This blog is managed by Song Hock Chye, author of Improve Your Thinking Skills in Maths (P1-P3 series), which is published and distributed by EPH.

## Monday, October 06, 2008

### The Three States of Water

Most PSLE students know that water exists in three states – solid, liquid and gas. Most students also know that the ‘magic numbers’ are ‘0’ and ‘100’ deg Celsius, because that is the temperature at which water changes its state.

Students also know that ice exists at 0 deg C and below, while steam exists at 100 deg C and above.

However, quite a few students get confused, when the temperature is at or between 0 and 100 deg Celsius. Some students get the impression that water exists in liquid form only when the temperature is at or between 0 and 100 deg C. This concept is wrong.

At or between 0 to 100 deg C, water can exist in two forms – liquid (as in lakes, rivers) and gas (water vapour). Below is a diagram to illustrate the states of water, in relation to the temperature.

Simply put,
Solid - At 0 deg C and below.
Liquid – At and between 0 and 100 deg C.
Gas – At 0 deg C and above.

Important and useful point to note

Because water can exist in 2 forms (liquid and gas) from 0 to 100 deg C, we have evaporation, condensation in this world and hence, the very critical Water Cycle, which is so important to life on earth.

If water exists only as liquid between 0 to 100 deg C, it will not evaporate to form water vapour, then condense to form clouds, and eventually fall back to earth as rain.

This wide range of 0 to 100 deg C, where water exists in 2 forms is unique, unlike many other substances, where at a given temperature, it exists only in one form.

Summary – Water exists in two forms (liquid and gas) from 0 to 100 deg C and NOT in liquid form only.

keentolearn said...

Dear Mr Song

From scarecrow in Cbox

What temperture does the volume of water start to increase when it changes into ice?

Is this in Primary Science syllabus?

Excel Eduservice said...

That is not part of PSLE syllabus. It is in secondary.

Tilted Earth said...

Dear Mr Song

From scarecrow in Cbox
Jo decided to make a bulb light up brighter. What should he do?

What is the more appropriate answer?

a)using more batteries in series(will it burn the filament)

b)use a higher wattage bulb(but if the batteries are not strong enough, does this help?)

Excel Eduservice said...

Answer is (a). As long as the voltage rating of the bulb is not less than the voltage of the circuit, it is OK.

For (b), using a higher wattage bulb will in fact make the bulb dimmer, if the same number of bulbs is used.

Excel Eduservice said...

Sorry, for (b), it should be...For (b), using a higher wattage bulb will in fact make the bulb dimmer, if the same number of batteries [not bulbs] is used

Anonymous said...

It shouldn't if it's exactly the same type of light bulb. If they're different.. then... it's a different story.

Light bulb wattages are rated as how much electricity they CONSUME, not how much light they give off or how bright they are.

All lights have certain levels of efficiency, meaning how much of the electricity they use is actually converted to LIGHT, instead of other wasted energy (like heat and sound).

A highly efficient 25 watt light may actually give off a lot of light, and only very little heat and sound (like fluorescent lights); whereas some 100 watt lights that are very inefficient, will actually give off very little light, but give off lots of heat (like incandescent lights).

Of course, if the lights are of the exact same type, the the higher the wattage, the brighter it should glow.

Excel Eduservice said...

You have made a point that made me re-look at my previous answer. Just realised that the answer I gave is technically wrong as well.

On 7 Oct I wrote “Sorry, for (b), it should be...For (b), using a higher wattage bulb will in fact make the bulb dimmer, if the same number of batteries [not bulbs] is used”

“It should be Sorry, for (b), it should be...For (b), using a higher VOLTAGE rated bulb will in fact make the bulb dimmer, if the same number of batteries [not bulbs] is used”

Thanks for the highlight.

As for the contents you posted, you are talking about the AC supply. That is not part of PSLE syllabus. The original Q posted in the comments was what would happen if a higher wattage (should actually be higher rated voltage bulb – I assume the poster quoted the original Q wrongly, since it is in context of PSLE) were to be used (in a DC circuit).

