Physical and Chemical Changes PDF

Physical and chemical changes 5.3.Physical change is a change in shape or appearance Key points:  Lattice: a three-dimensional arrangement of particles in a regular pattern  Vaporise: change state from a liquid to a gas; evaporate  Vapour: gaseous form of a substance that...

Physical and chemical changes 5.3.Physical change is a change in shape or appearance Key points:  Lattice: a three-dimensional arrangement of particles in a regular pattern  Vaporise: change state from a liquid to a gas; evaporate  Vapour: gaseous form of a substance that is normally solid or liquid at room temperature  (e.g. water vapour)  Volatile: describes a substance that easily becomes a gas boiling point (BP) the temperature at which a liquid boils and becomes a gas  Condense: to become a liquid, from a gas.  Sublimation: a change of state from a solid directly to a gas.  Melt: change state from solid to liquid Physical changes are reversible Most physical changes are reversible, which means the change can be undone and the substance goes back to how it was. When you put water (H 2 O) in the freezer, it turns to solid ice (H 2 O). When you take the ice out of the freezer, it melts back into liquid water. In this way we observe that a physical change has taken place because the water molecules are unchanged and no new substances have been created. Particles don’t change when they change state. The molecule of water that contains two hydrogen atoms and one oxygen atom (H 2 O) is exactly the same when it is a solid, a liquid or a gas. The only difference is how closely packed all the water molecules are and how much kinetic energy they have. In ice, all the water molecules are in a regular arrangement (rows, columns and layers). A three- dimensional arrangement of particles in a regular pattern is called a lattice. The water molecules in ice are constantly vibrating. This ice lattice is unique when compared to all other solids. Most solids are smaller and more compact than their liquid versions. The solid ice is different because it takes up more space than the liquid water. This is because the 3D bonds between the water molecules take up more space in the solid than the liquid. When heat energy is added, the water molecules vibrate faster. However, the molecules are still held in place in the lattice by other water molecules around them. As the ice warms up more, the water molecules gain more energy and vibrate even faster. Eventually they have so much energy that the water molecules break free of the others around them and are free to move around. The solid ice has melted to become liquid water. Changing state Substances can change between the three states. You are familiar with seeing water change state (when ice blocks melt), but other substances may only ever be seen in one state. Theoretically, all substances can be changed into different states if the temperature is hot (or cold) enough. Even gases, such as nitrogen, can be turned into a liquid at very low temperatures. ‘Dry ice’ is actually solidified carbon dioxide (CO2 ). Vaporisation and condensation When a liquid evaporates to become a gas, we say it has evaporated or vaporised. A vapour is the gaseous form of a substance that is normally a solid or liquid at room temperature. For example, when water is turned into a gas, it is referred to as water vapour. Vapours that are smelly are often called fumes. Vapours and fumes are gases and will behave like gases. Volatile substances, such as petrol, vaporise easily. Cooking oil does not vaporise if left at normal room temperatures. Cooking oil is not a volatile liquid. Boiling occurs when we heat a liquid to change it into a gas. Water left in the open at normal room temperature will evaporate very slowly. If the water is heated to its boiling point, the water molecules will quickly gain kinetic energy and evaporate or vaporise faster. When a gas cools down, we say it condenses into a liquid. The most common condensation you can observe is when your breath condenses on a cold surface. The kinetic energy of the water molecule passes to the surface as heat energy. The water molecules do not change, but they cool down, slow down and condense from a gas to a liquid. Melting and solidification When a solid is heated and changes state to become a liquid, we say it has melted. When the liquid loses heat and becomes a solid, it is called solidification. When solidification happens to water, it is sometimes called freezing. In both these examples the molecules do not change, only the amount of energy they have changes. Sublimation Some substances don’t ever exist as liquids. They just change state from a solid to a gas or from a gas to a solid. The process of a solid becoming a gas is called sublimation. Dry ice (CO2 ) changes directly from a solid into a gas when it warms up. Dry ice is often used to produce smoke effects on stage at concerts. However, the ‘smoke’ you see is not carbon dioxide, but clouds of water. When dry ice sublimes to form carbon dioxide gas, it cools the air quickly. This drop in temperature causes water vapour in the air to condense and form clouds of water. Diamond is the hardest known substance on Earth. It also sublimes, but only at extremely high temperatures (above 3500°C).