Daft sciencey type questions

[seren]

This is mine:

WHY (and/or HOW) does bicarbonate of soda relieve itching skin?

[skbuncks]

Didnt realise it did. Source please, or application which contains it.
In aqueous solutions it is a mild base (pH~8ish), and on reaction with an acid will liberate CO2, cant see how this would help though.

skb

[Trinoc]

Possibly something to do with the fact that bicarb is one of those odd compounds that can neutralise both acids and alkalis, so if the itching is caused by either of these then bicarb is likely to help it.

When bicarb encounters an acid it releases CO2 and becomes an alkali, forming the sodium salt of the acid. When it encounters an alkali it behaves as a weak acid, exchanging its hydrogen ion for an ion of whatever metal the alkali is made from, producing, say, sodium potassium carbonate.

A bottle of bicarb solution is always handy to have around when messing with chemicals. If you drop something corrosive on your hand you can pour bicarb on it without having to think first about whether what you spilled was acid or alkali.

This also means bicarb is good to rub into insect stings, and it doesn't matter whether the sting is acid or alkali.

[niggle]

My favourite from The Last Word:

Why do mirrors reverse left and right but not up and down?

[skbuncks]

My favourite from The Last Word:

Why do mirrors reverse left and right but not up and down?

[pedant mode] Concave mirrors will invert the image for objects further away than the focal point (S>F) [/pedant mode]

skb

ETA: I refuse to answer the actual question as it would cause my brain to explode from the stupid

[Mulder]

My favourite from The Last Word:

Why do mirrors reverse left and right but not up and down?

Because they are a flat surface and not a point ...

[Trinoc]

My favourite from The Last Word:

Why do mirrors reverse left and right but not up and down?

They don't. They reverse front and back. It is only our conventional assumption about the location of left and right on a human body that leads us to think left and right have been reversed.

[seren]

When bicarb encounters an acid it releases CO2 and becomes an alkali, forming the sodium salt of the acid. When it encounters an alkali it behaves as a weak acid, exchanging its hydrogen ion for an ion of whatever metal the alkali is made from, producing, say, sodium potassium carbonate.

Fascinating stuff, thank you! As for my source: my doctor!

My next question: please explain the cause of the Doppler effect, and (this is really the question I want answered) which is the real pitch of the siren/sound? The one you hear first, or the lower pitch heard afterwards?

Thank you!

[niggle]

I'll have a go if you don't mind.

Sound is waves in the air which our ears pick up and trasmit to our brains. The waves travel outwards from their source. So if that source - say, a siren - is moving, it is moving "towards" part of the waves and "away" from another part, as shown here, with the source mving towards the left:



You can see that the waves are smaller or narrower at the left and wider at the right. The pitch or "note" of the sound we hear is determined by its frequency - that is, the distance between the waves. So as the source approaches us we get the narrower waves we see at the left of the pic, which sounds higher to us, then once it passes us we get the wider waves at the right of the pic which sounds lower.

Neither pitch is the actual sound of the source - you can only get that if the source is stationary or it runs you over, giving you an instant of correct pitch.

This is a simplified explanation, but I hope it helps!

[Lord Muck oGentry]

My favourite from The Last Word:

Why do mirrors reverse left and right but not up and down?

They don't ( as Trinoc has already observed).

Stand in front of a mirror with a green ball in your right hand and a red ball in your left hand. Now draw the line from your green ball to the one in the mirror and do the same with the red balls. The lines don't cross. Neither do the lines from your head to the head in the mirror and from your feet to the feet in the mirror.

There is no reversal.

[Mulder]

There is no reversal.

So, are you saying that, if you take a photo of an object and it's mirror image then, they should look identical? Why do ambulances have backward writing on them?

[skbuncks]

.....sits back and awaits the impending semantics arguement over the word 'reversal'


skb

[Trinoc]

So, are you saying that, if you take a photo of an object and it's mirror image then, they should look identical? Why do ambulances have backward writing on them?

Ambulance writing looks reversed because you are, in effect, looking through it from behind.

When you look in a car mirror, how do you know the car behind is indicating to turn right? Because the indicator on your right is flashing, even though if you thought of the reflection as a real car it would appear to be indicating left.

As Lord Muck said, stand in front of a mirror. The reflection of your right hand is on your right (your right, not what you think is the right of the person in the mirror). The reflection of your left hand is on your left. The reflection of your head is at the top. The reflection of your feet is at the bottom. But ... the reflection of your chest is facing towards you when it ought to be facing away, and where you imagine the reflection of your back to be is facing away when it should be facing towards you. You have been inverted along a line at right angles to the surface of the mirror, which makes perfect sense. You have not been inverted left-right or top-bottom.