Anyway, thanks for your contribution, which is good info for those who wish to know more than just PSLE Science.

tilted earth said...

Dear Mr Song

Using the same number of batteries, does arranging two bulbs in parallel cause the bulbs to be brigher as compared to arranging the two bulbs in series?

Excel Eduservice said...

Parallel is brighter.

Tilted Earth said...

Dear Mr Song

For bulbs arranged in parallel, does it require more power to shine brightly as compared to bulbs arranged in series?

As for bubls in series/parallel, is this in the science syllabus for PSLE?

Tilted Earth said...

Dear Mr Song

As for bulbs in series/parallel, is this in the science syllabus for PSLE?

Anonymous said...

Most lamps are rated in watts at a specified voltage. Think it is hardly rated in voltage. The specified voltage is usually the normal working voltage that the lamp can withstand at all time.

In Singapore, the most common voltage is 230V for lamps to be used for general lighting service.. though they should have no problem working at 220V or 240V or even higher.

Lamps for various special uses have voltage ratings which range from 1.5 V for flashlight lamps, 6 V for projector lamps, 12 V for automotive lamps etc. The same lamp can be lighted up using both AC and DC power sources.

Excel Eduservice said...

Tilted Earth,

For bulbs in series/parallel, students must know:

1. If 1 bulb blows, will the rest still light up.
2. Which set up will the bulbs be brighter/dimmer.

You do not know which connection uses up more energy.

Anonymous,

I have to remind readers here that AC circuit is NOT part of PSLE Science. In fact, “wattage” is not even mentioned in PSLE Science textbooks.

All Qs on electricity will be on DC circuits only. Hence, for a bulb to blow in a DC circuit, we are not talking about the “wattage” but the voltage rating of the bulb, in relation with the DC voltage supply.

We are also not talking about the 230 V AC supply, but the 1.5V, 3V, 6V, 9V or 12V DC supply. As such, if the supply is 6V, and you use a bulb suited for 1.5V, there is a high chance that bulb will blow.

Conversely, if you use a 9V rated bulb, and the circuit is powered by a 1.5V battery, that bulb will be very dim, as compared to another bulb that is rated 1.5V, keeping voltage supply at 1.5V.

Likewise, a 9V rated bulb, in a 9V supply DC, will be brighter than the same 9V rated bulb connected to a 1.5V DC circuit.

As for lamps that can be used for AC and DC, that again is not part of PSLE syllabus.

Anonymous said...

Hope I am not confusing your readers. But just like to clarify that :

1. ALL incadecent lamps can work with an AC supply, as well as a DC supply. ie, your 1.5V lamp works with 1.5V AC as well as 1.5V DC supplies. Similarly, your 230V lamp works with 230V AC as well as 230V DC supply. Wattage is not meant only for AC circuits only ;-) It is equally applicable for your 1.5V flashlight bulb.

2. You have shown an example where your supply voltage is 1.5V, and when you connect it to a circuit with a 9V bulb compared to a 1.5V bulb. the one with a 9V bulb is likely to be dimmer.

However, if I supply 9V to a circuit with a 1.5V bulb, and then to a circuit with a 3V bulb, and next to a circuit with a 9V bulb, which circuit do you think will be brightest?
Does "using a higher VOLTAGE rated bulb will in fact make the bulb dimmer, if the same number of batteries [not bulbs] is used” sound correct in this case?

Anonymous said...

Hi Tilted Earth,

For bulbs arranged in parallel, does it require more power to shine brightly as compared to bulbs arranged in series?

Lets say you have 2 bulbs.

The effective resistance of the 2 bulbs in series is about 4 times the effective resistance of the 2 bulbs in parallel.

What this means is that the circuit with the bulbs in parallel effectively draws about 4 times MORE current compared to the one with the 2 bulbs in series.

This further implies that your battery in the circuit with parallel bulbs will last roughly 1/4 as long.

In short, your answer is YES. The circuit with the bulbs in parallel draws more current and hence uses more energy.

Excel Eduservice said...

