CHEMISTRY - STATES OF MATTER & PHYSICAL & CHEMICAL CHANGES - MODULE #3 B

SOLID
A solid is a state of matter in which the substance has a definite size and shape. The molecules that make up a solid are very close together and do not move about as they do in liquids and gases.

• A solid becomes a liquid when it is heated to its melting point. 
• A few solids change directly from a solid to a gas and this process is called sublimation.

• Some substances are called semi-solids. They are actually very thick liquids that seem like solids as they move much less than most liquids.

Some of the properties of solids are their 

• shape, color, hardness, density (thickness), brittleness (how easily it can be smashed or broken into pieces), malleability, ductility, elasticity, tensile strength.  

LIQUIDA liquid is a state of matter that has a definite size or volume but not a definite shape. 

• A liquid takes the shape of the container in which it is held and presses on the container in the same amount in all directions. 
• The molecules in a liquid are closer together than those in a gas but not as close together as those in a solid. 
• The molecules in solids are in a fixed position but those in liquids move around more.

• Although molecules in a liquid hang together through forces called cohesion. 

• The molecules at the surface of a liquid can also jump off from the liquid into the surrounding air in a process called evaporation. 
• The rate of evaporation depends on the humidity of the air, how much of the surface of a liquid is exposed to the air, and the temperature.

Most liquids expand or spread out when heated and contract when cooled. 

• When a liquid is heated to its boiling point, it changes to a gas. 
• When a liquid is cooled to its freezing point, it becomes a solid. 
• The melting point and boiling point are two ways to tell what a liquid is as liquids can differ in these measures. 
• Heating a mixture can be used to separate out the parts of a mixture.

• One property of liquids is diffusion. Diffusion is when the movement of a concentration in one part of a liquid that is higher to a part of the substance where the concentration is lower.

• Liquids are also difficult if not impossible to compress.

• Liquids also exert buoyancy on other objects placed within the liquid. The amount of buoyancy is equal to the amount of the liquid that is pushed out of place when the object is placed in the liquid.

GAS
A gas is a state of matter in which the substance has no definite shape or volume (size). 

• Gases take the shape and fill any container in which they are placed. 
• The molecules in gases move more quickly than those in liquids or solids and are spread out much more than in either liquids or solids.

• Gases change to liquids or solids if they are cooled or if enough pressure is applied to them. 
• The cooling of a gas so it becomes a liquid is called condensation. 
• If a gas changes directly to a solid, it is called sublimation.

There are several important scientific laws relating to gases. 

• One is Boyle's Law, named after Robert Boyle. This states that the greater the pressure upon a gas, the smaller the volume that gas will take up and the less the pressure, the greater the volume or space a gas will take up in a closed space. 

PHYSICAL & CHEMICAL CHANGES
There are several differences between a physical and chemical change in matter or substances.

• A physical change in a substance doesn't change what the substance is. 

•  In a chemical change where there is a chemical reaction, a new substance is formed and energy is either given off or absorbed. 
• For example, if a piece of paper is cut up into small pieces it still is paper. 
• This would be a physical change in the shape and size of the paper. 
• If the same piece of paper is burned, it is broken up into different substances that are not paper.

• Physical changes can be reversed

• Chemical changes cannot be reversed with the substance changed back without extraordinary means, if at all. 
• For example, a cup of water can be frozen when cooled and then can be returned to a liquid form when heated.

If one decided to mix sugar into water to make sugar water, this would be a physical change as the water could be left out to evaporate and the sugar crystals would remain. However, if one made a recipe for a cake with flour, water, sugar and other ingredients and baked them together, it would take extraordinary means to separate the various ingredients out to their original form.

• When heat is given off in a chemical change or reaction, it is called an exothermic reaction. 

•  When heat is absorbed in a chemical change or reaction, it is called an endothermic reaction. 

The speed at which chemical reactions take place depend on the temperature pressure and how concentrated the substances involved in the chemical reaction are. Sometimes substances called catalysts are used to speed up or help along a chemical reaction.