Science is a very straightforward subject because it is based on facts, not opinions. At the Primary School level, this is a double-edged sword— because there’s only one answer, you are not able to argue your thesis, like you could for a humanities subject. There are some concepts that may prove to be confusing and hinder your path to excellence. However, with the correct understanding and interpretation, it is easy to get an A* for Science.
After much interaction with students, teachers and viewing forum comments on Kiasuparents.com, I noticed five common mistakes. The following questions are often seen in school’s tests and the PSLE.
When water is heated to 100 degree C, bubbles can be seen rising through the water. What do these bubbles contain?
(1) Hot water vapour (2) Hot water droplets (3) Air dissolved in the water (4) Only oxygen is released from the hot water
Misconception: “(3) Air dissolved in the water.”
Correct: (1) Hot water vapour.
Although bubbles in boiling water contain some air that has been dissolved, most of the gas in the bubbles is water vapour, so (1) is the best answer among the four.
When water begins to heat up, bubbles that form on the side of the pot do consist of air that were dissolved in the water. This is because the solubility of gases decreases with increasing temperature of the water.
As the temperature gets hotter, more water molecules move into the air spaces of the bubbles. This is because the vapour pressure of the water increases, so the bubbles contain more water vapour.
When the water boils, the temperature of the water stays at 100 degrees. The water temperature does not increase anymore because the evaporating water absorbs heat energy.
Other interesting facts:
All liquids boil, not just water. Liquids boil when their vapour pressure is higher that the surrounding air pressure. The vapour pressure of water at 100 degrees is exactly equal to the atmospheric pressure. This is why water boils at 100 degrees at sea level. It is also why water boils at a higher temperature inside a pressure cooker and at a lower temperature at high altitudes.
Water can be superheated. In a very clean glass with few impurities, it is possible to heat the water beyond the boiling point. When the water is disturbed, it will explosively boil. This is why some people advise against heating water in a clean glass in the microwave – for there is a chance of that superheated water suddenly splashing and scalding. This happens because there is a lack of nucleation sites – small pockets of air that bubbles can form.
Evaporation of water at 0 degrees
Question: Does water only evaporate at 0-100degrees Celsius?
Misconception: Water only evaporates between 0-100 degrees Celsius. Water does not evaporate below 0 degrees.
Correct: Water continues to evaporate below 0 degrees, although the process of water escaping into the gaseous state from solid state is known as “sublimation.”
Sublimation of ice on the top of Mount Everest
Sublimation is the transition of a substance directly from the solid to the gas phase without passing through an intermediate liquid phase.
Water molecules can change into the gaseous state, leaving the surface of ice without first becoming water. Even though they are frozen, water molecules in ice still continue to vibrate and there will be some that have enough energy to vibrate or escape from the surface of the ice. This is why we tie up the bags of meat or fish in the freezer, for the exposed areas tend to become dry after some time.
freeze-drying, the process by which food is preserved, works by freezing the food, then subject it to reduced pressure to allow the water in the food to sublimate directly from the solid phase into the gas phase.
Compressible gas experiment
Question: A flask of air was placed above another jar containing brown gas. The two containers are separated by a piece of cardboard as shown below.
Five minutes after the cardboard was removed a light brown gas was seen in the two containers.
Which properties of the brown gas are shown by the above experiment? A: It can be compressed. B: It has no definite shape. C: It has no definite volume.
(1) A only. (2) A and B only. (3) B and C only. (4) A, B and C only.
Misconception: The answer key is (3).
Correct: The question above is poorly set. The answer is B only, so 1-4 are not accurate.
The experiment shows that the brown gas can mix with the air. And that it has no definite shape – since it can fit the shape of the flask and the jar. That is, the answer should be B only.
Even if (A) and (C ) may be correct, this is not validated in this experiment. We can substitute the brown gas with brown water and replace the air with colourless water. After some time, we can see the brown liquid has filled both the flask and the jar. Water is not compressible and has a definite volume. So (A) and (C ) are not proven with this experiment.
(B) is the only answer.
to test for compressibility, we can use a syringe to compress the gas.
Question: Insecticides are sprayed onto the plant. They entered the soil and get absorbed by the plant. The insecticides are found in the xylem-water carrying tubes. Insects eat the sugar (food) made by the leaves of the plant. This sugar is found in the phloem. How do the insecticides kill the insects?
Misconception: The insect consumes the sugars which were produced with contaminated water as a process of photosynthesis.
Correct: The insecticide coats the leaves of the plant, which the insects eat along with the leaves.
Different modes of action of pesticides
The question is meant to mislead. Do not be tricked by the question. The answer is quite straight-forward. Insecticides kill by coating the leaves of the plant, which the insects ingest.
In photosynthesis, only water molecules and carbon dioxide molecules make the sugars.
Insecticides consist of other molecules that do not get incorporated into the sugars that are produced.
It is possible for the insecticide to be carried by the plant and get absorbed into the tissue of the plant. And it is possible for the insects to eat the contaminated tissue and drink the contaminated water. But the sugar does not have insecticide.
Think about why we wash vegetables and the answer will become clear.
Some insecticides are made by plants as defences against insects. These are known as natural insecticides.
Question: what requires more force – pulling a load up an inclined plane or pulling the same load up vertically?
Wrong: Pulling it up an inclined plane because it needs to overcome gravity and frictional force; lifting it up requires overcoming the gravity only.
Correct: Pulling the load up vertically requires more force.
Ignoring the effects of friction, pulling the same load up vertically or over an incline results in the same amount of work done (same gravitational potential energy).
The work done = force x the distance travelled.
Force = work done / distance
Given the same work done, and that the load has to travel further along the incline, the force must be smaller for the incline.
Now, we need to add in friction
Friction is highest when the perpendicular force to the surface is the highest – i.e. when the incline is horizontal. In the typical case, the frictional force is less than the weight of the object, so pulling the object along the horizontal “incline” requires less force.
Friction decreases when the incline increases. When the incline is vertical, there is no frictional force and the force needed to pull the load is equal to the weight.
For inclines between horizontal and vertical, The force required to pull the load will be a function of the frictional force and the weight. As the incline becomes steeper, the force due to weight increases (from horizontal = 0 to vertical = full weight) and the frictional force decreases (from horizontal = full friction to vertical = no friction) – we can see that that the force required to pull the load over an incline will be no larger than the weight of the object provided the frictional force is no larger than the weight of the object (which is typically the case).
Other interesting facts
The inclined plane is one of the six classical simple machines defined by Renaissance scientists. It’s regularly used as an aid for raising or lowering a load.
The pyramids in Egypt are thought to have been built using ramps built out of mud brick and coated with chips of plaster to harden the surface.
There are instances of poorly-set examination questions and times when I have encountered teachers providing the wrong answers to students. I hope that this article can clarify some concepts and bring you closer to your A*.