Q Why does pastis (Pernod, Ricard etc) go cloudy when you add water? asks Geoff Crago

A Adding water to pastis tips a delicate chemical balance resulting in a rapid change in the appearance of the tipple, known as the "ouzo effect". Anise-flavoured liquors such as pastis and ouzo contain an oil, primarily composed of trans-anethol, which gives them their telltale taste. In neat pastis, the alcohol content is high enough to dissolve the oil, so the drink appears transparent. However, the oil is not soluble in water. Hence as water is added, and the ethanol diluted, the oil cannot stay dissolved. Instead it forms very tiny droplets dispersed in the water – an emulsion. Since these droplets are typically around one micrometre in diameter, they are just the right size to interact with visible light, scattering the light waves and so making the drink appear opaque.

Q Why can't all plastics be recycled? asks Timothy A

A Pretty much all plastics can be recycled in some way. But just what this entails depends upon the type of plastic. "There are two classes of plastics – thermosets and thermoplastics," reveals Dr Vannessa Goodship from the University of Warwick. "Many of the consumer items we use and recycle are thermoplastics," she explains. These include PET, PP, HDPE and PVC which can be re-melted and turned into something new. But for thermosets, a different approach is used. "Thermosets do not remelt but degrade," says Goodship. "These cannot be recycled like a milk bottle can, but are often sent for some kind of energy recovery instead." While such incineration processes destroy the material, they allow the energy such plastics contain to be harnessed. There are, however, practical and economic reasons that can create obstacles to recycling. "The families of plastics PET, PP, HDPE and PVC do not mix with each other and have to be separated. Here there are lots of added costs of cleaning, separating and reprocessing to consider," says Goodship. "It is easier to recycle if there is a large volume of the same type of waste which is why the cycling of HDPE (plastic milk bottles) and PET (clear carbonated drinks bottles) are examples of two successfully recycled plastics."

Q Why is ancient genetic material usually mitochondrial DNA, not chromosomal DNA? asks Luke R

A Scientists have recently extracted and sequenced mitochondrial DNA from a bone belonging to an early hominin, dating back 400,000 years – an astonishing feat. After such a long period, many biological molecules will have been damaged or have broken down, DNA included. But as Professor Michael Hofreiter from the University of York explains, mitochondrial DNA has an advantage.

"Mitochondrial DNA occurs in about 1,000 copies in each cell with a lot of variation, from about 500-5,000 copies," he says. Conversely, for cells within bone, which is the source of most ancient genetic material, there is far less chromosomal DNA. "Nuclear DNA occurs in just two copies per cell," Hofreiter points out. "So chances that some intact copies are left are higher for mitochondrial DNA – it's simply a number's game," he says.

Q What would happen if the world stopped spinning? asks Ali

A Besides disrupting the weather and ocean circulation, this would also stick a spanner in the works when it comes to navigation. As Dr Phil Livermore from the University of Leeds told me, the geographic poles would cease to be a logical point of reference since they would no longer be the only places on Earth that remain fixed in space. What's more, while the poles of the Earth's magnetic field are close to the geographical north and south poles, this might not be the case if the Earth were to stand still. "The Earth's magnetic field is generated in the liquid core of our planet, and is approximately aligned with the rotation axis of the Earth," explains Livermore.

"The reason why the magnetic field is so aligned is not straightforward, but it is linked to the "weather patterns" of the motion of liquid iron in the core that sustain the field," he says. "If rotation were to cease then these patterns would be destroyed and the magnetic field, if it were still generated, would likely be much more complicated than a dipole (a single north and south pole)."

The ramifications on the magnetic field could put the Earth at risk. "This field structure would be much weaker at the Earth's surface and would not protect us as well as the present dipole against harmful solar radiation," Livermore explains. But, while this could damage our satellites and wreak havoc with our communication systems, it may not mean curtains for life. "Such a scenario might be similar as that expected during a global magnetic reversal – there have been many of these in the past and life has survived, but the last was about 800,000 years ago," Livermore adds.

Q Why is the blood of a horseshoe crab not red like human blood? asks Lucinda McKay

A Humans and other mammals have red blood as a result of the metal-containing protein, or metalloprotein, haemoglobin. The colour arises because the "haem" part of the molecule, which is made up of a ring-shaped structure called a porphyrin, absorbs visible light. Oxygen in the blood latches on to the iron in the centre of this ring, and hence can be delivered to tissues around the body. Blood that contains oxygen is a brighter shade of red than the colour of deoxygenated blood because, as oxygen attaches to the iron, it changes the shape and electronic properties of the haem group and shifts the wavelengths of light absorbed.

However, other creatures use different molecules to transport oxygen around their body. Animals such as some snails and horseshoe crabs use metalloproteins called haemocyanins. The colour of haemocyanins is not due to a porphyrin, but arises from a very different structure which contains copper rather than iron. When there is no oxygen about, haemocyanins are colourless, however when oxygen is present they turn blue, resulting in creatures like the horseshoe crab having blue blood. This colour change occurs because each oxygen molecule acts like a bridge between pairs of copper atoms in the haemocyanin, thereby creating a structure that absorbs light in the visible region.

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