Page 8 - Physical chemistry understanding our chemical world

CONTENTS vii

Why is the water at the top of a waterfall cooler than the water at its base? 85
Why is it such hard work pumping up a bicycle tyre? 86
Why does a sausage become warm when placed in an oven? 87
Why, when letting down a bicycle tyre, is the expelled air so cold? 88
Why does a tyre get hot during inflation? 89
Can a tyre be inflated without a rise in temperature? 89
How fast does the air in an oven warm up? 90
Why does water boil more quickly in a kettle than in a pan on a stove? 91
Why does a match emit heat when lit? 94
Why does it always take 4 min to boil an egg properly? 95
Why does a watched pot always take so long to boil? 98
3.2 Enthalpy 99
How does a whistling kettle work? 99
How much energy do we require during a distillation? 102
Why does the enthalpy of melting ice decrease as the temperature
decreases? 104
Why does water take longer to heat in a pressure cooker than in an open
pan? 106
Why does the temperature change during a reaction? 107
Are diamonds forever? 109
Why do we burn fuel when cold? 111
Why does butane burn with a hotter flame than methane? 114
3.3 Indirect measurement of enthalpy 118
How do we make ‘industrial alcohol’? 118
How does an ‘anti-smoking pipe’ work? 120
Why does dissolving a salt in water liberate heat? 123
Why does our mouth feel cold after eating peppermint? 125
How does a camper’s ‘emergency heat stick’ work? 127

4 Reaction spontaneity and the direction of thermodynamic change 129
4.1 The direction of physicochemical change: entropy 129
Why does the colour spread when placing a drop of dye in a saucer of
clean water? 129
When we spill a bowl of sugar, why do the grains go everywhere and
cause such a mess? 130
Why, when one end of the bath is hot and the other cold, do the
temperatures equalize? 131
Why does a room containing oranges acquire their aroma? 133
Why do damp clothes become dry when hung outside? 134
Why does crystallization of a solute occur? 137
4.2 The temperature dependence of entropy 139
Why do dust particles move more quickly by Brownian motion in warm
water? 139
Why does the jam of a jam tart burn more than does the pastry? 139