?

• You boil the salty water when you just want to keep the salt.

• You distill the salty water when you want to purify/collect the pure water.

23. Do an informal table of the 4 methods of separation found in your textbook? Include a brief description and state for what each method is used.

Separation technique	Description	Uses
Sedimentation	Mixture is left to stand for several minutes. After a while, the heavier particles sink to the bottom of the beaker and form a sediment.	One of the steps in treating water in water-treatment plants – it allows the debris in the lake water to separate from the water.
Decantation	Used after sedimentation, it involves pouring out one of the layers into another container.	To separate water from mud or oil from water.
Filtration	The mixture is poured through filter paper which holds back the bigger particles, while the smaller particles pass through.	To separate particles that are suspended in a heterogeneous mixture (ex. Muddy water)
Distillation	The mixture is heated/boiled and the substance with the lower boiling point evaporates, enters the condenser, cools and condenses to a liquid and collects in a beaker/flask at the other end.	To separate homogeneous mixtures (ex. Salt and water, alcohol and water)

24. You must completely separate a mixture of salty water and fine sand in order to end up with the 3 substances at the end. Give the procedure to follow.

1. Filtration: Pour the mixture through filter paper to collect the sand.

2. Distillation: Distill the filtrate (salt water) to separate the salt from the water. The salt will remain in the round-bottom flask, the pure water will collect on the beaker at the other end of the condenser.

25. What’s an alloy? Give 2 examples.

An alloy is a homogeneous mixture of two or more metals or a metal and non-metal.

Examples: Steel, Brass, Bronze, 14K Gold, Sterling silver, pewter

26. Define the following in your own words. Include an example when possible.

a) Distillate: The substance obtained from condensation in the condenser of a distillation apparatus.

b) Filtrate: A liquid that has passed through a filter.

c) Condenser: A condenser is a piece of laboratory glassware used to cool hot vapors.It usually consists of a large glass tube containing a smaller glass tube running its entire length, within which the hot vapors condense into a liquid.

d) Residue: The substance that does notmove into another container after distillation

e) Sediment: Solid substances that deposit at the bottom of the beaker after sedimentation

27. Explain why the water that passes through the condenser does not mix with the mixture you are trying to separate. Because the cold water in the condenser is sealed off from the narrow tube inside the condenser where the distillate forms.

28. What is this water used for?

To cool and condense the hot vapours entering the condenser. It forms the distillate.

29. Look at the image above. The substance in the beaker is the distillate.

and the substance in the round-bottom flask is the mixture.

30. A bottle contains soapy water. The soap is the solute, the soapy water is the solution& the water is the solvent.

Soapy water is therefore categorized as a homogeneous mixture or a solution.