This is good discussion because it broadens the knowledge base of not only students but parents as well. However, I will have to put a disclaimer that what I post in this message is OUTSIDE PSLE syllabus, and students/parents are advised to be aware of that fact.

Yes, there is “wattage” in DC circuits as well. However, if a P5/6 student were to go to a hardware or electrical shop to purchase bulbs for DC circuits for conducting experiments, they will see that the ratings of those bulbs will be in volts and amps, and not in watts.

Of course, if you know Basic Electricity, all you need to do is to --- volts x amps = power, which is the “wattage”. That said, the Q is about using different V rated bulbs for different voltage circuits.

For sure we know that using an under-rated V bulb for a much higher voltage DC supply, will result in a higher probability that the bulb may blow.

As for using a higher rated V bulb in a lower supply DC circuit, the best thing to do is to test it yourself. You can set up the circuit using a 2 different rated bulbs, say 1.5V and 6V. Then connect them one at a time each, to a 1.5V circuit.

I have tried this some years back. The higher voltage rated bulb was much dimmer than the 1.5V rated bulb. I never really bothered to find out why this was so, but if you are interested in the topic of electricity, you can perhaps look into the possibility of resistance playing a part. Could it be that the higher voltage rated bulb has a higher resistance as well? I don’t know. Maybe you can help us find out.

Anonymous said...

VOLTAGE RATING
Voltage rating is the supply voltage required for normal brightness. If a slightly higher voltage is used the light bulb will be brighter but its lifetime will be shorter. With a lower supply voltage the light bulb will be dimmer and its lifetime will be longer. The light from dim light bulbs has a yellow-orange colour and the light from a bright light is more whitish in color. Typically, bulbs can operate at up to 3 to 4 times the rated voltage before it fuses.

POWER OR CURRENT RATING
This is the power or current for the light bulb when connected to its rated voltage. Low power bulbs are usually rated by their current and high power lamps by their power. It is easy to convert between the two ratings:

P = I × V
or
I= P/V where:P = power in watts(W)
I = current in amps (A)
V = voltage in volts (V)

Examples:

A lamp rated 3.5V 0.3A has a power rating
P = I × V = 0.3 × 3.5 = 1.05W

A lamp rated 6V 0.06A has a power rating
P = I × V = 0.06 × 6 = 0.36W

A lamp rated 12W 2.4W has a current rating
I = P / V = 2.4 / 12 = 0.2A

The brightness of a bulb is directly related to how much power (watts) it consumes, assuming we are talking about bulbs of similar efficiency & make. Of course resistance plays a part by limiting the amount of current, and also indirectly contributes to the efficiency of the bulb.

In short, when we talk about the brightness of a bulb, we should be talking about the combination of voltage AND current currently supplied to the light bulb or the amount of power it is consuming (in Watts). It should NOT be determined by the voltage rating of the bulb , but rather the actual voltage that is being supplied to the bulb.

Sorry if I confuse anyone out there. Good luck in your PSLE.

Anonymous said...

Just to highlight what I said in my previous post:

If we have 2 light bulbs with the following specs:

(a) A bulb rated 3.5V 0.3A
It has a power rating of
P = I × V = 0.3 × 3.5 = 1.05W

(b) A light bulb rated 6V 0.06A
It has a power rating of
P = I × V = 0.06 × 6 = 0.36W

Bulb(a) has a voltage rating of 3.5V and Bulb(b)has a voltage rating of 6V. Bulb (b) is dimmer in this case because the POWER RATING is lower than Bulb(a)0.36W vs 1.05W. It is NOT dimmer because of the higher voltage rating.

If I have a 3rd bulb(c) rated 6V 0.3A, its power rating will be
P = 6 x 0.3 = 1.8W
This bulb will be the brightest among the 3 bulbs, even though its voltage rating is also 6V.

In summary, when choosing a light bulb, the higher the POWER rating (by multiplying Voltage and Current), the brighter will be the bulb.

Excel Eduservice said...

“In summary, when choosing a light bulb, the higher the POWER rating (by multiplying Voltage and Current), the brighter will be the bulb.”

