Page 36 - Physical chemistry understanding our chemical world

WHAT IS PHYSICAL CHEMISTRY: VARIABLES, RELATIONSHIPS AND LAWS 3

But while asking questions concerning whether a series of
Complementary means
colours are complementary, we are in fact asking two questions
‘to make complete’.
at once: we ask about the colour in relation to how dark or light
it is (‘What is the brightness of the colour?’); but we also ask a
more subjective question, saying ‘Is the pink more red or more white: what kind of
pink is it?’ We are looking for two types of relationship.
In any investigation, we first look for a qualitative relationship. In effect, we ask
questions like, ‘If I change the variable x, is there is a response in a different variable
y?’ We look at what kind of response we can cause – a scientist wants to know about
the qualities of the response, hence QUAL-itative. An obvious question relating to
qualitative relationships is, ‘If I mix solutions of A and B, does a reaction occur?’
Only after we know whether or not there is a response (and of what general kind)
does a physical chemist ask the next question, seeking a quantitative assessment. He
asks, ‘How much of the response is caused?’ In effect, physical chemists want to
know if the magnitude (or quantity) of a response is big, small or intermediate. We
say we look for a QUANT-itative aspect of the relationship. An obvious question
relating to quantitative relationships is, ‘I now know that a reaction occurs when I
mix solutions of A and B, but to what extent does the reaction occur; what is the
chemical yield?’

Does my radio get louder if I vary the volume control?
Observed and controlled variables

We want to turn up the radio because it’s noisy outside, and we want to hear what is
broadcast. We therefore turn the volume knob toward ‘LOUD’. At its most basic, the
volume control is a variable resistor, across which we pass a current from the battery,
acting much like a kettle element. If we turn up the volume control then a larger
current is allowed to flow, causing more energy to be produced by the resistor. As
a listener, we hear a response because the sound from the speakers becomes louder.
The speakers work harder.
But we must be careful about the way we state these relationships. We do not ‘turn
up the volume’ (although in practice we might say these exact words and think in these
terms). Rather, we vary the volume control and, as a response, our ears experience
an increase in the decibels coming through the radio’s speakers. The listener controls
the magnitude of the noise by deciding how far the volume-control knob needs to be
turned. Only then will the volume change. The process does not occur in reverse: we
do not change the magnitude of the noise and see how it changes
the position of the volume-control knob. We consciously, care-
While the magnitude of the noise and the position of the volume
fully, vary the magni-
knob are both variables, they represent different types, with one tude of the controlled
depending on the other. The volume control is a controlled variable
variable and look at
because the listener dictates its position. The amount of noise is the the response of the
observed variable because it only changes in response to variations observed variable.
in the controlled variable, and not before.