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  1. What floats – what sinks? Ayesha was waiting for dinner. Today Ammi was making her favourite food – puri and spicy potatoes. Ayesha watched as her mother rolled out the puri and put it in the hot oil. She saw that at first the puri sank to the bottom of the pan.
  2. As it puffed up, the puri came up and started floating on the oil. One puri did not puff up and did not float like the others. On seeing this, Ayesha took some dough and rolled it into a ball. She flattened it and put it in a bowl of water. Alas! it sank to the bottom and stayed there.
  3. In the evening Ayesha went for a bath. She had just come out when her mother called, “Ayesha, you have dropped the soap in the water again. Take it out and put it in the soap case.” Ayesha was in a hurry and the soap case fell out of her hands. It started floating on water. Ayesha gently put the soap in the soap case. She saw that the case continued to float, even with the soap in it. A wooden boat in water will float. But a needle will sink! Why does this happen?
  4. Water density: Water is an odourless, tasteless, transparent, and colourless chemical substance, which is the main constituent of this planet’s lakes, oceans, streams, and the fluids of most living organisms. Density is the mass per unit volume of a substance.
  5. Density of Water? The density of a material is defined as its mass per unit volume. It’s a measurement of how tightly matter is packed together. The density of a substance can be explained as the relationship between the mass of the substance and volume it takes up.
  6. Water density changes with temperature and salinity. Density is measured as mass (g) per unit of volume (cm³). Water is densest at 3.98°C and is least dense at 0°C (freezing point).
  7. Factors Affecting Water Density There are several factors which can affect the density of a substance. Some factors which affect the density of water is given in the points below. The density of water is around approximately 1 gram/ cubic centimetre (1 g/cm3).
  8. It is temperature-dependent, but this relation is said to be non- linear and also it is unimodal in nature rather than monotonic. When it is cooled from the room temperature, the liquid water tends to become increasingly dense, as with another kind of substances, but approximately at about 4°C, pure water is said to reach its maximum density.
  9. As it gets cooled further, it tends to expand and becomes less dense. This kind of unusual negative thermal expansion is related to strong, intermolecular forces, orientation-dependent, or interactions and it is observed in the form of molten silica.
  10. Density Vs Temperature Water does not have an absolute density as its density varies with temperature. It has a higher density in the liquid state than the solid. Check the Density Vs Temperature graph given below to understand how density changes with temperature.
  11. Archimedes' Principle Archimedes' principle indicates that the upward buoyant force that is exerted on a body immersed in a fluid, whether fully or partially submerged, is equal to the weight of the fluid that the body displaces. Because the reason that a ship floats is that it displaces a lot of water. The displaced water wants to return to it's original location, where the ship is now, and this pushes the ship upwards. The force which pushes the ship up is called the buoyancy force.
  12. Archimedes continued to do more experiments and came up with a buoyancy principle, that a ship will float when the weight of the water it displaces equals the weight of the ship and anything will float if it is shaped to displace its own weight of water before it reaches the point where it will submerge.
  13. This is kind of a technical way of looking at it. A ship that is launched sinks into the sea until the weight of the water it displaces is equal to its own weight. As the ship is loaded, it sinks deeper, displacing more water, and so the magnitude of the buoyant force continuously matches the weight of the ship and its cargo.
  14. But a needle, light as a leaf, thin as a pin, will sink right in! Why does this happen? The density of nail (as of iron) is much larger than the water. So it sinks easily. The weight of the water displaced by the ship is equal to its weight, so it floats. Whereas the weight of the water displaced by the iron nail is less than its weight so the iron nail sinks
  15. Why does ice float in water? Each water molecule is made up of two hydrogen (H) atoms and one oxygen (O) atom. The bonds between water molecules are called hydrogen bonds. As water cools to 3.98°C, its mass stays the same but volume decreases – the same mass fits into a smaller space so it is more compact.
  16. When water freezes at 0°C, the mass stays the same but its volume expands by 9 percent. In liquid water, molecules are attracted to each other and temporarily held together by hydrogen bonds.
  17. When water freezes at 0°C, a rigid open lattice (like a web) of hydrogen-bonded molecules is formed. It is this open structure that makes ice less dense than liquid water. This is why icebergs float.
  18. Why does a lemon float in salty water? When salt is dissolved in water, as it is in ocean water, that dissolved salt adds to the mass of the water and makes the water denser than it would be without salt. Because objects float better on a dense surface, they float better on salt water than fresh water.
  19. Why do things float in salty water? The fact that seawater has salt dissolved in it makes things more able to float in it. The salt in seawater causes it to weigh more per unit volume than freshwater. The higher weight causes saltwater to have a higher density, as density is directly related to the mass of the substance.
  20. Things dissolve in water: Some materials seem to disappear when added to water. We say they have dissolved. They are still there, but the particles are so small you can no longer see them. There are some materials that do not dissolve in water.
  21. A soluble substance is one that dissolves in a liquid, usually water. It might look like it's simply disappeared, but in fact, it's still there - it's just mixed in to form a liquid called a 'solution'.
  22. The solid that dissolves is called the 'solute'. The liquid that dissolves the solute is called the 'solvent'. An insoluble substance is a solid that doesn't dissolve, even if you warm up the water. Things like salt, sugar and coffee dissolve in water. They are soluble. They usually dissolve faster and better in warm or hot water.
  23. Pepper and sand are insoluble, they will not dissolve even in hot water. Substances that do not dissolve in water are called insoluble. Sand and flour are examples of insoluble substances.
  24. Dead Sea All oceans and seas have salty water. The saltiest of all is the Dead Sea. How salty? Imagine 300 grams of salt in one litre of water! Would you be able to even taste such salty water? It would be very bitter. Interestingly, even if a person does not know how to swim, she would not drown in this sea. She will float in water, as if lying down on it! Remember the lemon you floated in salty water?
  25. Dandi March This incident took place in 1930, before India became independent. For many years the British had made a law that did not allow people to make salt themselves.
  26. They had also put a heavy tax on salt. By this law people could not make salt even for use at home. “How can anybody live without salt?” Gandhiji said, “How can a law not allow us to use freely what nature has given!” Gandhiji, with several other people, went on a yatra (long walk) from Ahmedabad to the Dandi seashore in Gujarat, to protest against this law.
  27. Do you know how salt is made? The sea water is collected in shallow beds dug in the sand. Water is allowed to dry in the sun. After the water dries the salt remains on the ground.