I have to repeat and re-emphasise that the above statement has to be qualified. The above is definitely true for AC circuits, where the AC supply is very high (110V for countries like US and Canada or 230V for UK and Singapore). In the REAL WORLD however, a 110V/230 V AC supply behaves very, very differently from a 1.5V DC supply. Your calculation is accurate, only on paper. It does not hold true in actual real life.

I appreciate your sharing of views. However, in the REAL WORLD, it is not that simple. You have left out a very critical aspect, which affects DC circuits tremendously – resistance.

In the REAL WORLD, most DC circuits are usually anywhere from 1.5V to 12V only. As such, a slight resistance increase, be it internal or external, will have a MAJOR IMPACT on the output of the load (in this case the brightness of the bulb). This is unlike the 110V/230V AC supply, where a slight increase in the resistance can be virtually ignored, because of the very much higher voltage and current involved.

As I have pointed out earlier, the best way is to carry out the experiment yourself. Get a 1.5V rated bulb and a 6V rated bulb. Connect them one at a time each, powered by a 1.5V DC supply. See it with your own eyes, that the higher rated voltage bulb is dimmer than the lower rated voltage bulb. I have carried out the experiment before, so I can say that with conviction.

My take is that the 6V rated bulb has a higher internal resistance than the 1.5V rated bulb. Because the DC supply is small (1.5V), a slight increase in the internal resistance of the bulb will affect the performance. This is unlike the AC supply, where the voltage is in the hundreds of volts and any internal resistance can be overcome easily.

I will also qualify that the above explanation is my opinion only, because I have never really bothered to find out why the 6V rated bulb is dimmer than the 1.5V rated bulb in a DC circuit. However, I am quite confident that internal resistance of the bulbs has a lot to do with it.

Anonymous said...

Please refer to my last posting with the 3 bulbs.

You may have chosen a 6V bulb that has a lower power rating than a 1.5V bulb. This of course is related to the internal resistance. You probably should have chosen a 6V bulb with similar power rating ;-)

FYI, resistance applies to both AC and DC circuits ;-) Of course in AC there is an additional component and we call it impedance instead.

This will be my last post on this topic.

Have a good day.

Anonymous said...

I re-read your post this morning on the way your experiment was conducted and I concur with your general observation though not with the conclusion drawn ....

A 1.5V rated bulb is meant to work at around 1.5V, and a 6V bulb is meant to work at 6V, and a 230V bulb is meant to work at 230V.

If we supply 1.5V to a 1.5V rated bulb, we should get the 'normal' brightness that the bulb is supposed to work. It will get even brighter if we increase voltage supply until it fuses.
On the other hand, if we supply 1.5V to a 6V bulb, it is going to be very dim since it is not suppose to give off its 'normal' brightness at that low voltage. It is meant to work at 6V. Similarly, the 230V lamp is designed to work at 230V, and hence will most likely not light up at all at 1.5V.

However, if we had conducted another experiment with 2 bulbs with EXACTLY the same voltage rating but DIFFERENT power ratings, the one with the higher power rating will be brighter (assume same efficiency).
Bulb(a) - 3V 0.3A
Bulb(b) - 3V 0.6A
Bulb(b) should light up brighter at 3V supply even though the voltage ratings are the same. If we further change the specs of Bulb(b) to 3.5V 0.6A, I suspect it will also be brighter than Bulb(a) at 3V input voltage.

Also, if each of the bulbs are working at their respective rated voltages, (ie the 1.5V bulb with 1.5V supply, the 6V bulb with 6V supply etc) ,their relative brightness can be gauged by their power rating. (assuming similar efficiency)

As mentioned before, the actual amount of brightness is determined by its actual power consumption (ie voltage across the bulb X actual current going through the bulb).

If we supply a 1.5V bulb with 230V, it should theorectically be super-bright. But chances are it would have fused way before that.
Similarly, if we supercharge the 230V bulb and supply it with 1000V, it would be even more super-duper-bright (again provided it doesn't fuse by then). Potentially though, the 230V bulb can reach much higher level of brightness than a 1.5V bulb (ie the max brightness before it fuses).

