What are the differences between concentrated and dilute acids?
A concentrated acid is an acid which is in either pure form or has a high concentration. Laboratory type sulfuric acid (about 98% by weight) is a concentrated (and strong) acid.
A dilute acid is that in which the concentration of the water mixed in the acid is higher than the concentration of the acid itself. For instance, 5% sulfuric acid is a dilute acid.
A dilute acid, unlike a concentrated acid, will ionize to a greater degree in their solution (higher percent dissociation with decreasing concentration). However, if an aqueous acid mixture (such as sulfuric acid, mentioned above) is added to water, the resultant pH from adding a dilute one would be higher (lower acidity) than for a concentrated one.
The difference is concentration, i.e. the % of water in the mixture.
Dilute acids will be, for example, 80 or 90% water, with only 10 - 20% of the weight being the acid itself. Concentrated acids have much lower water contents - concentrated sulphuric acid is typically 4% water, 96% acid. Concentrated hydrochloric acid is 37% HCl (HCl itself is a gas, so 37% w/w is about the maximum you can dissolve in water).
Not to be confused with "strong acid", which refers not to concentration but to degree of ionic dissociation in water.