GCSE science 6-mark questions, explained

The cell cycle starts with interphase. The cell grows and makes more sub-cellular structures, such as ribosomes and mitochondria, so there will be enough for two cells. The DNA is then copied, so that each chromosome has an identical copy of itself. Next comes mitosis. In prophase the chromosomes condense and become visible, and in metaphase they line up across the middle of the cell. In anaphase one copy of each chromosome is pulled to each end of the cell. In telophase a new nuclear membrane forms around each set of chromosomes, so the nucleus has divided into two. Finally, in cytokinesis, the cytoplasm and the cell membrane divide, producing two daughter cells. Because the DNA was copied exactly, the two daughter cells are genetically identical to each other and to the parent cell, with the same number of chromosomes. Mitosis is important because it lets multicellular organisms grow, by increasing the number of cells, and it replaces damaged or worn-out cells, for example when skin heals after a cut. Some organisms also use mitosis for asexual reproduction.

On the carton, “pure” is used in its everyday sense: it means nothing has been added, so the juice is just as it came from the oranges. In chemistry, a pure substance is a single element or a single compound that is not mixed with any other substance. Orange juice is a mixture of water, sugars, acids, vitamins and many other substances, so in chemistry it is not pure at all. The chemist could check the aspirin in two ways. First, she could heat a small sample slowly and measure the temperature at which it melts. A pure substance melts at one sharp, specific temperature, so if the aspirin melts sharply at the known melting point it is pure. If it is impure, it will melt gradually over a range of temperatures, usually below the known value. Second, she could use chromatography. She would dissolve a little of the aspirin, spot it onto chromatography paper, run a suitable solvent up the paper and then use a locating agent to show the spots, because aspirin is colourless. A pure substance gives only one spot, whereas an impure sample gives two or more spots because the different substances move different distances.

At first, atoms were thought to be tiny solid spheres that could not be divided. When the electron was discovered, scientists knew atoms must contain smaller particles, so the plum pudding model was suggested: a ball of positive charge with negative electrons spread through it. Then, in the alpha particle scattering experiment, positively charged alpha particles were fired at a very thin sheet of gold foil. Most of the alpha particles went straight through, which showed that most of the atom is empty space. A few were deflected through large angles and a very small number bounced straight back, which showed that the positive charge and most of the mass must be concentrated in a tiny nucleus at the centre. The plum pudding model could not explain these results, so it was replaced by the nuclear model. Niels Bohr then suggested that the electrons orbit the nucleus at fixed distances, and his calculations agreed with experimental results. Later experiments showed that the positive charge of the nucleus is carried by protons, and James Chadwick showed that the nucleus also contains neutrons. Each time, the model changed because new evidence could not be explained by the old one.