I agree with the observations you made in your experiment. If we operate a 1.5V bulb at above 1.5V (say 6V), it is like an athlete on steriods, and it can outperform a much stronger athlete like a 6V bulb with higher power rating. If we pump it with even more steriods, it will most likely collapse, like the case of the bulb where it fuses. However, if we also give steriods to the 6V bulb (operate it at maybe 24V), it will no doubt set the new record.

Hence, we should choose a bulb with voltage rating that matches with the voltage of our torch or supply. And I would like to stress that the actual brightness depends on the actual power consumption of the bulb.

keentolearn said...

Dear Mr. Song

Under respiratory systems, there are two popular types of questions concerning the increase in breathing rates and heartbeat while doing exercises. What is the most appropriate information to put in while answering these types of questions?

Q1) After playing basketball with his friends, Tom checked his breathing rate again. He found that his breathing rate was 20 breaths per minute. Why was his breathing rate higher?

Q2) Jerry ran around the track for 20 minutes.
(a) What would happen to his heartbeat after 20 minutes? (b)Why is this so?

Tilted Earth said...

Dear Mr Song

When placed overnight in an air-tight container,the fine sugar coatings on sugar rolls were observed to disappear or became wetter.

Excel Eduservice said...

Keentolearn,

Important points for Q1 -
The body needs more oxygen and hence, the breathing increases.

For Q2a) Just mention that his heartbeat rate increases. No need to explain further. Q2b) Need to explain that his body needs more oxygen and food for energy. Hence, his heartbeat increases, so that the blood will be able to transport the required oxygen and food to the different parts of the body.

Tilted Earth, where did you get that Q from? Never seen it before. Is it from assessment papers or some "out of the box" thinking science books?

Tilted Earth said...

Dear Mr Song

Actually, my wife asked me about it.We bought sugar rolls on Sat Evening, and she noticed the difference while having them for breakfast today.

Last year, there was a question regarding chestnut for PSLE Science, so I thought, maybe it's useful to know more about this issue.

Anonymous said...

The humidity of the air in Singapore is relatively high.

If we leave the sugar roll outside on a hot humid day, it doesn’t take long for it to get lumpy or even dissolves.

If we had kept it in sugar an air-conditioned room, it will probably be just fine.

Inside the air-conditioned room, the humidity is low.

But outside the room, the humidity is high and the sugar absorbs moisture directly from the humid air.

Sugar is hygroscopic.
This means that it has a tendency to attract moisture.

But different hygroscopic chemicals can vary in the power of that tendency to attract moisture. Chemical like Calcium Chloride can absorb moisture even in very low humidity.

ping an said...

Dear Mr Song

Lily filled one foam cup with cold water. She then filled another foam cup with the same amount of very hot water. Immediately she placed both cups in the freezer. What Freezes First, Hot Water or Cold Water? Explain.

ping an said...

Dear Mr Song

During one clear morning, Tricia observed that the Sun appeared to be of the same size as the moon she saw the night before. She then concluded that both the Sun and the Moon are of the same size. However, her classmate, Ron told her that she is incorrect.

If you were Ron, how would you explain to Tricia that the sun is larger than the Moon even though they appear to be of the same size?

Excel Eduservice said...

Ping An,

I came across the hot/cold water in freezer some years back. The answer I got from discussion with people who have lived in cold countries is that the hot water freezes first in winter.

Many people have tried to explain this phenomenon, but I think the most convincing one is that the hot water will evaporate faster than the cold, hence, taking away more latent heat.

The net effect is that more energy is lost by the hot water than the cold in a shorter time, causing the hot water to freeze first.

I won’t argue with those who have tried out the experiment in real life.

I believe your other Q (the sun/moon) has been answered in cbox.

Anonymous said...

IS STEAM A GAS OR LIQUID,SOME SAID IS GAS SOME SAID IS liquid.

ExcelEduservice said...

Steam is a gas.